Newbie's First Questions RCA TS 721 Restoration

[Penthode]

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.

[Chris K]

Originally Posted by Penthode

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!

[Penthode]

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.

[Chris K]

Originally Posted by Penthode

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?

[Chris K]

Originally Posted by Penthode

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 ) 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.

[Penthode]

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.

[Kevin Kuehn]

Originally Posted by Penthode

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.

[Penthode]

Originally Posted by Kevin Kuehn

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.