RCA 8-PT-7014 Sound issue...

[ZackN920]

Before anyone ask's, This set has been restored!...before I bought it.

Anyway's I've been using this little thing semi-regularily. Good little desk tv. It has developed an issue with squeeling when the volume is set above half. Just out of knowwhere it will do it, and freak ya out. Especially if ya don't know about the problem. It's intermittant and not heat related. It will do it at start up and after being on for 2 hour's or so. The audio quality will also take a dip whenever it squeels.

I'm not sure if it's just the audio output tube/audioamp/soundif(6U8) or an actual component that's bad, and I don't want to open it up until I get a general idea of what it is. This set's not like other's where I can just take the back off. I gotta take the whole damn thing apart... I'd just test the tube if it were easier to do so, but im a little lazy and want to hear what you all would think off hand.

[old_coot88]

Possibly a microphonic tube causing a feedback loop. If possible with the set in operation, try tapping each of the audio tubes with something like a screwdriver handle, and see if the squeel occurs.
Is the pic affected in any way when it happens?

[Gregb]

How sure are you that the person that restored it didn't just patch it up to make it work for a sale? I have seen many a set that was supposed to have been restored and wasn't even close. I have restored one of these sets and I know what your saying, it has to come all apart to check anything. I think you might have to get in there and have look.

Gregb

[SwizzyMan]

Originally Posted by Gregb

How sure are you that the person that restored it didn't just patch it up to make it work for a sale? I have seen many a set that was supposed to have been restored and wasn't even close. I have restored one of these sets and I know what your saying, it has to come all apart to check anything. I think you might have to get in there and have look.

Gregb

What he said... Open it up and check if It even was repaired ever. I have one of these all original that works (sort of) these little things are surprisingly tough so its easy to say it could have been "restored"

[ZackN920]

Old Coot- its not easy to get to the tubes and I'm being lazy. If it were, I'd probably would have allready done that and test it too. Oh, and the picture is not affected by it. Actually the set doesn't even need a signal sent to it to squeal.

Greg, Swizzy, I bought it from a member here at the forum, he restored it around 2007. There's a thread about it here somewhere, probably in the archives. He really used it for awhile as well before deciding to put it in the classifieds.
Anyway's I know for sure it's been redone as well, because I had to pull it apart to readjust screen position(yoke assembly) after getting shipped to the house. It looked like a good job, and he also replaced the celeniums with diodes.
And Swizzy, you have one that's original and still works?-even sort of working...Holy Crap! These things run hot as hell, that's a surprise. I would never expect one of these to still work.

[jr_tech]

Originally Posted by ZackN920

Old Coot- its not easy to get to the tubes and I'm being lazy. If it were, I'd probably would have allready done that and test it too.

Well, you are not going to be able to fix it without popping it out of the can anyway... likely a microphonic 6U8 (a easy guess because 6U8s are used in the tuner, video IF, audio IF and audio stages.

So:
1. Remove the 2 large thumb-nuts that attach the stand (if your model has one).
2. Pull off the larger knobs.
3. Remove the 3 screws that attach the knob cover.
4. lift out the knob cover.
5. Remove the 2 screws that attach the handle.
6. Remove the screw under the bezel (center front).
7. Place face down on soft mat.
8. Lift the can off.

Five minutes, tops, and you should be able to replace any of the tubes without further disassembly (other than a few cardboard pieces that are tucked under the side rail screws and perhaps the speaker)

jr

[Tubejunke]

Originally Posted by ZackN920

Swizzy, you have one that's original and still works?-even sort of working...Holy Crap! These things run hot as hell, that's a surprise. I would never expect one of these to still work.

I don't know if you mean these RCAs or TVs that old in general. I have a 56 21" Philco tabletop and a 17" Sparton (early 50s) unrestored and still playing fine. The Sparton takes a few minutes to get width properly, but the Philco has always played likely as good as new. It has been through 1 crt in it's nearly 60 years of service.

It's all about dormancy time I think. If a set never served 20-30 years in an attic or barn, but rather was kept around and periodically used, I believe that it; no I know that the caps are far less likely to fail. Radios have a better chance of being kept around and in service I think. I have originals going back to the 30s.

[ZackN920]

I've gotta pick up a few 6U8's. Don't have any in my stash. Hopefully that's all it is then. (Anybody got some good one's they want to sell?)

Tubejunke- I ment these models of RCA tv's. I know there's situations where a good number of the caps are still good. I've got a bunch of working radio's that I haven't touched that are running on mostly original parts. It's better though to recap them if I were to use them regularily. I've been kinda bad when it comes to that with my '41 RCA 29k. I use that one too much for having original parts...

