I fed an audio signal through the grid of the 6AT6 and the sound system from there forward works...speaker, AO transformer and tube and the volume control. I fed a 21.5 MHz RF signal through the plate for both pin 2 and 7 of the 6AL5 and didn't get a signal through the speaker with either. Am I doing something wrong here? I tried about 4 different 6AL5s but nothing changed. I'm sure I'm doing something stupid! :worried:

The 6AL5 Sound Discriminator(detector) plate circuits are not tuned to the 21.25 mhz sound IF. To hear sound you would need to inject your 21.25 mhz carrier with FM audio modulation into the grid of the 1st or 2nd audio IF amplifiers.

If you have Sams 70 folder 7, on page 6 there are instructions for sound alignment using a AM signal generator and VTVM. You should be able to substitute a digital VOM or scope for the vacuum tube volt meter.

The sound IF is tuned to 21.25 MHz.
The bigger issue in tracing the signal back is that FM discriminators don't work like AM detectors. A discriminator derives the sound by a method involving things like phase comparison which is difficult to break down into smaller blocks of operation and get audio.

Gotcha...thanks everyone for the advice and insights. This is a great place to learn... particularly if you're willing to embarrass yourself!!! Seriously, this restoration and the interaction with the experts here has taught me so much. It's very appreciated!

What are you using for a,signal generator? Where,are you injecting the signal? Can you outline what you are doing with the equipment used?

The AM signal generator method kind of relies on old TV/radio service tube era signal generators with crude dirty modulators (at least if you want the detector to have output to hear and measure). Basically the tube era modulators distort the carrier enough to create a form of FM modulation along with the AM modulation.* Modern SS generators often don't have that useful distortion, and even some of the high end laboratory grade stuff from the tube era lacked it.

Easy way to check for it is to take an analog FM radio and feed it 10.7MHz (nearly universal FM IF frequency) or something in the FM band if the generator goes that high and see if the radio produces the modulation tone.

* In some cases video buzz in the sound of vintage sets is created from a similar effect where over modulation of the AM video carrier creates FM noise the TV can demodulate...I've described this in-depth in other threads before and don't feel like doing it again right now ( after midnight).

What is being used as a signal generator? How are you achieving frequency accuracy?

How do you know the generator is producing a signal?

Is it modulated?

I would like to see justification why you feel the audio channel is not working.

What is being used as a signal generator? How are you achieving frequency accuracy?

How do you know the generator is producing a signal?

Is it modulated?

I would like to see justification why you feel the audio channel is not working.

For the crosshatch pattern I'm using a Leader LCG-396 that I have tested on a Zenith 12" Sidekick I restored. There's an audible tone that accompanies the crosshatch pattern. I'd have to look up the frequency...probably 1KHz. Last week, I injected an audio output tone from a Knight KG 650 RF generator (range 160 KHz to 115 MHz for the RF...I don't know what the audio tone frequency is) all the way back to the output of the discriminator 6AL5 and got a loud audio tone from the speaker that responded to volume pot changes so everything from the volume control forward is working. The problem has to be in the RF circuit correct? Now, this morning I ran the pattern generator into the set and I could hear a very low 1KHz tone coming through the speaker that responded to volume control inputs but when I say low output, I mean I needed to put my ear close to the speaker to hear it...but it was there and that's the first time I've heard it.

As for injecting carrier signals with information into the RF circuitry, that's about where my knowledge base stops. I made some idiotic attempts to get something out of a carrier signal input to the RF circuit but they are not worth mentioning for the purpose of troubleshooting analysis.

Another potential issue for the future...I did try to input a digital over the air broadcast signal through a digital to analog converter box into the TV. It's a commercial unit I've used before on my other vintage TVs and it works OK. On the 721, I wasn't getting a discernable image just some image jumps and flashes when I changed channels on the converter box. The pattern generator injects images fine but my guess is I'm going to need some sort of a signal boost if I'm going to watch anything other than a crosshatch pattern. That's for another day.

For the crosshatch pattern I'm using a Leader LCG-396 that I have tested on a Zenith 12" Sidekick I restored. There's an audible tone that accompanies the crosshatch pattern. I'd have to look up the frequency...probably 1KHz. Last week, I injected an audio output tone from a Knight KG 650 RF generator (range 160 KHz to 115 MHz for the RF...I don't know what the audio tone frequency is) all the way back to the output of the discriminator 6AL5 and got a loud audio tone from the speaker that responded to volume pot changes so everything from the volume control forward is working. The problem has to be in the RF circuit correct? Now, this morning I ran the pattern generator into the set and I could hear a very low 1KHz tone coming through the speaker that responded to volume control inputs but when I say low output, I mean I needed to put my ear close to the speaker to hear it...but it was there and that's the first time I've heard it.

As for injecting carrier signals with information into the RF circuitry, that's about where my knowledge base stops. I made some idiotic attempts to get something out of a carrier signal input to the RF circuit but they are not worth mentioning for the purpose of troubleshooting analysis.

Another potential issue for the future...I did try to input a digital over the air broadcast signal through a digital to analog converter box into the TV. It's a commercial unit I've used before on my other vintage TVs and it works OK. On the 721, I wasn't getting a discernable image just some image jumps and flashes when I changed channels on the converter box. The pattern generator injects images fine but my guess is I'm going to need some sort of a signal boost if I'm going to watch anything other than a crosshatch pattern. That's for another day.

I thought you were attempting to inject a 21.25MHz IF signal to test the audio chain.

Do you understand how the RF IF system of this set works. It is a split sound design and if the tuner local oscillator is way off, you will get no sound.

The alignment of a split sound set is more critical than the later intercarrier sets as the sound IF/ discriminator has to be precisely aligned with the Video IF otherwise you get good sound and lousy picture or good picture with no sound. Also in the split sound receiver, you always use the tuner fine tuning control to adjust for best sound and not best picture.

I you are not familiar with split sound vs intercarrier sound systems, this forum will help enlight you.

It would be good to review the rf/if design of the split sound televisions manufactured by RCA Victor in the late 1940s. The first post WW2 design separated the sound from the video at the tuner output. You will find that the video if carrier is 25.75MHz and the audio if is 21.25MHz. The 21.25 MHz sound IF signal goes through 2 or 3 stages and it is fed to a Foster Seeley FM discriminatory. The alignment procedure is very standard but you at least need a crystal calibrated accurate 21.25 MHz signal.

In a split sound design where the audio is branched off prior to the video detector, achieving the 21.25MHz sound from the tuner requires the tuner local oscillator to be tuned very precisely. The range of the fine tuning control is only about +/-250kHz and simply changing the 6J6 oscillator tube can affect tuning as much as +/- 2000kHz which means so my changing the oscillator tube or even removing the tube shield will be sufficient to lose audio! The later KRK 5 tuner has an adjustment to offset the effects of the minor capacitance changes when replacing the oscillator tube. I believe the earlier KRK2 tuner used in your set has the same. If so it would only be a,matter of making minor adjustments to it to bring the local oscillator in line. Since the tuner uses three 6J6 tubes, you may try swapping them to find one that brings in the sound.

If you are getting a picture through the tuner and can hear,audio if applied to the volume control, that leaves only the discriminatory and two sound if stages. Providing you nor anyone else has disturbed the adjustments, the likely only case for the path to fail is open filaments or lack of power to the tube elements. I would first check if the anode and screen voltages are correct. If so and the if and discrimator tubes are lit, I think your only problem is tuning the local oscillator in the tuner.

Lastly, a word about intercarrier sound. Most people are used to on later sets adjusting the fine tuning for best picture with the sound always present. This is because in the intercarrier design, the am video carrier beats with the FM audio carrier to produce a 4.5MHz signal, or the difference between the video and sound carriers. That means that if you adjust the fine tuning control, the difference between sound and audio if carriers remain the same and you will always get the 4.5MHz diiference. RCA was one of the last manufacturers to adopt the intercarrier design until 1952 I suspect because they wanted to hold out paying a royalty fee and because of customer dissatisfaction forcing their hand.

It would be good to review the rf/if design of the split sound televisions manufactured by RCA Victor in the late 1940s. The first post WW2 design separated the sound from the video at the tuner output. You will find that the video if carrier is 25.75MHz and the audio if is 21.25MHz. The 21.25 MHz sound IF signal goes through 2 or 3 stages and it is fed to a Foster Seeley FM discriminatory. The alignment procedure is very standard but you at least need a crystal calibrated accurate 21.25 MHz signal.

