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 Post subject: RCA 115 - Volume independant static
PostPosted: Apr Sat 04, 2020 5:14 pm 
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Joined: Aug Tue 13, 2013 3:18 am
Posts: 1089
Location: Milwaukee, Wisconsin
I have a RCA 115 that I restored, probably close to 7 years ago. It was the first radio I ever worked on. I don't play it a lot, its more of a sentimental piece and I have better sounding ones for daily driving. But I've noticed it has some intermittent static that is volume independent. I've been over looking it for a while since its not that loud of static, and the music can mostly drown it out. But now that I have some extra time on my hands due to the current environment, I thought i might take a look at it.

Normally I would focus any troubleshooting on the output section since it is volume independent. But this radio controls the volume by (as far as I can tell) changing the resistance in on the cathodes in the RF stage. So I think that could widen the scope of where I need to look for the source of the static. But I'm not sure about that.

I haven't opened the radio up yet to check anything. But from what I recall, I replaced all the dog bones (as most were either out of spec, or close to it). And I left the mica caps alone. Seven years ago, the wisdom was to leave mica caps alone. I know in recent years people have started to see an uptick in mica failures. I've been away from the radio scene for at least a year, maybe 2. So I'm not sure if that trend has continued? That being said, would it be worth replacing the mica caps in this radio to try and fix it? Or any other suggestions to look at? Hoping to get some ideas and input before I throw it on the bench and start poking and prodding.

http://www.nostalgiaair.org/pagesbymode ... 014721.pdf

Thanks!

-Nick


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 Post subject: Re: RCA 115 - Volume independant static
PostPosted: Apr Sat 04, 2020 7:07 pm 
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Location: 42001 KY
could have a tube that is arcing.

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 Post subject: Re: RCA 115 - Volume independant static
PostPosted: Apr Sun 05, 2020 1:03 am 
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Tubes, transformers, and mica caps with high voltage on them are all suspect.

I suggest isolating this to either the audio stages or the IF/RF stages first. You could pull the 2A7 tube which pretty much leaves the detector and audio stages. This will start localizing the source.

I didn’t see any Candohms in this set, and since you replace the resistors, I don’t think that’s it. I suppose the volume control could get noisy because of where it is in the circuit.

Mica caps C14 and C15 are good candidates for noise. If suspected, you could lift one side to see. I have replaced ones in this position before because of noise and muffled audio.

Isolate first, then dig deeper.

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People may not remember how fast you did a job, but they will remember how well you did it.


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 Post subject: Re: RCA 115 - Volume independant static
PostPosted: Apr Sun 05, 2020 7:46 pm 
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Location: Milwaukee, Wisconsin
Good idea Tony, I didn't think of pulling tubes to isolate stages despite doing that in the past on other radios. Like I said, I'm a bit rusty. I pulled the 2A7, still had static. Pulled the 57 (and in the process the grid cap pulled off, so now I need to reattach that), still had static. I removed the wire that connects L9/C14/C15 to R4/C16. Which should effectively take the last of the mica caps out of the equation. Which leaves just the audio stage. And there isn't much there besides the 2A5, my tone control, and the OPT.

Could I use my signal tracer to check the grid of the 2A5 to see if the static is there? Then check the plate? If testing either point, should I leave the 2A5 in, out, or repeat with it in and out? Or should I start by disconnecting C30 from the tone wiper to take it out of circuit?


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 Post subject: Re: RCA 115 - Volume independant static
PostPosted: Apr Sun 05, 2020 10:09 pm 
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Location: Milwaukee, Wisconsin
Since it was easy to get to, I took C30 off the tone wiper. Still had static. So I guess that rules out the tone control causing it.

If I'm going to use my signal tracer on the 2A5, it looks like I'll probably have to do it through a test socket adapter. The bottom of the tube socket isn't easy to get to.


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 Post subject: Re: RCA 115 - Volume independant static
PostPosted: Apr Mon 06, 2020 12:01 am 
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You could lift one side of C16 which will isolate the output section from the rest of the radio.

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Tony

People may not remember how fast you did a job, but they will remember how well you did it.