My 59' Zenith Space Commander(see avatar) is mostly all original. (It's got a repair shop sticker on the back from years ago) It works great other than the remote functions. I want to redo that one soon though because some issues are arising. Picture waves like a flag(noticeable if video game is paused),audio is really getting buzzy with certain images and the highvoltage sounds like a burning cigarrette...

[Tim]

Originally Posted by ZackN920

I've gotta pick up a few 6U8's. Don't have any in my stash. Hopefully that's all it is then. (Anybody got some good one's they want to sell?)

I'm pretty sure I've got some good pulls. PM if interested.

[Phil Nelson]

This article has a link to the RCA factory service manual for this model:

http://antiqueradio.org/rca16.htm

It includes disassembly instructions and loads of other info.

As old_coot88 suggested, you can try some tube tapping once you have the chassis out. I doubt that a tube tester will tell you whether a tube is microphonic, but if you find a microphonic 6U8, you could try swapping it with the other 6U8s in the set. A tube that's microphonic in one position (say, RF amp) might work fine in another position. Since the TV was working normally apart from the squeal, I see no reason to replace all of the 6U8s.

While you have the chassis out, it couldn't hurt to clean the tube pins and sockets, or at least wiggle each tube and make sure that it's firmly seated. The previous owner might have "restored" it, but who knows what was really done. You'll find out after you open it up.

My RCA T-100 TV had a thick lead collar around its oscillator tube to prevent microphonics. Nowadays you can buy aftermarket microphonic dampers to fit around tubes. They are collars made of brass, copper, or even plastic.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

[dtvmcdonald]

My TRK-12 radio's audio section is seriously microphonic. I've never ever noticed this before in any tube radio I ever had.

[Phil Nelson]

It was not uncommon to mount radio chassis on springs or rubber washers to reduce microphonics.

My Zenith 12-S-471 radio had spring mounts on the chassis; tapping certain tubes would give you a brief chime, but it never grew into a squeal. My Stewart Warner 1865 radio is mounted on rubber washers. My Philco 42-1240 TV is mounted on rubber "feet," presumably for the same reason. The Philco feet were used on various TV chassis, commonly enough that someone now makes repros.



If microphonics are a serious problem with your TRK-12 radio, you could try adding damper rings. You'll find lots of info about them in an Internet search, and not all of the info is snake oil. The lead damper on my T-100 tube was there for a reason.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

[dtvmcdonald]

My TRK12 radio IS on rubber mounts, but they are too stiff to
damp microphonics. They are not noticeable during operation, just
with zero signal.

[Adam]

That set was mine. This is the history of this set as long as I had it: I bought it an estate sale in the Los Angeles area back in the fall of 2006. I had replaced all the wax/paper caps and electrolytics, swapped the seleniums for 1N4007s, and replaced a few resistors that were causing trouble with the vertical output back then. Then I used it semi-regularly before I sold it to Zack last year. From 07-09 it was my bedroom TV. From 09-11 it was my desk TV. But I hadn't used it much since 2011. It sat on display on top of some other sets mostly unused 2011-12. 2012-13 it sat packed up in a box. Then 2013-14 it sat on top of some other sets in the basement until I sold it.

[ZackN920]

There ya go guy's. I told ya... Wow Adam, this set's realy traveled, CA to MI, now residing in IL. This thing's been getting good use on my desk now.

Anyway's if i'm not too busy tomorrow, I plan on opening it up. I'll test the tube and if it tests good/ok, i'll swap it to another spot on the chassis with another 6U8 and go from there. See how it runs and hopefully have it back together that night. If it tests low or the one I intend to swap tests low, ill have to wait for a spare good one to come in the mail.

Originally Posted by Phil Nelson

This article has a link to the RCA factory service manual for this model:

http://antiqueradio.org/rca16.htm

It includes disassembly instructions and loads of other info.

As old_coot88 suggested, you can try some tube tapping once you have the chassis out. I doubt that a tube tester will tell you whether a tube is microphonic, but if you find a microphonic 6U8, you could try swapping it with the other 6U8s in the set. A tube that's microphonic in one position (say, RF amp) might work fine in another position. Since the TV was working normally apart from the squeal, I see no reason to replace all of the 6U8s.

My RCA T-100 TV had a thick lead collar around its oscillator tube to prevent microphonics. Nowadays you can buy aftermarket microphonic dampers to fit around tubes. They are collars made of brass, copper, or even plastic.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

That's good to know Phil. I was figuring that I could probably swap them around. And I don't have plans to replace all 6U8's but I figure it's good to have a couple in my stash as extra back up. I probably got something else around here that uses them as well...
I've read your article. Very good as well as your other restoration's. Huh, thats very interesting about the "dampers" for these and other select few tube's. May look into them, if they're cheap.