In a split sound design where the audio is branched off prior to the video detector, achieving the 21.25MHz sound from the tuner requires the tuner local oscillator to be tuned very precisely. The range of the fine tuning control is only about +/-250kHz and simply changing the 6J6 oscillator tube can affect tuning as much as +/- 2000kHz which means so my changing the oscillator tube or even removing the tube shield will be sufficient to lose audio! The later KRK 5 tuner has an adjustment to offset the effects of the minor capacitance changes when replacing the oscillator tube. I believe the earlier KRK2 tuner used in your set has the same. If so it would only be a,matter of making minor adjustments to it to bring the local oscillator in line. Since the tuner uses three 6J6 tubes, you may try swapping them to find one that brings in the sound.

If you are getting a picture through the tuner and can hear,audio if applied to the volume control, that leaves only the discriminatory and two sound if stages. Providing you nor anyone else has disturbed the adjustments, the likely only case for the path to fail is open filaments or lack of power to the tube elements. I would first check if the anode and screen voltages are correct. If so and the if and discrimator tubes are lit, I think your only problem is tuning the local oscillator in the tuner.

Lastly, a word about intercarrier sound. Most people are used to on later sets adjusting the fine tuning for best picture with the sound always present. This is because in the intercarrier design, the am video carrier beats with the FM audio carrier to produce a 4.5MHz signal, or the difference between the video and sound carriers. That means that if you adjust the fine tuning control, the difference between sound and audio if carriers remain the same and you will always get the 4.5MHz diiference. RCA was one of the last manufacturers to adopt the intercarrier design until 1952 I suspect because they wanted to hold out paying a royalty fee and because of customer dissatisfaction forcing their hand.

I so appreciate the wealth of information you have provided here! I did read through almost all of your 721 restoration thread and it was very informative (...those old, reformed electrolytics still hanging in there?...). I do know the carrier signal splits into 2 paths for the audio and video and I have not messed with the converter transformer T3...in fact, the adjustment screw is soldered so you know you shouldn't move it! From the tracing I've done, the issue is in the 6BA6-6AU6-6AL5 IF chain. Video is fine and an audio signal applied to the grid of the 6AT6 1st audio is loud, clear through the speaker and is adjustable via the volume control. All these tubes are NOS and lit...filament voltage is spot on at 6.3V AC for all tubes. I did screw around with both the top and bottom of sound IF transformer (T107) and the sound discriminator transformer (T108) a little bit. No change in getting any sound and I believe I returned the adjustments to approximately their original positions. When I read the service data on how to precisely adjust the oscillation of these transformers as a function of adjusting both the top and bottom precisely, I thought I screwed it up for good. I have not checked the pin voltages on all of these tubes yet but I do remember the 6AL5 having some really low pin voltages specked out...-0.5V...stuff like that that I did not get when I did some random voltage checks. I will do a more comprehensive voltage check and report back. Thanks again so much for taking the time to reply!

Oh you shouldn't have touched the discriminator or if transformers!! To align it requires proper equipment!!

Never mind for now. There was nothing wrong with the IF adjustments. The problem is most likely the local oscillator in the tuner is off frequency.

Because you have messed with the adjustments, you may now have lost the reference between sound and video. The best thing is to not touch any of the IF adjustments and try and get sound by slowly adjusting the tuner oscillator frequency adjustment.

So long as the discriminator and IF adjustments are back to where they were, you should be okay.

I looked at the 721TS notes and I do not see a master oscillator adjustment. I suggest you first try exchanging the oscillator with the other 6J6 tubes in the tuner to see if that brings back the audio.

Oh you shouldn't have touched the discriminator or if transformers!! To align it requires proper equipment!!

Never mind for now. There was nothing wrong with the IF adjustments. The problem is most likely the local oscillator in the tuner is off frequency.

Because you have messed with the adjustments, you may now have lost the reference between sound and video. The best thing is to not touch any of the IF adjustments and try and get sound by slowly adjusting the tuner oscillator frequency adjustment.

So long as the discriminator and IF adjustments are back to where they were, you should be okay.

I looked at the 721TS notes and I do not see a master oscillator adjustment. I suggest you first try exchanging the oscillator with the other 6J6 tubes in the tuner to see if that brings back the audio.

Where is the tuner oscillator adjustment located? Is this the T3 transformer I mentioned or is it somewhere in the tuner shielded box?

Oh you shouldn't have touched the discriminator or if transformers!! To align it requires proper equipment!!

Never mind for now. There was nothing wrong with the IF adjustments. The problem is most likely the local oscillator in the tuner is off frequency.

Because you have messed with the adjustments, you may now have lost the reference between sound and video. The best thing is to not touch any of the IF adjustments and try and get sound by slowly adjusting the tuner oscillator frequency adjustment.

So long as the discriminator and IF adjustments are back to where they were, you should be okay.

I looked at the 721TS notes and I do not see a master oscillator adjustment. I suggest you first try exchanging the oscillator with the other 6J6 tubes in the tuner to see if that brings back the audio.

I pulled up the schematic (I'm at work :nono:) and there are oscillator coil adjustments for each channel. Are you saying adjust for each channel or is there a general tuner oscillator adjustment available?? I don't see one.

On the older KRK2 tuner used in this set, there is a screw for each channel. But before you touch those, it is worthwhile to swap 6J6 tubes around. The inter-electrode capacitance difference between tubes may be sufficient to bring the tuning within range of the audio. Try exchanging the oscillator with the two othe 6J6s to check that out.

You do not have to worry about the front end RF tuning because the tuning is very broad. The oscillator is the critical one and you have to be within a few hundred kHz to hear anything.

On the older KRK2 tuner used in this set, there is a screw for each channel. But before you touch those, it is worthwhile to swap 6J6 tubes around. The inter-electrode capacitance difference between tubes may be sufficient to bring the tuning within range of the audio. Try exchanging the oscillator with the two othe 6J6s to check that out.

You do not have to worry about the front end RF tuning because the tuning is very broad. The oscillator is the critical one and you have to be within a few hundred kHz to hear anything.

Couldn't one hook his digital to analog converter box set to Ch 3 or 4 rf carrier(no antenna) to the set's ant input and monitor the DC voltage at the output(pin 1) of 6AL5(V116) sound discriminator? With the TV's ch selector set to the corresponding carrier frequency it seems pin 1 should be at or very near 0v DC when everything is aligned correctly. If this works I'd set the fine tuning to center and adjust the Ch 3 or 4 oscillator slug in attempt to read 0 DC volts at pin 1. This assuming the sound IF and discriminator adjustments are very near their previous tampered with settings.

Couldn't one hook his digital to analog converter box set to Ch 3 or 4 rf carrier(no antenna) to the set's ant input and monitor the DC voltage at the output(pin 1) of 6AL5(V116) sound discriminator? With the TV's ch selector set to the corresponding carrier frequency it seems pin 1 should be at or very near 0v DC when everything is aligned correctly. If this works I'd set the fine tuning to center and adjust the Ch 3 or 4 oscillator slug in attempt to read 0 DC volts at pin 1. This assuming the sound IF and discriminator adjustments are very near their previous tampered with settings.

Yes you could. However you will hear the audio as you are tuning the slug long before you will have anything intelligible on the meter. Further you can hear when it is tuned as you will be tuning between the two distortion peaks if the discriminator is properly aligned for a perfect s response. The alignment remains suspect now because the settings have been changed. But even if the alignment is bad, you should hear something.

So the first rudimentary step is to try and bring the local oscillator back in line. I suspect a simple swap around of the tubes will yield some audio. Then we can focus on its precise final adjustment.

Which reminds me that split sound TV tuning is no different from an FM radio tuning which uses the discriminatory circuit. That is as you approach the correct tuning, the audio builds in level, becomes distorted then the loudest and clearest audio is received. Tuning further you reach a second distortion peak and the audio clears and is weaker. Many forget that either side of the best audio point is slope detection which is weaker and slightly distorted.

But here, we first need to hear any audio and the home in on it.

Yes you could. However you will hear the audio as you are tuning the slug long before you will have anything intelligible on the meter. Further you can hear when it is tuned as you will be tuning between the two distortion peaks if the discriminator is properly aligned for a perfect s response. The alignment remains suspect now because the settings have been changed. But even if the alignment is bad, you should hear something.

So the first rudimentary step is to try and bring the local oscillator back in line. I suspect a simple swap around of the tubes will yield some audio. Then we can focus on its precise final adjustment.

