Newbie's First Questions RCA TS 721 Restoration

[Chris K]

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

[Chris K]

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.

[Kevin Kuehn]

Originally Posted by Chris K

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?

[Chris K]

Originally Posted by Kevin Kuehn

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.

[Penthode]

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

[Chris K]

Originally Posted by Penthode

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?

[Notimetolooz]

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?

[Chris K]

Originally Posted by Notimetolooz

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

[Kevin Kuehn]

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?

[Chris K]

Originally Posted by Kevin Kuehn

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

[Penthode]

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).

[Notimetolooz]

Originally Posted by Chris K

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.

[Penthode]

Originally Posted by Notimetolooz

...
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.

[Chris K]

Originally Posted by Notimetolooz

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

[Chris K]

Originally Posted by Penthode

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.