Rca 721ts

[Adam]

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]

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]

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]

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]

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]

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]

Originally Posted by Steve McVoy

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]

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]

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]

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]

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]

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]

well done Adam, beautiful cabinet as well!