Which reminds me that split sound TV tuning is no different from an FM radio tuning which uses the discriminatory circuit. That is as you approach the correct tuning, the audio builds in level, becomes distorted then the loudest and clearest audio is received. Tuning further you reach a second distortion peak and the audio clears and is weaker. Many forget that either side of the best audio point is slope detection which is weaker and slightly distorted.

But here, we first need to hear any audio and the home in on it.

I'm with you guys listening in earnest. I will do the tube swapping later this evening as I'm working late today. I have about a dozen 6J6 tubes so I can spend the next 2 days doing tube swaps if that's what's needed. We'll see. I'll update in a few hours. Thanks again so much for the interest and help!

Cool. If you have a whole bunch, you'll have a better chance of finding one that works. Of course the tube shield must be on the tube for if it is missing, it too can cause the tuning to be way off.

Cool. If you have a whole bunch, you'll have a better chance of finding one that works. Of course the tube shield must be on the tube for if it is missing, it too can cause the tuning to be way off.

Been swapping for about an hour...no change. I'm going to double check my audio input test just to be certain I'm getting what I said I was getting from the volume control forward.

Nice clean tone injecting an audio signal into the volume control.

As I said before, I can hear a very faint 1KHz tone that responds to volume changes when injecting video and audio through the antenna from the Leader pattern generator but it is very, very faint. I have to put my ear up to the speaker to hear it but it is there.

As I said before, I can hear a very faint 1KHz tone that responds to volume changes when injecting video and audio through the antenna from the Leader pattern generator but it is very, very faint. I have to put my ear up to the speaker to hear it but it is there.

At any point throughout fine tuning does the audio get louder even though the picture may worsen, or does the best picture seem to track with the loudest sound?

At any point throughout fine tuning does the audio get louder even though the picture may worsen, or does the best picture seem to track with the loudest sound?

I get no change in the very faint sound no matter what I do to the fine tuning. I think I might need to get a list together of the "special" equipment Penthode mentioned I would need to get the transformers I messed with back to where they need to be. Nothing I've done so far has affected the picture. It's still rock solid.

You are getting a picture? What are you feeding the set?

You are getting a picture? What are you feeding the set?

Leader LCG-396 pattern generator crosshatch and 1KHz tone through coax into a 75 ohm to 300 ohm adapter to the 300 ohm antenna wire into the set. Picture is fine...just no noteworthy sound. Maybe I've got a bad tube socket in the tuner? I'll check. I've wiggled them but I'll clean them and take a closer look. We had talked about pin voltages in the audio IF before. Is there anything to gain by doing that at this point?

I didn't want to interfere in the effort but I took a look at the Leader LGC-396 manual and it does not mention a 1KHz sound signal.
If it did that could be used to get the discriminator at least in the ballpark.

The way a NTSC RF signal is produced is that the sound signal (1KHz lets say) is FM modulated onto a 4.5MHz carrier, then that sound (sub)carrier is added to a video signal, that combined video signal is AM modulated onto a RF channel frequency carrier. The result is that there is a FM sound subcarrier that is offset from video carrier by 4.5MHz.

Do you have an oscilloscope?

I didn't want to interfere in the effort but I took a look at the Leader LGC-396 manual and it does not mention a 1KHz sound signal.
If it did that could be used to get the discriminator at least in the ballpark.

The way a NTSC RF signal is produced is that the sound signal (1KHz lets say) is FM modulated onto a 4.5MHz carrier, then that sound (sub)carrier is added to a video signal, that combined video signal is AM modulated onto a RF channel frequency carrier. The result is that there is a FM sound subcarrier that is offset from video carrier by 4.5MHz.

Do you have an oscilloscope?

When I hooked the Leader up to my Zenith for testing, there was a tone. I'll check again this evening. My Conar generator definitely has one. Yes I have a Tektronix and a Heathkit scope

If there was an intentional audio tone on the Leader it would be highly unlikely that they didn't include a switch and/or volume control marked as such so it could be turned on/off or level adjusted. Do you have an over the air digital to analog converter you can use as a signal source?

If there was an intentional audio tone on the Leader it would be highly unlikely that they didn't include a switch and/or volume control marked as such so it could be turned on/off or level adjusted. Do you have an over the air digital to analog converter you can use as a signal source?

OK I'll check this evening

The effectiveness of the Leader is how close the Leader output is to the channel carrier frequencies. Let us assume it is close.

What channel are you using with the Leader? Have you tried other channels?

Stepping back, let's reason this out. It cannot be a tube socket in the tuner because the tuner carries both video and audio. However the video is broadband and the audio is extremely narrow bandwidth which is why you may see video but hear no audio if the tuning (local oscillator) is slightly off.

You have exchanged the oscillator tube with other 6J6's? (The oscillator 6J6 is the one closest to the front of the chassis. The other two do not matter). You have the proper tube shield fitted? (If the shield is missing, the tuning will be off).

The audio video split is immediately after the tuner. You have monkied with the alignment so that remains an unknown. Have you confirmed there is plate and screen voltages on the IF stages?

Here is another gotcha on RCA sets: Is the discriminator tube lit? There is a 5.1 ohm resistor in the 6AL5 filament path that carries a fair bit of current and often opens. It is inserted in the path to allegedly improve the discriminator linearity hence fidelity. Check to see if the 6AL5 is actually lit and if not you can simply bridge the 5.1 ohm resistor if it is open.

Next, are you familiar with analog TV receiver fine tuning where you do not tune for brightest picture but for most detail? This is because analog TV used vestigial sideband transmission and the video carrier must be tuned off to one side of the the video band to 50% up the slope of the video bandpass on one side. That is the point you will ind the sound. If the picture is a bit smeary and lacking detail, that is a sure sign the fine tuning is off and why you may not have sound.

To recap, tell us what channel you are using, ensure the 6AL5 is indeed lit and see if the fine tuning has sufficient range to lose the video when you adjust to the end stop of the fine tuning control. (On the lower VHF channels, because split sound sets have a narrow tuning range, you may not see much difference. But try it and see).

When I hooked the Leader up to my Zenith for testing, there was a tone. I'll check again this evening. My Conar generator definitely has one. Yes I have a Tektronix and a Heathkit scope

Some other generators do have sound modulation. The LGC-396 doesn't seem to.
That generator can only output channels 5 and 6.
That may be a problem if you want to adjust channels 3 or 4 on the tuner so that you can use it with the digital TV converter.

The suggestion that you might be able to use the output from the digital converter to align the sound could be a good one. If you arrange to not receive a station on the convertor it may put out a sound carrier without modulation.
You then might be able to peak the sound IF and adjust the discriminator for zero volts output. Using a scope would be the best way to see the results.
You would first need to make sure the tuner was adjusted correctly.

...
You then might be able to peak the sound IF and adjust the discriminator for zero volts output. Using a scope would be the best way to see the results.
You would first need to make sure the tuner was adjusted correctly.

Unfortunately that will not work here because we are dealing with a split sound set and not intercarrier! Because the set top box goes through the tuner, the fine tuning (local oscillator) in the tuner will change discriminator zero. In fact, when the set is properly aligned, each time you change channels, you are always adjusting the fine tuning for discriminator zero!

You must use only a crystal calibrated 21.25MHz oscillator applied to the IF input to initially set discriminator zero. You cannot do it with anything applied to the antenna terminals.

I would not worry about discriminator zero at this point because the ball is out of the ballpark. Let us find the audio first and then find a way to set discriminator zero.

I have a deep hunch the 5.1 ohm filament resistor is bad.

Some other generators do have sound modulation. The LGC-396 doesn't seem to.
That generator can only output channels 5 and 6.
That may be a problem if you want to adjust channels 3 or 4 on the tuner so that you can use it with the digital TV converter.

The suggestion that you might be able to use the output from the digital converter to align the sound could be a good one. If you arrange to not receive a station on the convertor it may put out a sound carrier without modulation.
You then might be able to peak the sound IF and adjust the discriminator for zero volts output. Using a scope would be the best way to see the results.
You would first need to make sure the tuner was adjusted correctly.

The Leader has a channel 3 or 4 option for output...not 5 or 6. Its LCG 396...not LGC 396. Maybe that's the issue

The effectiveness of the Leader is how close the Leader output is to the channel carrier frequencies. Let us assume it is close.

What channel are you using with the Leader? Have you tried other channels?

Stepping back, let's reason this out. It cannot be a tube socket in the tuner because the tuner carries both video and audio. However the video is broadband and the audio is extremely narrow bandwidth which is why you may see video but hear no audio if the tuning (local oscillator) is slightly off.

