Newbie's First Questions RCA TS 721 Restoration

[Penthode]

Cool. If you have a whole bunch, you'll have a better chance of finding one that works. Of course the tube shield must be on the tube for if it is missing, it too can cause the tuning to be way off.

[Chris K]

Originally Posted by Penthode

Cool. If you have a whole bunch, you'll have a better chance of finding one that works. Of course the tube shield must be on the tube for if it is missing, it too can cause the tuning to be way off.

Been swapping for about an hour...no change. I'm going to double check my audio input test just to be certain I'm getting what I said I was getting from the volume control forward.

[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?

[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.