Newbie's First Questions RCA TS 721 Restoration

[Chris K]

Originally Posted by kvflyer

Just a thought, if the CRT is not connected, it can't act as a capacitor (aquadag coating). Could that cause the voltage to appear lower? You know, if you have an open filter capacitor, the unsmoothed voltage would be lower.

I changed all the filter caps...all new electrolytic caps (Nichicon)

[Electronic M]

Originally Posted by Chris K

Hi Tom...would something like this work? I do have an HDMI output port from my computer.

https://www.amazon.com/HDMI-Converte.../dp/B07W58PNPP

I bought one of those to try a while ago. I can't recommend it because the one I got would regularly black out and come back to normal. Maybe I got a rare defective unit, but if a good portion are like mine (likely for no-name Chinese stuff) it's not a great gamble.

Also IIRC the ad claims it will transmit...It doesn't seem to have the power for anything other than a direct connection and even then was going dark regularly.

[Chris K]

Originally Posted by Electronic M

I bought one of those to try a while ago. I can't recommend it because the one I got would regularly black out and come back to normal. Maybe I got a rare defective unit, but if a good portion are like mine (likely for no-name Chinese stuff) it's not a great gamble.

Also IIRC the ad claims it will transmit...It doesn't seem to have the power for anything other than a direct connection and even then was going dark regularly.

OK I guess I'll stay away for now and rely on the pattern/sound generator and the crappy over the airways digital, converted to analog, signals. On these projects, I try to maintain a conservative, step by step diagnostic and analysis approach but sometimes, at a point of "problem solved!", I want to race to the end game finish line ASAP. I need to concentrate on getting sound next.

[Notimetolooz]

That HV meter looks OK. I haven't seen that model before. Yeah, it could be the capacitor smoothing effect.

A quick and dirty way to check the audio output circuit is to touch the wiper on the volume control with a screwdriver with a finger touching the blade. Don't have the volume control turned down however. You should hear a hum. If this was a hot chassis set you definitely want it plugged into an isolation transformer to do this.
You would need a FM modulated 4.5MHz audio signal to inject to trace the signal further up (from the volume control towards the tuner).
A oscilloscope could enable you to trace the audio from the tuner toward the volume control.
Without those you are left with checking voltages, resistors and tubes, etc.

[Chris K]

Originally Posted by Notimetolooz

That HV meter looks OK. I haven't seen that model before. Yeah, it could be the capacitor smoothing effect.

A quick and dirty way to check the audio output circuit is to touch the wiper on the volume control with a screwdriver with a finger touching the blade. Don't have the volume control turned down however. You should hear a hum. If this was a hot chassis set you definitely want it plugged into an isolation transformer to do this.
You would need a FM modulated 4.5MHz audio signal to inject to trace the signal further up (from the volume control towards the tuner).
A oscilloscope could enable you to trace the audio from the tuner toward the volume control.
Without those you are left with checking voltages, resistors and tubes, etc.

I'll check the HV with the CRT in place, grounded and plugged in. I managed, after a 6 month wait, to get my hands on some Aquadag graphite with binder spray from Ted Pella Associates and I can spray any chipped or scratched areas although the coating on this CRT is pretty complete and undamaged.

I do have a scope and a signal generator. I can give those a try. Maybe Mr. Andersen would know if this is a hot chassis but I do have the hot and neutral blades of the power cord marked so I can be sure it acts as a keyed plug.

On the topic of the power cord, it's in terrific condition. Is there anything I can do or a treatment I can apply to help preserve the rubber and keep it from becoming hard and brittle?

Thanks
Chris

[Electronic M]

Originally Posted by Notimetolooz

That HV meter looks OK. I haven't seen that model before. Yeah, it could be the capacitor smoothing effect.

A quick and dirty way to check the audio output circuit is to touch the wiper on the volume control with a screwdriver with a finger touching the blade. Don't have the volume control turned down however. You should hear a hum. If this was a hot chassis set you definitely want it plugged into an isolation transformer to do this.
You would need a FM modulated 4.5MHz audio signal to inject to trace the signal further up (from the volume control towards the tuner).
A oscilloscope could enable you to trace the audio from the tuner toward the volume control.
Without those you are left with checking voltages, resistors and tubes, etc.

This is a early post war split sound set so the sound IF is 21.25MHz NOT 4.5MHz that intercarrier sound sets use.

Also he may be able to get away with a tube AM modulated signal generator...Most tube TV/radio repair grade generators distort the the RF carrier enough to create FM/PM modulation that TV audio systems can detect. I've done this with 2-3 different generators so far.

[Chris K]

Originally Posted by Electronic M

This is a early post war split sound set so the sound IF is 21.25MHz NOT 4.5MHz that intercarrier sound sets use.

Also he may be able to get away with a tube AM modulated signal generator...Most tube TV/radio repair grade generators distort the the RF carrier enough to create FM/PM modulation that TV audio systems can detect. I've done this with 2-3 different generators so far.

I have an old Knight Model KG-650 RF generator with an output range from 100 kilocycles to 100 megacycles. We might be getting into territory here where my comprehension database is running full throttle and losing ground!

[Electronic M]

Originally Posted by Chris K

I have an old Knight Model KG-650 RF generator with an output range from 100 kilocycles to 100 megacycles. We might be getting into territory here where my comprehension database is running full throttle and losing ground!

If it's a working generator odds are it will work for audio IF signal injection...IIRC a friend of mine has the same generator and has used it for FM work. You can check it by using an FM radio set to a dead channel in the 88-95MHz range and trying to tune the generator to make a signal in that range. If the FM radio tunes it then there should be enough FM slop in the generator for it to work with TV audio.

Before you start with IF injection first try audio injection at the first audio stage grid. That knight generator should have an audio output you can use. If the set passes audio from the 1st audio grid decently then IF injection is the next troubleshooting step. If it doesn't pass audio injected at the 1st audio grid then troubleshoot the audio stages and make them work before messing with the audio IF.

We can walk you through audio IF injection if you end up needing to do it.

[Yamamaya42]

Originally Posted by Chris K

I have an old Knight Model KG-650 RF generator with an output range from 100 kilocycles to 100 megacycles. We might be getting into territory here where my comprehension database is running full throttle and losing ground!

I get what EM is saying, that most tube frequency generators ( like my Eico324) and so on, will have just enough intermodulation / distortion in them to trigger the sound discriminator circuit in the TV when you feed the generator via input AM modulation.
IE, you should hear some pretty weird stuff when you get it right!