You have exchanged the oscillator tube with other 6J6's? (The oscillator 6J6 is the one closest to the front of the chassis. The other two do not matter). You have the proper tube shield fitted? (If the shield is missing, the tuning will be off).

The audio video split is immediately after the tuner. You have monkied with the alignment so that remains an unknown. Have you confirmed there is plate and screen voltages on the IF stages?

Here is another gotcha on RCA sets: Is the discriminator tube lit? There is a 5.1 ohm resistor in the 6AL5 filament path that carries a fair bit of current and often opens. It is inserted in the path to allegedly improve the discriminator linearity hence fidelity. Check to see if the 6AL5 is actually lit and if not you can simply bridge the 5.1 ohm resistor if it is open.

Next, are you familiar with analog TV receiver fine tuning where you do not tune for brightest picture but for most detail? This is because analog TV used vestigial sideband transmission and the video carrier must be tuned off to one side of the the video band to 50% up the slope of the video bandpass on one side. That is the point you will ind the sound. If the picture is a bit smeary and lacking detail, that is a sure sign the fine tuning is off and why you may not have sound.

To recap, tell us what channel you are using, ensure the 6AL5 is indeed lit and see if the fine tuning has sufficient range to lose the video when you adjust to the end stop of the fine tuning control. (On the lower VHF channels, because split sound sets have a narrow tuning range, you may not see much difference. But try it and see).

The discriminator is lit, in fact, it's a NOS tube.
I've tried both channels 3 and 4 with the Leader input and turned the fine tuning through the full range for both. I also scrolled through the other channels in case something could be heard.
I changed the oscillator tube multiple times with different 6J6s. I have the shield in fact, this chassis has an inner steel shield and an outer lead shield over the tuner oscillator that I put back in place with every tube I tried.
I have ohmed out the discriminator resistor and it is fine. I can go back and recheck this evening. It's an odd looking resistor as it almost looks like ceramic.

Yes it is ceramic because it dissipates a lot of heat.

And the tube,shield is in place? The fine tuning range is suspect... is the range sufficient to adjust the video extinguishing at one end of its range?

The Leader has a channel 3 or 4 option for output...not 5 or 6. Its LCG 396...not LGC 396. Maybe that's the issue

My typo.

The manual I found doesn't mention options. Perhaps yours is a later version.
http://bama.edebris.com/manuals/leader/lcg396/

You could use a sig-gen with AM modulation (1KHz) on 21.25 MHz to inject the signal at the output of the tuner. Trace it thorough the sound IF with a detector probe attached to a scope. You could not get the sound any further than the discriminator, however you could confirm the sound IF transformers were peaked correctly. You could check the 21.25MHz with a frequency counter if need be.

Yes it is ceramic because it dissipates a lot of heat.

And the tube,shield is in place? The fine tuning range is suspect... is the range sufficient to adjust the video extinguishing at one end of its range?

I'll check the fine tuning tonight. Yes, I put the shield back in place each time I tried a different tube.

You mention the fine tuning adjusts full range. On ch4 on the 721TS the full fine tuning range is only about 0.5MHz where each channel is 6MHz. So the fine tuning is very narrow.

So I switched on just now my 721TS on channel 4 and adjusted the fine tuning across its full range. The pictures attached below depict what you would normally see on a properly aligned set.

The first image is with the fine tuning fully clockwise. Note the picture is brightest because the video carrier is at the top of the response curve. As a result the picture will lack fine detail. And there is no sound.

The second image is with the fine tuning roughly center with the picture a bit dimmer but with full detail. This is the correct point where the video carrier is at the 50% point on the video bandpass slope or nyquist slope for vestigial sideband demodulation. As this is a spilt sound set, you must adjust for best sound which would be the discriminator zero position.

The third image is with the fine tuning control fully counter clockwise and the picture is very weak. It has trouble locking because the carrier is at the bottom of the slope. There is no sound.

So as you adjust the fine tuning control counter clockwise, you should see the picture grow gradually dimmer if the tuner's local oscillator is set correctly within range of the tv signal. Either end of the fine tuning control there will be little or no sound.

If the image brightness remains the same across the tuning range or if the image grows a bit dimmer when the fine tuning control is clockwise, then the local oscillator is off frequency and you will get no sound.

You mention the fine tuning adjusts full range. On ch4 on the 721TS the full fine tuning range is only about 0.5MHz where each channel is 6MHz. So the fine tuning is very narrow.

So I switched on just now my 721TS on channel 4 and adjusted the fine tuning across its full range. The pictures attached below depict what you would normally see on a properly aligned set.

The first image is with the fine tuning fully clockwise. Note the picture is brightest because the video carrier is at the top of the response curve. As a result the picture will lack fine detail. And there is no sound.

The second image is with the fine tuning roughly center with the picture a bit dimmer but with full detail. This is the correct point where the video carrier is at the 50% point on the video bandpass slope or nyquist slope for vestigial sideband demodulation. As this is a spilt sound set, you must adjust for best sound which would be the discriminator zero position.

The third image is with the fine tuning control fully counter clockwise and the picture is very weak. It has trouble locking because the carrier is at the bottom of the slope. There is no sound.

So as you adjust the fine tuning control counter clockwise, you should see the picture grow gradually dimmer if the tuner's local oscillator is set correctly within range of the tv signal. Either end of the fine tuning control there will be little or no sound.

If the image brightness remains the same across the tuning range or if the image grows a bit dimmer when the fine tuning control is clockwise, then the local oscillator is off frequency and you will get no sound.

My God...what a beautiful picture. Let me do a little more orienting. I have a digital to analog converter box and an over the air digital broadcast antenna that works OK on my other vintage TVs and I don't get anything approaching a picture on the 721 with this setup. The only picture I've gotten is with the pattern generator...crosshatch...dots etc...through the Leader 396. I did double check and it does not produce a tone. Maybe I should stop focusing so much on getting the sound and concentrate more on getting a broadcast picture. I don't know if my digital to analog converter box has enough "oomph" to get a picture on the 721. What are you pumping into your set to get that beautiful picture?

To get the picture I am using a Zenith set top DTV box and rabbit ears. The station is TV Ontario from the Paris Ontario transmitter about 20 miles away.

To get the picture I am using a Zenith set top DTV box and rabbit ears. The station is TV Ontario from the Paris Ontario transmitter about 20 miles away.

I'm right outside Philadelphia PA USA and we're pretty right with OTA digital stations. The unit I'm using as a converter was very inexpensive. Let me see if I can find something with more muscle

To get the picture I am using a Zenith set top DTV box and rabbit ears. The station is TV Ontario from the Paris Ontario transmitter about 20 miles away.

Actually what I would like to do the most, and how I would use this TV most of the time, is to use my computer as a signal input source and play old movies and black and white TV shows from YouTube and other Internet based broadcasts. I've seen Bob Andersen and Buzz1151 do this without much trouble but I really don't know where to start with equipment etc.

Actually what I would like to do the most, and how I would use this TV most of the time, is to use my computer as a signal input source and play old movies and black and white TV shows from YouTube and other Internet based broadcasts. I've seen Bob Andersen and Buzz1151 do this without much trouble but I really don't know where to start with equipment etc.

There is a lot of stuff available for video sources. Besides a DVD player, I also have a HDMI to composite video convertor. I also watch YouTube videos on my set via a Roku1.

I received an email from Roku yesterday telling me the Roku 1 has reached end of life and will no longer be furnished with software updates. Well it is 10 years old....

I have a digital to analog converter box and an over the air digital broadcast antenna that works OK on my other vintage TVs and I don't get anything approaching a picture on the 721 with this setup. The only picture I've gotten is with the pattern generator...crosshatch...dots etc...through the Leader 396. I did double check and it does not produce a tone. Maybe I should stop focusing so much on getting the sound and concentrate more on getting a broadcast picture. I don't know if my digital to analog converter box has enough "oomph" to get a picture on the 721. What are you pumping into your set to get that beautiful picture?

I would agree focus on getting a broadcast picture using the converter box. First verify everything is working correctly using another analog TV. Next verify you're set to the same channel and center the fine tuning control on your 721, then try adjusting the oscillator slug associated with that same channel. It is curious that the Leader produces a picture on ch 3 or 4 but not the converter box. I believe the Leader produces it's signal using digital logic, but it is possible it's master clock frequency has wondered slightly.

