Newbie's First Questions RCA TS 721 Restoration

[bandersen]

Possibly. I'm also suspicious of the vertical oscillator, output tube and all associated components.

Symptom is very bad linearity. The oscillator should produce a linear sawtooth wave which is then amplified by the vertical output stage. If either is defective, it will distort the deflection.

[Chris K]

Originally Posted by bandersen

I'd say a lot closer Yes, it is a vertical issue. Looks like foldover and you have way too much height. Remember the viewing area is a 4:3 aspect ratio rectangle - it's not a porthole set. You should have about 1-1/2" blank at the top and bottom.

Reduce the height and adjust the vertical linearity and see where that gets you.

HOLY CRAP! The height pot was the last link in the fold over chain of events. A few hours with Bob's advice and here it is. Thank you so much! Thanks to all who helped over the past several weeks!

[Spinning Head]

Wow! You stayed with it through all those bad parts.
Break out your favorite adult beverage and celebrate, just don't mix with HV.

[old_tv_nut]

Wonderful!

[Chris K]

Thanks guys! It feels pretty good. It needs some additional tweaking but where it is now compared to where it was...all I can say is Yippee!

[Notimetolooz]

Yeah, you just have to stay with it. TVs are complex and there can be more than one bad part.
You do have to make sure you are doing things correctly like cleaning pots.
You don't want to make more problems. You might not have done anything wrong but the guy that worked on it before may have.

[kvflyer]

Yes, that looks "Mo Bettah"! Congratulations. I had a problem with my Admiral set that turned out being tin whiskers in the control. A good tooth brushing and it fixed that. First time I had experienced it and I was surprised, but pleased.

[Chris K]

Originally Posted by kvflyer

Yes, that looks "Mo Bettah"! Congratulations. I had a problem with my Admiral set that turned out being tin whiskers in the control. A good tooth brushing and it fixed that. First time I had experienced it and I was surprised, but pleased.

Those Admirals grew whiskers like Santa Claus! I wonder what kind of metal alloy they used where that was such a common problem across so many TVs for so many years? Thanks for the congrats!

[Chris K]

Originally Posted by Notimetolooz

Yeah, you just have to stay with it. TVs are complex and there can be more than one bad part.
You do have to make sure you are doing things correctly like cleaning pots.
You don't want to make more problems. You might not have done anything wrong but the guy that worked on it before may have.

Oh I screwed the pooch...no question. The set was virgin. You can only access the insides of the rear adjustment pots for spray cleaning by removing them. There's no access from the front and I ended up not having to replace the vertical height control. About half an hour of spraying and twisting did the job. It's clean and smooth now from 2.4 megs all the way down to next to nothing. The entire chassis is now rewired, recapped and clean. Next mountain to climb is the sound. It's working as I get crackle but no processing of a signal from the generator. This set's a bear! Fighting me to the last tube, coil and transformer!

[Chris K]

Originally Posted by Notimetolooz

The voltages you measured around the 6K6 seem pretty good. Have you tested and replaced tubes in this set?
What about the voltages on the 6AT6 pins.

A bit of clarification on how the sound stages work. 6K6 is the audio output tube, it is responsible with developing the power to operate the speaker. Most of the 6AT6 is used as a audio amplifier, it increases the voltage size (amplitude) of the audio signal. The 6AL5 is the FM discriminator (detector or demodulator). Before the 6AL5 the signal is 21.25 MHz with FM audio modulation, after the discriminator the sound is in the audio range. That makes the volume control a good place to try injecting audio. The audio stages are easier to troubleshoot than the IF.
You need a bigger audio signal at the grid of the 6K6 than the 6AT6 grid to hear the same volume.
The general way to determine the wattage required for a resistor is to use the resistor value and the voltage difference across it. That 1K resistor that feeds the screen of the 6K6 and then feeds the plate through the output transformer has 25 volts across it. Voltage squared, divided by the resistance is the wattage given off (dissipated), that come out to 0.625W. You always want to use a resistor rated two or more times the dissipated wattage so it stays fairly cool. So a 2W should be OK.

I fed an audio signal through the grid of the 6AT6 and the sound system from there forward works...speaker, AO transformer and tube and the volume control. I fed a 21.5 MHz RF signal through the plate for both pin 2 and 7 of the 6AL5 and didn't get a signal through the speaker with either. Am I doing something wrong here? I tried about 4 different 6AL5s but nothing changed. I'm sure I'm doing something stupid!

[Kevin Kuehn]

Originally Posted by Chris K

I fed an audio signal through the grid of the 6AT6 and the sound system from there forward works...speaker, AO transformer and tube and the volume control. I fed a 21.5 MHz RF signal through the plate for both pin 2 and 7 of the 6AL5 and didn't get a signal through the speaker with either. Am I doing something wrong here? I tried about 4 different 6AL5s but nothing changed. I'm sure I'm doing something stupid!

The 6AL5 Sound Discriminator(detector) plate circuits are not tuned to the 21.25 mhz sound IF. To hear sound you would need to inject your 21.25 mhz carrier with FM audio modulation into the grid of the 1st or 2nd audio IF amplifiers.

[Kevin Kuehn]

If you have Sams 70 folder 7, on page 6 there are instructions for sound alignment using a AM signal generator and VTVM. You should be able to substitute a digital VOM or scope for the vacuum tube volt meter.

[Notimetolooz]

The sound IF is tuned to 21.25 MHz.
The bigger issue in tracing the signal back is that FM discriminators don't work like AM detectors. A discriminator derives the sound by a method involving things like phase comparison which is difficult to break down into smaller blocks of operation and get audio.

[Chris K]

Gotcha...thanks everyone for the advice and insights. This is a great place to learn... particularly if you're willing to embarrass yourself!!! Seriously, this restoration and the interaction with the experts here has taught me so much. It's very appreciated!

[Penthode]

What are you using for a,signal generator? Where,are you injecting the signal? Can you outline what you are doing with the equipment used?