View Full Version : Rca 721ts


Adam
07-26-2007, 04:10 AM
This is the next set I'm starting to work on. Pictures are after I cleaned up the chassis a bit, it was full of lint. The insulation is bad on the wires going to the yoke and the volume on/off switch. The plastic piece which goes inbetween the neck of the crt and the focus coil was totally disintegrated. Otherwise everything looks ok. I do have the cover to the hv cage and all the knobs.

John Folsom
07-26-2007, 07:45 PM
That looks like a nice clean chassis, no corrosion. The wires on the early RCAs always gets stiff as a board and cracks when bent. Nice thing about 721s is that the glass front keeps the front view of the set looking good, even if the sides and top are not.

Adam
07-27-2007, 04:00 AM
A note from someone who once worked on this set, the red piece of paper in the picture below. It says "wrong part", it's tied around a resistor going to the vert. lin. control. It's supposed to be 560ohms, and it reads green-blue-brown-silver, which I think is 560ohms 10%, so I don't see how it can be a "wrong part", but I tested it and it tested about 1.6k so it is bad. I wonder how long ago that note was put in there.

ohohyodafarted
07-27-2007, 04:45 PM
Color code is correct, but is it possible that it was replaced with a resistor of insuficient wattage at some time? Maybe it was overworked. That may explain why it is reading 1.6K.

Does the schematic call for the wattage which is currently installed?

kbmuri
07-29-2007, 03:26 PM
Adam,

I picked up a 721 last spring at the ETF Convention. I'm really anxious to get to it, it's next up in the in-basket after the Haunted Roundie Admiral from Hell. I thought I might do another step-by-step series here with it, but you beat me to it :thmbsp:

I'd be happy to doublecheck anything on mine if you want, and compare notes. Datasheet is SAMS 70-7, I have one handy.

- Kirk

Adam
07-29-2007, 04:38 PM
The schematic does call for a 1/2w resistor, which is what's in there.

The I have the sams 70-7 and it is actually the schematic for the 730tv which is very close but it doesn't have the audio amp on the tv chassis, and has a few other small differences. Just last night I found a copy of the original service information for the 721ts here: http://www.njarc.org/manuals.htm I ordered all my capacitors based on the parts list in the sams and then later found out I didn't have all the ones I needed.

The power supply is odd in this set with that -85v supply and not just tying all the filter caps to ground, it makes figuring out how to put the new electrolytics under the chassis a bit more difficult.

I've been closely following that Admiral thread, strange tv problem, I've learned some interesting stuff reading that.

kbmuri
07-29-2007, 06:48 PM
GREAT catch, finding the RCA manuals. Thanks.

All of my Capeharts have the filter caps isolated from the chassis, at roughly the same -90V. Jordan did something really brilliant with an isolated terminal strip to securely mount his electrolytics. Thread is here:
http://audiokarma.org/forums/showthread.php?t=96894

But I haven't opened up my 721 yet to know how much room there is under there.

Yeah, I didn't really figure I wanted to learn how to do alignment work but that Admiral has made me stretch my brain cells. It's been a tough one.

Adam
08-07-2007, 12:45 PM
I fired this set up probably for the first time in 50 years this morning, at first I got a very dim picture / no sound, but I just had the ion trap in backwards. Now I get a good picture / no sound. I get some buzzing if I turn the fine tuning control. But it does get sound on channel 1 - only it's picking up 92.3 FM on channel 1 - very strange, some kind of allignment trouble? It's also picking up FM radio on ch 3, but I don't know where because that one didn't announce the station number. No FM radio on any of the other channels.

kbmuri
08-07-2007, 01:39 PM
Adam -

NICE!

If you could tune an FM radio to hear the same audio you'd know what the frequency is you're getting on Ch3.

Keep notes on how you troubleshoot your lack of sound. I'm obviously interested.

Great job so far!

Steve K
08-07-2007, 02:39 PM
Adam:

You do really nice work. I'm sure that you'll get the sound working as nice as that picture. You're getting closer to that Mercury set!

Steve

Adam
08-09-2007, 05:51 AM
I want to try the alignment for the sound on this set, I've been reading through the allignment instructions, but I didn't have all the necessary equipment, so I picked up a signal generator and a sweep signal generator. The signal generator actually works as is, I was able to pick it up on the frequency it said it was on on over an AM radio station (640 Khz), and I plugged it into a scope to see what the output looked like and everything looks ok, some of the controls stick and are dirty, but that's all. As for that sweep signal generator, it looks like someone started to recap it and didn't finish.