I would agree focus on getting a broadcast picture using the converter box. First verify everything is working correctly using another analog TV. Next verify you're set to the same channel and center the fine tuning control on your 721, then try adjusting the oscillator slug associated with that same channel. It is curious that the Leader produces a picture on ch 3 or 4 but not the converter box. I believe the Leader produces it's signal using digital logic, but it is possible it's master clock frequency has wondered slightly.

I'll take that route Kevin. Thanks very much.

Yes I agree follow Kevin's route. However be aware of the process of adjustment of the local oscillator to bring the set in tune and yield a picture like my set.

First, the way the tuner is designed is the inductances are cascaded. That is when you adjust, it was originally suggested that you start with channel 13 and move downward. If you do not need the other channels you need not really need to worry except to be aware the reason you may not be able to tune channels 6 , 5, 4 and 3 is that there is something wrong with the tuner oscillator coils above these channels. Or there may be a bad tuning capacitor in the tuner.

I have found with each of my late 40's sets which had sat unused for decades that the whole chain was off and I needed to adjust the higher channels in order to bring channel 4 correctly within range. Study the service notes as they will infer the same adjustment process.

Be aware that if you adjust the screw slugs initially counter clockwise, you have the danger of adjusting them so they will drop out from where they are threaded. Inspecting the tuner, you may find someone has been there before you and some of the slugs are missing for some of the higher channels. That may be why you are not able to fine tune properly. It is always worthwhile inspecting the holes where the screws are to see if any are missing.

I always begin by adjusting clockwise until snug and the back off the screws. Follow my suggestion in my previous comment to find the correct response slope to place the carrier at 50% which is proper tune for best picture detail and where you will find the audio. If you cannot adjust the fine tuning to where the image begins to fade, means the local oscillator frequency is too high. Brass slugs are used which means in screwing the slugs lowers frequency.

To fix the video to make it sharp as well as retrieve the audio, you must get the local oscillator tuning right.

Yes I agree follow Kevin's route. However be aware of the process of adjustment of the local oscillator to bring the set in tune and yield a picture like my set.

First, the way the tuner is designed is the inductances are cascaded. That is when you adjust, it was originally suggested that you start with channel 13 and move downward. If you do not need the other channels you need not really need to worry except to be aware the reason you may not be able to tune channels 6 , 5, 4 and 3 is that there is something wrong with the tuner oscillator coils above these channels. Or there may be a bad tuning capacitor in the tuner.

I have found with each of my late 40's sets which had sat unused for decades that the whole chain was off and I needed to adjust the higher channels in order to bring channel 4 correctly within range. Study the service notes as they will infer the same adjustment process.

Be aware that if you adjust the screw slugs initially counter clockwise, you have the danger of adjusting them so they will drop out from where they are threaded. Inspecting the tuner, you may find someone has been there before you and some of the slugs are missing for some of the higher channels. That may be why you are not able to fine tune properly. It is always worthwhile inspecting the holes where the screws are to see if any are missing.

I always begin by adjusting clockwise until snug and the back off the screws. Follow my suggestion in my previous comment to find the correct response slope to place the carrier at 50% which is proper tune for best picture detail and where you will find the audio. If you cannot adjust the fine tuning to where the image begins to fade, means the local oscillator frequency is too high. Brass slugs are used which means in screwing the slugs lowers frequency.

To fix the video to make it sharp as well as retrieve the audio, you must get the local oscillator tuning right.

All good info above. I realize it will make the purists cringe, but I often wonder if some novice collecting folks would not be happier simply adding a composite video and audio inputs to their vintage TV. That ultimately eliminates all this tuner and IF alignment confusion along with the specialized test gear, knowledge base, and frustration. Basically it turns the vintage TV into a video and sound monitor. Bottom line is you need to become borderline obsessed with this hobby to endure doing this the hard way, when possibly all the individual really wished for was to enjoy watching their old TV. :scratch2:

All good info above. I realize it will make the purists cringe, but I often wonder if some novice collecting folks would not be happier simply adding a composite video and audio inputs to their vintage TV. That ultimately eliminates all this tuner and IF alignment confusion along with the specialized test gear, knowledge base, and frustration. Basically it turns the vintage TV into a video and sound monitor. Bottom line is you need to become borderline obsessed with this hobby to endure doing this the hard way, when possibly all the individual really wished for was to enjoy watching their old TV. :scratch2:

Ill give it the old fashioned way try but I hadn't thought of injecting composite video. Hmmmmmmm

Probably won't be able to use the suggestions here until tomorrow night. Busy weekend. I'll post with my progress.

I just had an idea to set accurately the channel 4 local oscillator setting in your TV. Further you will be able to determine where it is before you start! I tried it out on my 721TS and worked a treat.

The local oscillator beats with the channel 4 video and audio carriers to create the difference frequencies used for the IF: eg 93.0 - 67.25 = 25.75MHz and 93.0 - 71.75 = 21.25MHz. Below, I have drawn a little pictorial diagram depicting the IF amplifier frequencies, the Channel 4 frequencies and lastly the local oscillator frequency generated in the tuner. The tuner always radiates via the antenna terminals a small portion of the oscillator signal. In the old days it was a problem and was difficult to suppress and would cause interference on radios and other TVs. We can take advantage of this. (Note: The IF frequency move from the 20 MHz range to the 40MHz range in the 1950's was to minimize the problem).

Fortunately for us, the frequency of the local oscillator for channel 4 falls smack dab in the middle of the FM band at 93MHz! So all you have to do is to connect one of the television antenna terminals to an FM radio antenna of which you can trust the dial calibration. The frequency of the oscillator must be exactly 93.0MHz so that is where you tune the radio. The radio needs a signal indicator as you wont hear anything as the oscillator is supposed to be unmodulated.

For tuning reference, I see in Philly there is a bunch of low power 92.9MHz stations and a high power WMMR a Rock station at 93.3. So the local oscillator will be found between these two stations.

I used my Sony portable radio with a digital display and a signal strength light for my test. I switched on my 721TS with the antenna connections made and the power light was brightest at 93MHz.

Because your oscillator is off frequency, you will be able by this method to find where the oscillator is currently tuned. If it is low, you turn the channel 4 screw clockwise and if high counterclockwise until you reach exactly 93 MHz.

Remember the signal is a pure unmodulated sinusoid so you won't hear anything on the radio.

Once you find 93 MHz, I will bet you a 6BG6-G (my Avatar) you will then find a decent picture and audio! I suspect the discriminator will be off tune slightly but once you have established exactly the 93MHz local oscillator and feeding a channel 4 TV signal to the set, you can touch up the discriminator to get best sound.

I just had an idea to set accurately the channel 4 local oscillator setting in your TV. Further you will be able to determine where it is before you start! I tried it out on my 721TS and worked a treat.

The local oscillator beats with the channel 4 video and audio carriers to create the difference frequencies used for the IF: eg 93.0 - 67.25 = 25.75MHz and 93.0 - 71.75 = 21.25MHz. Below, I have drawn a little pictorial diagram depicting the IF amplifier frequencies, the Channel 4 frequencies and lastly the local oscillator frequency generated in the tuner. The tuner always radiates via the antenna terminals a small portion of the oscillator signal. In the old days it was a problem and was difficult to suppress and would cause interference on radios and other TVs. We can take advantage of this. (Note: The IF frequency move from the 20 MHz range to the 40MHz range in the 1950's was to minimize the problem).

Fortunately for us, the frequency of the local oscillator for channel 4 falls smack dab in the middle of the FM band at 93MHz! So all you have to do is to connect one of the television antenna terminals to an FM radio antenna of which you can trust the dial calibration. The frequency of the oscillator must be exactly 93.0MHz so that is where you tune the radio. The radio needs a signal indicator as you wont hear anything as the oscillator is supposed to be unmodulated.

For tuning reference, I see in Philly there is a bunch of low power 92.9MHz stations and a high power WMMR a Rock station at 93.3. So the local oscillator will be found between these two stations.

I used my Sony portable radio with a digital display and a signal strength light for my test. I switched on my 721TS with the antenna connections made and the power light was brightest at 93MHz.

Because your oscillator is off frequency, you will be able by this method to find where the oscillator is currently tuned. If it is low, you turn the channel 4 screw clockwise and if high counterclockwise until you reach exactly 93 MHz.

Remember the signal is a pure unmodulated sinusoid so you won't hear anything on the radio.

