Newbie's First Questions RCA TS 721 Restoration

[Chris K]

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.

[Penthode]

Originally Posted by Chris K

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.

[Chris K]

Originally Posted by Penthode

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?

[Penthode]

Originally Posted by Chris K

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.

[Chris K]

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.

[Penthode]

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.

[Penthode]

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?

[Kevin Kuehn]

Originally Posted by Penthode

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?

[Penthode]

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.

[Kevin Kuehn]

Originally Posted by Penthode

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.

[Penthode]

That is a good point.
So many variables.

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

[bandersen]

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.

[Penthode]

Originally Posted by bandersen

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

[Electronic M]

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.

[Penthode]

Originally Posted by Electronic M

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.