Steve McVoy
08-09-2007, 07:08 AM
Before you do a sound alignment, you should make sure the local oscillator is on the right frequency. These sets are tricky, since they don't have intercarrier sound. You can have an excellent picture and no sound if the local oscillator is off by as much as 1 mHz. The best way to tell is with a frequency counter.

Adam
08-09-2007, 07:09 PM
I went and picked up a frequency counter today, I tested it with that method with the signal generator and the AM radio outlined over in the Admiral no sound thread and it seems to work fine. The only problem with it seems to be that the number 2 doesn't light up in the leftmost nixie tube.

Dave S
08-11-2007, 09:42 PM
Before you do a sound alignment, you should make sure the local oscillator is on the right frequency. These sets are tricky, since they don't have intercarrier sound. You can have an excellent picture and no sound if the local oscillator is off by as much as 1 mHz. The best way to tell is with a frequency counter.

I have an excellent picture and no sound on my 721. It happened right after I moved it from my living room to our club's museum.

A frequency counter is one toy I don't have. Any recommendations / specifications for these purposes?

Thanks,
--Dave

kbmuri
08-11-2007, 11:20 PM
Before you do a sound alignment, you should make sure the local oscillator is on the right frequency. These sets are tricky, since they don't have intercarrier sound. You can have an excellent picture and no sound if the local oscillator is off by as much as 1 mHz. The best way to tell is with a frequency counter.

I measured mine (I think) on the Admiral. Given I have an accurate frequency counter and semi-accurate o'scope, what's the best procedure for measuring it? I was able to get a clip-on probe into the tuner area thru a service port, but touching anything in there is pretty disturbing to the picture reception. Is there a less invasive method?

I asked the same question on my thread, sorry to hijack Adam's. It's just that here is where Steve M's quote is.

Dave S, I got my counter on eBay for 130 bucks and I'm thrilled with it. It's utterly accurate at 15 Mhz and 1190 KHz and at all the TV RF and IF frequencies. Presumably everywhere else. It apparently goes into the GHz too if I ever need it. The eBay link is still there and is on the Admiral no-sound thread. The heathkit I got is also accurate, but only up to 20MHz -- not enough for TV work. I guess make sure it goes to 100MHz at least.

Adam
08-12-2007, 07:02 AM
I've been having similar problems. I just tried the method over on the Admiral thread with the loops of wire around the oscillator tube. First I tried just a few turns of bare wire, and got nothing. I could only get it to work after putting as many turns of wire as would fit on there, using insulated wire, so I wouldn't have to worry about the different turns touching each other, and then taped in all in place. I could get a reading on channels 1, 2 and 3. With the higher frequencies for the other channels, it either would be erratic, or I got nothing at all.

Adjusting the fine tuning from one end to the other, I got 71.70-71.06 MHz for ch 1, 80.42-79.70 ch 2 , and 85.65-83.00 ch 3. Channel 1 is nearly right on, channel 2 and 3 read a bit low. But I'm not sure how accurate this is anyway, I don't think the frequency counter can deal well with something at such a low voltage. And because channel 1 reads nearly right on, and channel 1 is not working correctly, it is recieving 92.3 FM, I would think the problem would be elsewhere.

Is there any way to improve this set up to get more accurate readings? Here's a pic of what I did below.

Steve McVoy
08-12-2007, 08:39 AM
The fact that you are getting 92.3 means that your audio IF (in fact the entire audio section) is working correctly. Your problem is almost certainly the LO frequency.

You are receiving 92.3 FM on channel one because there are two products that are created in the mixer. Usually, you would get a product of LO minus the audio IF frequency (87.0 - 21.25 = 65.75). Here you are getting LO plus the audio IF frequency (71.05 + 21.25 -= 92.3).

I use a tube shield over the LO tube, connected to the frequency counter input. Pull the shield up so it doesnt touch the chassis. Be sure to connect the ground lead of the counter to the chassis near the tube shield. This should produce an accurate result.

I would use a VCR with channel 3 output as your signal source. Then, adjust the LO slug for channel 3 for 65.75 plus the audio IF frequency (LO should be 87.0 mHz). Then, listen for audio. Move the LO slightly above and below the proper frequency, since the IF may have been peaked for something slightly different from the specs.