Once you find 93 MHz, I will bet you a 6BG6-G (my Avatar) you will then find a decent picture and audio! I suspect the discriminator will be off tune slightly but once you have established exactly the 93MHz local oscillator and feeding a channel 4 TV signal to the set, you can touch up the discriminator to get best sound.

Wow...thank you! Amazing information. This weekend got away from me but...I'm off tomorrow and after I watch the SLS/Artemis launch in the morning at 8:30am EDT, I'm going to try all of these generous ideas and techniques. You guys are incredibly kind for taking the time you have spent on trying to help this all thumbs newbie get this beautiful antique working and bringing pleasure to myself and my family. The former owners have been in contact with me asking how it's been going and they will be thrilled if WE succeed! They owned the TV since Daddy first brought it home in 1948 so I owe it to them and all of you to reboot myself and do this right. Fingers crossed!

I've gotten it about as good as I can. I do have a decent image from an HDMI to RF Coax feed from my computer. Unfortunately, my new cell phone blanks out the image every time I take a photo. I haven't done well with the Android to iPhone transition so far!
Just for the information because it was mentioned here before by Kevin...how would I install the means to inject composite video and sound into the unit???

Did you follow my instructions to set the local oscillator? What did you find?

To facilitate injecting video directly you will need to build a 75 ohm interface. It could be connected to the grid of the first video input stage. Then you could inject audio to the volume control. It will take a bit of fiddling as you have to ensure the video polarity is correct.

It appears you were very close connecting by the antenna. What happened?

Did you follow my instructions to set the local oscillator? What did you find?

To facilitate injecting video directly you will need to build a 75 ohm interface. It could be connected to the grid of the first video input stage. Then you could inject audio to the volume control. It will take a bit of fiddling as you have to ensure the video polarity is correct.

It appears you were very close connecting by the antenna. What happened?

I need to go on ebay and get a radio with a signal strength meter. I thought I had one but I didn't. Couldn't work on the oscillator just yet. I did get an RF modulator that injects excellent, clear video in my Zenith B&W TV into the antenna from my computer's HDMI output and I fed that into the antenna on the RCA and I got a decent picture...a little snowy but decent...but still no sound.

Could I go into the 12AU7 grid with a composite video signal? The modulator I got has RCA video and sound outputs and I can mount 2 RCA female connectors on the back of the 721 chassis and run the signals to the volume pot and the video amp grid. But if this TV has some funky video system, this might not work.

You don't need a fancy radio. Don't you have an FM radio kicking about? What kind of FM radio do you have?

You don't need a fancy radio. Don't you have an FM radio kicking about? What kind of FM radio do you have?

I have an old channel master super fringe FM radio

I have an old channel master super fringe FM radio

Tube model? That should work in a pinch. What does it look like? Model number?

It doesn't have to be fancy. You may hear the silent carrier as you tune to it.

Right now we a just fishing for the local oscillator. Use as your reference the radio stations I indicated and the oscillator should be right next to the 92.9 KHz low power station in Philly. It is probably well off from that. Just connect the TV antenna terminal with the set on channel 4 directly to the radio antenna and tune until you hear what would be like a silent radio station.

OK will do. Unfortunately, I’ve got a 430 wake up call tomorrow! If it’s possible, I’ll pick this up around 8 PM Eastern daylight time tomorrow night. Thank you so much for sticking with me through all of this. I’ll get everything set up and be ready to go. See you tomorrow!

That's okay. You have gone this far and it would be a shame not to figure this out.

I guess the most important aspect of this hobby I have learnt through the years is patience and perseverence.

I just had an idea to set accurately the channel 4 local oscillator setting in your TV. Further you will be able to determine where it is before you start! I tried it out on my 721TS and worked a treat.

The local oscillator beats with the channel 4 video and audio carriers to create the difference frequencies used for the IF: eg 93.0 - 67.25 = 25.75MHz and 93.0 - 71.75 = 21.25MHz. Below, I have drawn a little pictorial diagram depicting the IF amplifier frequencies, the Channel 4 frequencies and lastly the local oscillator frequency generated in the tuner. The tuner always radiates via the antenna terminals a small portion of the oscillator signal. In the old days it was a problem and was difficult to suppress and would cause interference on radios and other TVs. We can take advantage of this. (Note: The IF frequency move from the 20 MHz range to the 40MHz range in the 1950's was to minimize the problem).

Fortunately for us, the frequency of the local oscillator for channel 4 falls smack dab in the middle of the FM band at 93MHz! So all you have to do is to connect one of the television antenna terminals to an FM radio antenna of which you can trust the dial calibration. The frequency of the oscillator must be exactly 93.0MHz so that is where you tune the radio. The radio needs a signal indicator as you wont hear anything as the oscillator is supposed to be unmodulated.

For tuning reference, I see in Philly there is a bunch of low power 92.9MHz stations and a high power WMMR a Rock station at 93.3. So the local oscillator will be found between these two stations.

I used my Sony portable radio with a digital display and a signal strength light for my test. I switched on my 721TS with the antenna connections made and the power light was brightest at 93MHz.

Because your oscillator is off frequency, you will be able by this method to find where the oscillator is currently tuned. If it is low, you turn the channel 4 screw clockwise and if high counterclockwise until you reach exactly 93 MHz.

Remember the signal is a pure unmodulated sinusoid so you won't hear anything on the radio.

Once you find 93 MHz, I will bet you a 6BG6-G (my Avatar) you will then find a decent picture and audio! I suspect the discriminator will be off tune slightly but once you have established exactly the 93MHz local oscillator and feeding a channel 4 TV signal to the set, you can touch up the discriminator to get best sound.

I have the Channel Master tuned between WMMR 93.3 and WXTU 92.5. What's the next step? I have a working, good Tektronix oscilloscope too if that'll help with this.

Actually, the scope won't help...it's a 60MHz scope

Connect one TV antenna terminal to the radio antenna. Turn on the TV, tune to channel 4 and center the fine tuning control.

We have identified where the TV local oscillator is supposed to be at exactly 93 Mix. Try start tuning the radio between 92 and 94MHz until you pick up the local oscillator. The oscillator is inoculated and once you think you have located it, you can change the fine tuning slightly to confirm you have indeed found it.

Connect one TV antenna terminal to the radio antenna. Turn on the TV, tune to channel 4 and center the fine tuning control.

We have identified where the TV local oscillator is supposed to be at exactly 93 Mix. Try start tuning the radio between 92 and 94MHz until you pick up the local oscillator. The oscillator is inoculated and once you think you have located it, you can change the fine tuning slightly to confirm you have indeed found it.

So what should I be hearing on the radio when I have this hooked up? I hear a buzz and a "wee-oh-weee-oh-weee-oh" but the wave of the wee-oh isn't very fast. It's about half a MHz below the 92.5 tuning of WXTU where I get this

When I flip the channel to 3 or 5, the radio blasts in. I think the oscillator for channel 4 is tuned to about 91.5 or 92 MHz

IF...I'm going to attempt to adjust the slug on the channel 4 coil, do I need to have the CRT hooked up? I would think not but what the hell do I know?! It's a PIA to have the CRT in place and this chassis on its side so I can get to the bottom of the tuner compartment.

I have the radio volume all the way up...the TV tuned to channel 4...and the radio tuned so the speaker is quiet. We might be as low as 91MHz. No wonder this thing didn't work. I haven't done anything to the TV yet as far as messing with the channel coils. Waiting until I hear from you guys regarding the next step.

IF...I'm going to attempt to adjust the slug on the channel 4 coil, do I need to have the CRT hooked up? I would think not but what the hell do I know?! It's a PIA to have the CRT in place and this chassis on its side so I can get to the bottom of the tuner compartment.

I have the radio volume all the way up...the TV tuned to channel 4...and the radio tuned so the speaker is quiet. We might be as low as 91MHz. No wonder this thing didn't work. I haven't done anything to the TV yet as far as messing with the channel coils. Waiting until I hear from you guys regarding the next step.

This is good.

If it is as low as 91MHz that would explain why you are getting picture and no sound. Confirm the quiet spot on the radio dial is indeed the tv oscillator by turing back and forth the fine tuning control. You should hear a change.

The screw is made of brass so that means turning the screw clockwise should bring the frequency up. However, because the inductances for all of the channels are in series, the higher channels, likely 5 or 6 may be out. I would start with turning clockwise the 4 screw. If the screw becomes tight before reaching 93MHz, let us know.

This is good.