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 Post subject: Re: RCA 115 - Volume independant static
PostPosted: Apr Mon 06, 2020 3:08 am 
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Joined: Aug Tue 13, 2013 3:18 am
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Location: Milwaukee, Wisconsin
I lifted the side of C16 that connects to R6, and no more static. Which I thought was odd, as I had lifted the wire connecting the plate of the 57 to the 2A5, which is accomplishing almost the same thing and had static. I put C16 back in, and powered up, static was still gone. Which is puzzling. until i remembered one other thing that happened since my last test.

I had removed and reinserted the output tube when I went to find my test socket in the event i needed to use my tracer. So maybe there was some corrosion/dirt/poor contact on the 2A5 to begin with? I know I had cleaned the pins for sure on the tubes 7 years ago. Back then I only had the Deoxit 5% spray I would have sed. Now I have the Deoxit 100% in a needle bottle. I went around on each pin applied some, and rubbed it with a q-tip. I did seem to get some dirt off. Did the same to the caps and the cap leads as well. And I put some on a pipe cleaner and tried to clean the sockets. I'm not a fan of these early sockets, at they really only make contact on one side. And not sure how well the pipe cleaner worked. Did seem to get some dirt off though.

So, once my 57 tube is repaired, I'll give it a final test tomorrow. I decided to use some sensor safe RTV I had laying around to glue the cap back down. I like the idea of something that will remain a little flexible with heat expansion that may occur. And being sensor safe, it shouldn't off gas and corrode the grid wire like regular RTV. Just have to give it 24 hours to dry and I can solder it back on. Tomorrow night around this time I should hopefully be able to confirm it was as simple as dirty/poor contact. I just can't believe that in 7 years, it got enough dirt somehow to make poor contact.


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 Post subject: Re: RCA 115 - Volume independant static
PostPosted: Apr Mon 06, 2020 10:55 am 
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That good news that it’s isolated to the audio section. Tube sockets have been known to cause issues, sometimes they can get carbon tracks, especially on rectifiers.

Any component that was heated up is also suspicious. A carbon resistor or maybe a new cap that got hit on the side with an hot iron could be a problem.

Let’s see what happens.

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People may not remember how fast you did a job, but they will remember how well you did it.


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 Post subject: Re: RCA 115 - Volume independant static
PostPosted: Apr Tue 07, 2020 1:14 am 
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Joined: Aug Tue 13, 2013 3:18 am
Posts: 1089
Location: Milwaukee, Wisconsin
Well, it will be another day before I can test. While going to solder the grid cap back on the 57, the roll of solder slipped out of my hand when I grabbed it off my shelf. And true to my luck, it took the perfect trajectory/bounce to hit the speaker cone.

The only upside is the wile the tear ran most of the radius, it was fairly clean. So I think the thin layer of fabric glue I put on both sides of the cone across the tear should do the trick. It did a little more damage around the frame though, where the U shaped part of the cone is. Hopefully it doesn't cause too much of an issue.


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 Post subject: Re: RCA 115 - Volume independant static
PostPosted: Apr Tue 07, 2020 10:55 pm 
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Joined: Aug Tue 13, 2013 3:18 am
Posts: 1089
Location: Milwaukee, Wisconsin
Looks like my patch luckily did the trick. And as upsetting as the accident was, I think it led me to find the real cause of the issue. My mind immediately jumped to worst case scenario mode late last night and I turned the chassis over to see how hard it would be to remove the speaker if I needed to send it off for re-coning.

While I had it flipped over, I noticed I had a component lead that I forgot to solder 7 years ago. The end of R7 that goes to ground was just wrapped around the base of the terminal, somewhat free to move. I suspect that, not a dirty tube connection, was the culprit. R7 connects through R6 to the audio path into the 2A5 grid. I'm guessing an intermittent connection here could cause the issue. I don't know how I missed it when I was looking at it the past couple days. Flipping it back and forth along with the other troubleshooting i was doing probably happened to jostle it enough to make a good connection.

One last question before I button it up. The schematic says the secondary on the OPT says one side should go to chassis ground when the speaker is chassis mounted. Mine is chassis mounted, but it looks like this never was done. I see a loose ground bus wire hanging down near the OPT, like it was meant to connect there. But as far as I can tell, it doesn't look like it was ever soldered to it. The solder joint on the OPT looks untouched, and I don't seem to see any loose solder on the bus wire like if it were de-soldered.

What is the purpose of this, and should I make the connection?


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