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 Post subject: RCA 721TCS Revisited - Screeching, weak audio, some snow.
PostPosted: Dec Wed 21, 2016 11:14 pm 
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I have the 721TCS back on the bench. It was a previous restoration from some time back. NOT the 721TS which I just wrapped up.

The Audio has never been good - just adequate. Plus, the TV has a tendency to start screeching after it's been on for awhile. Sometimes it just screeches with no audio present, or no screeching OR audio at all. Sometimes it screeches when the volume is raised.

The video has some snow. Not much, just a hint in some bars across the screen. Plus, it has some wierd vertical bars that look like rasters. Only, again, they're vertical.

Here's a short video of the symptoms:

https://www.youtube.com/watch?v=dBr5YB3 ... e=youtu.be

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 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Dec Wed 21, 2016 11:35 pm 
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Lee, your video is private, we can't see it, or at least I can't.


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 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Dec Thu 22, 2016 3:47 am 
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Neither can I Lee


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 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Dec Thu 22, 2016 4:46 am 
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Huh!

Try this:

https://youtu.be/dBr5YB3QohU

You can hear the screeching as I lower the volume.

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 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Dec Wed 28, 2016 9:03 pm 
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Doesn't a 721 need that lead tube shield on the tuner local oscillator tube (6J6).

As I recall that was to silence microphonics from the LO that got into the audio.

Jim


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 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Dec Thu 29, 2016 3:07 am 
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jimmc wrote:
Doesn't a 721 need that lead tube shield on the tuner local oscillator tube (6J6).

As I recall that was to silence microphonics from the LO that got into the audio.

Jim


The lead shield is in place. 8)

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 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Dec Sat 31, 2016 10:42 pm 
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Hmmmm....no takers, eh?

Not even a hint? 8)

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 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Dec Sat 31, 2016 11:13 pm 
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Holidays.

Happy 2017.


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 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Jan Sun 01, 2017 1:09 am 
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Yes. It can wait.

On my way to a party myself.

Happy New Year, All. 8)

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 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Jan Sun 01, 2017 10:12 pm 
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My bet is on a microphonic tube or tube socket. Craig


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 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Jan Mon 02, 2017 1:37 am 
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analog.tv wrote:
My bet is on a microphonic tube or tube socket. Craig


So.....start with tube swaps? What section would you start with?

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 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Jan Mon 02, 2017 4:19 am 
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Thats what I also told you Lee. Thats a good start.


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 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Jan Mon 02, 2017 5:18 am 
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Anything in the tuner, sound IF, or audio stages could cause the unwanted noise. You could try gently tapping each tube with something like the eraser end of a pencil and see if you can duplicate the screeching noise. The 6J6 in the tuner can do that even with the lead shield in place.

I recall a chassis many years ago where moving the ground connection on a bypass cap in the sound IF during restoration to a more convenient point caused it to occasionally oscillate at a frequency you couldn't hear directly but caused all sorts of weird beats and noises in the audio. Took quite a while to figure that one out, and moving that connection back to the point where it had originally been grounded solved it immediately. Usually, things aren't that critical but I bring this up just because once in a great while physical position of a part, or where a certain part is grounded can make all the difference in the world in a circuit that was designed for a very specific component placement and lead dress.

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 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Jan Mon 02, 2017 3:39 pm 
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I Don't recall which FM discriminator the 721 uses but the 621TS and other early RCA sets used the Foster-Seely circuit which is a crappy discriminator circuit. I believe the lead shield on the tuner tube was suposed to help with microphonic problems.

The later sets used the Ratio Detector circuit which looks anlmos identical to the R-S.

Any tube socket equipped with a shield needs the shield in place.

My 621 used to howl until I found a lead tube shield.

I had a problem with a tube socket in a set that had a chassis ground that got connected by way of the rivet. Even thought it looked clean and good it was not making contact.

Jim


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 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Jan Tue 03, 2017 1:53 am 
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Found the screecher. V115 6AU6 2nd Sound IF. I put the tv into it's screeching state by adjusting the volume control then wiggled the top of all the tubes until V115 reared it's ugly head. Replaced it. No more screeching.

Now, I'm doing voltage checks on all the sound related tubes. I'll check back with you all when I'm done.

Max Volume varies from soft to moderate, depending on the movie soundtrack. It is by no means loud at maximum volume.

NOTE: Bridging Pins 2 and 6 on the 6AT6 (1st Audio) does NOT increase volume level. Interesting. 8)

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 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Jan Tue 03, 2017 2:41 am 
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Did voltage checks on V117 6AT6 and V118 6K6GT

V118 is fine. Voltage checks at or slightly above specs.

V117 Pin 7 reads 90.1 Volts. The Schematic on the 721TS (not TCS, I'm assuming they're the same) reads 195 Volts.

Seems I have the same problem as the 721TS I just finished, only bridging Pins 2 and 6 doesn't help or affect performance one way or the other.

Strange...

Anybody have the 721TCS Schematic and see any difference between the two? 8)

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 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Jan Tue 03, 2017 4:48 am 
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While chasing some video bugs, I went through the Channel Contacts again with some cleaner. Picture quality has improved.

It's amazing how much maintenance these old warriors need. Lots of contacts.

Small wonder TV Repair was so lucrative in the early days.

Wish this beast would play a bit louder through it's 12" loudspeaker. 8)

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 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Jan Tue 03, 2017 6:23 pm 
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M3-SRT8 wrote:
Anybody have the 721TCS Schematic and see any difference between the two? 8)


I just picked up the original RCA service supplement including schematic. I'll scan and upload ASAP.


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 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Jan Tue 03, 2017 6:34 pm 
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Tweaking T108, the Sound Discriminator Trans, has yielded a considerable increase in volume. More than I need. That problem is gone.

Now, I'm working on a newfound snow and smear problem. I replaced C18, a 270pF mica that measured 514pF out of circuit. It's replacement measures 269pF, and no leakage at 600V. I usually measure new parts, especially in critical circuits.

This video is driving me nuts. Mein Gott, what a hobby. 8)

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Last edited by M3-SRT8 on Jan Tue 03, 2017 7:13 pm, edited 1 time in total.

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 Post subject: Re: RCA 721TCS Revisited - Screeching, weak audio, some snow
PostPosted: Jan Tue 03, 2017 6:55 pm 
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Replaced V103 6AG5 3rd Pix IF. Picture is back to fine and dandy. Sounds great.

It's like caring for a spoiled brat. 8)

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