If it is as low as 91MHz that would explain why you are getting picture and no sound. The screw is made of brass so that means turning the screw clockwise should bring the frequency up. However, because the inductance for each channel is in series, the higher channels, likely 5 or 6 may be out. I would start with turning clockwise the 4 screw. If the screw becomes tight before reaching 93MHz, let us know.

So I should tune the radio back to 93MHz and adjust the slug for channel 4 clockwise until the radio is silent? Also, what about my CRT question?

So I should tune the radio back to 93MHz and adjust the slug for channel 4 clockwise until the radio is silent? Also, what about my CRT question?

Yes adjust it until it is at 93MHz as close as possible. You do not need the CRT to do this.

Once the oscillator is close as possible to 93MHz, you can re install the CRT and connect the set top DTV box or whatever has the channel 4 modulator and get best video plus sound. That is if the video and audio IF alignment is not too far out.

Also important: when handling thge CRT, always handle or hold the CRT from the bell, not the neck. And when installing in the chassis for testing out of the cabinet, ensure the bell of the tube is properly supported. It must not be supported just by the neck. I generally hold the bell of the tube up with a few books under the bell of the tube.

Fail. Did it exactly the way we schemed out. I tuned the L60 adjustment screw for channel 4 until the radio quieted completely at 93MHZ. Then I reassembled the rig and fed my RF modulated, computer video into the set. Same meh picture and no sound. This thing has silver mica disease in the RF or something else way outside my ability to fix. I think it's time for some Ben and Gerry's and then bed. Thanks my friend for trying. I really don't know where we go from here. I think doing the composite setup and pulling some tubes is the way this is going to go at this stage in my journey with this TV. Maybe at some point I'll have the insight and skills to get this going from the rooter to the tooter but I don't think it's that time just yet. Right now, a decent picture and sound no matter how it's obtained is how I'm going to go with this. It'll sit in the corner in the living room playing YouTube recordings of The Honeymooners and I Love Lucy and I don't think anyone will care if I'm injecting video and sound right in it's heart.

Or maybe I'll feel different tomorrow! Who knows.

Sorry to hear it did not work. There are a few checks you can actually make to ensure the local oscillator is correctly on frequency.

Remember the three screenshots I posted last week? When you adjust the fine tuning control across its range, do you get the bright smeary image with the control fully clockwise, the slightly less bright but sharper image in the middle setting and at the full counter clockwise the weak image? This is very important for the proper vestigial sideband tuning I outlined and it would confirm the oscillator tuning is correct. This is how all properly operating 621TS and 721TS sets should work.

You are very close to getting it to work and we only have to methodically work it through.

Bypassing the RF and IF and feeding the video directly to the video amplifier I do not think is a viable option for you. There is no conventional contrast or gain control and you may have to construct a transistorized gain contolled amplfier to invert the video and ensure the video level is correct. It won't be a matter of directly connecting the video to the grid of the tube. If you cannot diagnose this simple fault do you think you are up to some electronic design and construction work?

I have gone through great pains to outline tests which you have not responded to. Do you understand what I have said about the tuning?

Sorry to hear it did not work. There are a few checks you can actually make to ensure the local oscillator is correctly on frequency.

Remember the three screenshots I posted last week? When you adjust the fine tuning control across its range, do you get the bright smeary image with the control fully clockwise, the slightly less bright but sharper image in the middle setting and at the full counter clockwise the weak image? This is very important for the proper vestigial sideband tuning I outlined and it would confirm the oscillator tuning is correct. This is how all properly operating 621TS and 721TS sets should work.

While I'm sure your 721 has been aligned to perfection, I think you're assuming the video IF in Chris's set is also in perfect alignment. I don't recall any mention that the image has ever respond to the fine tuning control, but Chris can confirm. It certainly would be reasonable that the Video IF on a nearly 75 year old TV could be compromised to the extent that the fine tuning control will not show such subtle fine tuning indications?

There a number of variables at play here! Yes the alignment may be out. But the oscillator may not be at 93MHz. Maybe Chris tuned to an image or harmonic? How certain are we the oscillator is correct?

Certainly the fine tuning test will show if we are in the ballpark. If the video carrier is not on the slope at 50%, the picture will be smearyand awful anyway.

I see impatience here. Too often I see the knowledge we have gathered taken for granted.

If the alternative is to feed the video amplifier, fine. But will no contrast control in this set to set level and assuming the output composite analog video polarity of say a DVD player is negative sync you may not need to invert video. Is Chris up to devising a way to interface the video and manage signal level?

It seems simpler to me to resolve the relative simple problem the set has rather than introduce another problem. Maybe the best way is to let somebody else work on it.

There a number of variables at play here! Yes the alignment may be out. But the oscillator may not be at 93MHz. Maybe Chris tuned to an image or harmonic? How certain are we the oscillator is correct?

Certainly the fine tuning test will show if we are in the ballpark. If the video carrier is not on the slope at 50%, the picture will be smearyand awful anyway.

I see impatience here. Too often I see the knowledge we have gathered taken for granted.

If the alternative is to feed the video amplifier, fine. But will no contrast control in this set to set level and assuming the output composite analog video polarity of say a DVD player is negative sync you may not need to invert video. Is Chris up to devising a way to interface the video and manage signal level?

It seems simpler to me to resolve the relative simple problem the set has rather than introduce another problem. Maybe the best way is to let somebody else work on it.

I went back and reread all the posts. What strikes me odd is the Leader test pattern gets a signal through to the output of the second video amp, enough to drive the CRT well with a high contrast test pattern, however Chris has said his over the air box gets little to no legible picture. We have not been shown what the signal from the over the air box looks like, but he hints it has a poor picture on other TV's as well. I would assume the RF output has something wrong with it or it's signal is much weaker than the Leader. Is the over the air box outputting on the same rf channel as the Leader, or are we comparing reception on two different channels on the 721 tuner? Given the digital signal source, a properly working converter box should not produce a weak or snowy rf signal - they more less lock in or not. So without a decent rf modulated video source it's very difficult to observe the fine tuning detail you're looking for. I would prefer to see a signal from a known good DVD player feeding an rf modulator outputting on Ch 3 or 4.

That is a good point.
So many variables.

More screen shots and photographs of the chassis and the setup would help.

I've been thinking you have poor gain and the only reason the Leader works is that is has a greater output level.

I suggest you test the four 270pF mica coupling caps in the IF. I've found quite a few bad ones in early RCA sets.

I suggest you test the four 270pF mica coupling caps in the IF. I've found quite a few bad ones in early RCA sets.

I would refrain from touching anything in the IF strip at the moment. It will upset a alignment and we not have even a gain reference point at the moment.

I would suggest first Chris disconnect anything from the antenna switch on the tv set yo channel 4 and advance the contrast control fully clockwise and see if he can see snow on the screen.

It with the Leader connected with the contrast control fully clockwise if the picture loses sync and goes negative.

The CRT can tell many things if only we had a few photographs

I wonder if he could have kept the set upright with the CRT in while adjusting the oscillator. And accomplish that by hanging the tuner corner of the chassis off the bench and crouching below the bench to access it from the bottom.

If alignment is off it's possible there may have been interesting changes if it had signal injected into it.

I agree with the assessment that the signal source may be weak and or the set may have gain issues.

I wonder if he could have kept the set upright with the CRT in while adjusting the oscillator. And accomplish that by hanging the tuner corner of the chassis off the bench and crouching below the bench to access it from the bottom.

If alignment is off it's possible there may have been interesting changes if it had signal injected into it.

I agree with the assessment that the signal source may be weak and or the set may have gain issues.

The tuner oscillator screws are all available from the front of the chassis. Chris says he has the oscillator at 93MHz although that still has to be fully ascertained.

What would show if the set is healthy is to see the snow on the screen and rush of noise from the speaker with the contrast and volume controls fully advanced.

Going back to the suggestion on the IF coupling capacitors, the sound take off is before the video IF.

Also this set has no agc so the contrast control sets the RF and Video IF gain. The snow and noise from the speaker on channel4 with no modulator will tell us if there is gain AND if the oscillator is close due to the rd stages tracking.