If you have no sound after this, check the plate, screen grid, and cathode voltages on the audio IF tubes. Obviously you should also check the tubes themselves. After all this if there is still no audio, then do the alignment.

kbmuri
08-12-2007, 09:52 AM
The Sams-sheet alignment instructions said to make an insulated tube shield for the alignment procedure, so I had one handy. Using the tube shield, my results:
Ch2: 79.53 - 79.94
Ch3: 85.71 - 86.16
Ch4: 91.02 - 91.50
Ch5: 101.50 - 102.04
Ch6: 106.50 - 107.08

Steve McVoy
08-12-2007, 10:39 AM
The proper LO frequencies are:

Ch 2: 81.0
Ch 3: 87.0
Ch 4: 93.0

Yours are all too low. There may be a trimmer that will bring them all up, but, if not, adjust the individual trimmers for each channel.

Your sound will then work fine.

John Folsom
08-12-2007, 11:29 AM
Adam, keep in mind that for the 721, you need to start the LO frequency alighnemt at channel 13 and work your way down, as each lower channel adds inductance to the one above to lower the frequency. If you cannot get an accurate count, try using an active ch. 13 signal and tune the LO for best sound, then step down to 12 and repeat... all the way down to 2.

Adam
08-12-2007, 04:42 PM
Now I understand why I was getting the FM radio, that means earlier on ch 3 I must have been picking up something around 108 MHz.

I tried connecting the frequency counter to the tube shield, it still wouldn't give a good reading on anything but ch 1-3.

I tried adjusting for sound on each channel I could receive over the air, + the DVD player on ch 3, so I adjusted channels 13, 11, 9, 7, 5, 4, 3, 2. I can now get sound on channels: 5, 4, 3, and 2 - but with some buzzing. I still can not get sound on any of the other channels, but I did get some buzzing.

In order to go through the whole procedure with all the channels I'm either going to have to find a frequency counter that will work with the lower input level, or I saw another procedure in the service manual which just involves hooking a VTVM up to the sound discriminator but with that I need to find another signal generator that goes all the way up to 215.75 MHz for ch 13, this one I have goes no higher than 50 MHz.

kbmuri
08-12-2007, 08:49 PM
The proper LO frequencies are:

Ch 2: 81.0
Ch 3: 87.0
Ch 4: 93.0

Yours are all too low. There may be a trimmer that will bring them all up, but, if not, adjust the individual trimmers for each channel.

Your sound will then work fine.

I'm sure that was one of my problems. I set Ch3 to exactly 87.0. Costs me a little picture quality, no sound. Nice to have that section undertood and working though. I set the fine tuning at the middle of its range and set the trimmer at exactly 87.0. So I can tune 1/4 MHz to either side of it.

Adam
08-14-2007, 09:19 PM
I have good sound on all the channels now. I still get some slight buzzing, but only when watching a DVD and then mostly only when it's of a color program. I borrowed a later model Heathkit frequency counter and it had no problems with the low input level. I adjusted the local oscillator for 21.25 + the sound frequency for each channel starting at channel 13 and going down to channel 1.

Adam
08-16-2007, 10:29 PM
I decided to try the rest of the sound alignment to see if it would improve anything, I did the sound discriminator alignment, and it didn't seem to make much difference. When I tried the IF allignment, I just got some weird images on the scope, I think the sweep generator must not be working properly. The sound is actually ok on this set now, I just wanted to try the alignment. I'll probably just touch up the cabinet a bit and put this back together now, rather than messing more with the sweep generator. Here's a picture of the weird image I got on the scope and of the underside of the chassis with all the new caps, not very original looking though, one of these days I'll try stuffing the old caps, seeing all new caps under 60s and mid-late 50s sets doesn't bother me, but I think they look sort of out of place here.

Steve McVoy
08-16-2007, 10:38 PM
You really don't need a sweep generator to align the audio IF, if you have a scope that has a bandwidth of 30 mHz or so. Just make sure the LO is right, hook up a signal source, and put the scope probe on one of the plates of the audio detector tube. Peak the audio IF transformers for maximum level. Don't mess with the discriminator transformer secondary if you can avoid it, or just touch it up to minimize the buzz.

Adam
08-17-2007, 01:24 AM
I went back and tried that method of alignment, it improved the sound a bit, still get some buzzing on some dvd's but not on others, not at all over the air unless during one of those commercials where there is a lot of writing on the screen. I also replaced the 6K6 audio output tube with a stronger one, and that improved the sound too, made it a bit louder. But none of audio IF tubes tested weak, only the output tube. At this point it is working equally well as many of my other sets.

Adam
08-18-2007, 11:40 PM
Put it all back together today, here's some pics... Thanks for all the alignment information in this thread, I learned some new stuff fixing this one. This is now my oldest working set, I have one older, a 630ts, but it's far from being operational. Now on to the next set, that 12" Mercury is up next...

jpdylon
08-19-2007, 02:45 AM
well done Adam, beautiful cabinet as well!