Thanks so much guys for this fantastic and informative discussion. I really appreciate all the interest and all of the knowledge that you guys have shared with me through this rather arduous journey. It’s quite obvious that I do not speak the language well enough to be able to communicate most of the issues and let you guys know exactly what’s going on. My plan is to sit down tomorrow in front of the computer and write it as a scenario complete with pictures and hopefully I can recap exactly what situation the set is in at the moment. Thank you again so much for everything you’ve done. It’s really very kind and generous. I’ll just make one comment about what I read here. I think Bob is probably correct. My first impression when I went over to an over the air signal through my digital converter box I just felt like the leader was putting a much stronger signal into the television. When I had hooked up the digital box, the picture was very weak. Some of these generator boxes are notorious for putting out very strong signals so I could’ve been part of the problem

Btw I did the composite hookup and it looks very good. All I’m doing to run it is pulling the tuner oscillator tube. It works fine with my generator too through the tuner when I put the oscillator back in. The screen shot is of the composite feed. Of course my camera is blanking part of it

And of course the image is rotated

... I think Bob is probably correct. My first impression when I went over to an over the air signal through my digital converter box I just felt like the leader was putting a much stronger signal into the television. When I had hooked up the digital box, the picture was very weak. Some of these generator boxes are notorious for putting out very strong signals so I could’ve been part of the problem

Did you rotate the contrast control? The 721TS does not have an automatic gain control. If the signal is weaker from the digital box, did you rotate the contrast control clockwise to compensate?

The adjustment of the contrast control in the earliest RCA receivers may be a little tricky. You must adjust it for each rf signal fed to this set. You did not say if the contrast control was set to maximum.

Sure the Leader may be putting out a higher level signal but you left out mention of the contrast control.

Btw I did the composite hookup and it looks very good. All I’m doing to run it is pulling the tuner oscillator tube. It works fine with my generator too through the tuner when I put the oscillator back in. The screen shot is of the composite feed. Of course my camera is blanking part of it

I went to the trouble of posting three images showing the fine tuning behavior. But you have still not answered my question.

First, the picture looks washed out. Did you advance the contrast control? Secondly, does the fine tuning react as depicted in the three photos I posted? This is very important if you want to find the sound. Can you please answer this?

That composite video injection trick is a good thing for anyone working on old TV's to have in their toolbox. Once you get proficient at using an oscilloscope you can look at the video signal at 1st video input and compare it to the 2nd video output. According to RCA service data the in going signal should be about 1 volt peak to peak, and likewise about 30 volts peak to peak at the second video output to the CRT grid. The sync pulse needs to be negative going into the 1st video grid. Each amplification stage inverts the signal, which is why the polarity is important going in. Obviously it's working correctly the way you have it. As far as I know negative going sync is standard for most video component's, but it's not always the case where you're injecting into various TV circuitry. For instance when the video is feeding the cathode of the CRT in needs to be inverted compared to going to the grid.

That composite video injection trick is a good thing for anyone working on old TV's to have in their toolbox. Once you get proficient at using an oscilloscope you can look at the video signal at 1st video input and compare it to the 2nd video output. According to RCA service data the in going signal should be about 1 volt peak to peak, and likewise about 30 volts peak to peak at the second video output to the CRT grid. The sync pulse needs to be negative going into the 1st video grid. Each amplification stage inverts the signal, which is why the polarity is important going in. Obviously it's working correctly the way you have it. As far as I know negative going sync is standard for most video component's, but it's not always the case where you're injecting into various TV circuitry. For instance when the video is feeding the cathode of the CRT in needs to be inverted compared to going to the grid.

One should add a 75 ohm termination should be added to prevent reflections if the cable is more than a few feet long. The video level needs to be a couple of volts peak to peak in the 721TS to provide proper contrast. There is no gain control in the video amplifier to control contrast. Hence the the picture will lack contrast unless further additional amplification is added which includes an adjustment for contrast.

If you want to take photos try turning the screen brightness down and select a longer shutter speed on your camera, something longer than a 30th of a second. If the camera doesn't give you access to shutter speed then try a lower sensitivity number (lower ASA in old film terms)

Peter

One should add a 75 ohm termination should be added to prevent reflections if the cable is more than a few feet long. The video level needs to be a couple of volts peak to peak in the 721TS to provide proper contrast. There is no gain control in the video amplifier to control contrast. Hence the the picture will lack contrast unless further additional amplification is added which includes an adjustment for contrast.

From my own experimenting with my Magnavox converter box I'm seeing closer to 1.5, possibly 2 volts peak to peak. Without a contrast control the contrast is way too much in a low light setting. Of course one could add video signal attenuation, but for my testing purposes there was no need.
Agree cable length is critical.

OK...I think I'd like to make a statement here and hopefully exit all of this gracefully and with a degree of satisfaction I hope for all involved.

As I stated in the title of this 23 page, 340 comment thread, I am a novice at TV restoration. Radios...there I think I have developed a level of competence but TVs are a brand new adventure for me. With the help of everyone here, this project has come along so far, much further than I ever thought it would in my hands. I owe so much to the experts here for their continuing guidance and input. I've tried, to the best of my limited experience and skill set to employ all of the suggestions here. Unfortunately, my lack of a good electronic vocabulary and experience has frustrated some of my efforts and now is apparently beginning to frustrate some who have reached out to help me.

The discussion has taken a turn that is starting to make me feel uncomfortable. I thought I was doing an adequate job of being grateful and humble for the generous input of everyone here but I guess, for some, I haven't done that. Let me state I have never sought to take advantage of anyone's time or knowledge. If I have given the impression I'm here to steal some advice and disappear, I apologize for any discomfort I have caused. This is a hobby for me. I have 2 jobs, a wife, a new house and an ageing and challenged 93 year old father to care for. I don't have hours to sit at the bench and work on this and if my lack of responding, or being able to remember to do everything that has been suggested has angered anyone, I hope my explanation of my circumstances will give some perspective. Some of you have been so generous and have invested a lot of your time trying to help and if my responses have made you feel unappreciated or taken advantage of, again, I'm very sorry.

I think it's in the best interests of everyone we bring this thread to a close. I hope to be back with other projects and perhaps give back some of what I have learned to other newcomers. I am awed by the level of expertise and the generous way everyone has contributed. I didn't in any way want to make anyone feel uncomfortable.

Again, thanks so much to all of you. All the best.

Chris

OK...I think I'd like to make a statement here and hopefully exit all of this gracefully and with a degree of satisfaction I hope for all involved.

As I stated in the title of this 23 page, 340 comment thread, I am a novice at TV restoration. Radios...there I think I have developed a level of competence but TVs are a brand new adventure for me. With the help of everyone here, this project has come along so far, much further than I ever thought it would in my hands. I owe so much to the experts here for their continuing guidance and input. I've tried, to the best of my limited experience and skill set to employ all of the suggestions here. Unfortunately, my lack of a good electronic vocabulary and experience has frustrated some of my efforts and now is apparently beginning to frustrate some who have reached out to help me.

The discussion has taken a turn that is starting to make me feel uncomfortable. I thought I was doing an adequate job of being grateful and humble for the generous input of everyone here but I guess, for some, I haven't done that. Let me state I have never sought to take advantage of anyone's time or knowledge. If I have given the impression I'm here to steal some advice and disappear, I apologize for any discomfort I have caused. This is a hobby for me. I have 2 jobs, a wife, a new house and an ageing and challenged 93 year old father to care for. I don't have hours to sit at the bench and work on this and if my lack of responding, or being able to remember to do everything that has been suggested has angered anyone, I hope my explanation of my circumstances will give some perspective. Some of you have been so generous and have invested a lot of your time trying to help and if my responses have made you feel unappreciated or taken advantage of, again, I'm very sorry.

I think it's in the best interests of everyone we bring this thread to a close. I hope to be back with other projects and perhaps give back some of what I have learned to other newcomers. I am awed by the level of expertise and the generous way everyone has contributed. I didn't in any way want to make anyone feel uncomfortable.

Again, thanks so much to all of you. All the best.

Chris

Personally, I think you've done fine so far, there IS a learning curve with any hobby, especially technological ones, and you have displayed patience above what could be expected from a novice facing a tough problem. I agree with your plan to put this set aside for a while, sharpen your skills on some sets of lesser desirability as your time allows, and then some day in the future come back to this set armed with the newfound wisdom of what your experiences with the other TVs will teach you...... :thmbsp:

Thanks so much!

Yes sorry Chris. You are doing fine. I didn't want to make you feel pressured. On the contrary, I was hoping you would ask more questions.

Thanks. I’m just not sure of the form yet. I don’t wanna make a bad impression!

Forum not form. I switched from 25 years with an android phone to an iPhone. I’m kind of a mess with it!

It sometimes happens that people that want to help get over-invested in a project. It sometimes happens to me.
There also can be an element of pride, "I can talk this guy through landing the plane!".