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 Post subject: Capital Stereo Phonograph 926 balance control.
PostPosted: Feb Sat 17, 2018 5:56 am 
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Joined: Apr Wed 13, 2016 1:58 pm
Posts: 238
Location: Bartlett, TN
I have weak output on the T1 Transformer side. I have a suspicion that it has to do with the balance control. I have stared at this for a while and I can't seem to figure out how this balance control works. It does make some difference but it seems to change the tone rather than the volume between speakers. Any suggestions on where to look? Electrolytics and paper caps have been replaced. I assume this is to set the stereo volume between speakers correct? Pots have been cleaned. Plate voltages are close, plate signal is close. Once it makes it to the speakers, one side has twice the volume as the other. Could this be as simple as me needing a stereo record?


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Last edited by Tdlunsfo on Feb Sat 17, 2018 4:34 pm, edited 1 time in total.
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 Post subject: Re: Capital Stereo Phonograph balance control.
PostPosted: Feb Sat 17, 2018 6:21 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 17432
Location: Albuquerque, NM 87123
I doubt the problem is the balance control. Can you check voltages at the plates of the 50EH5 tubes? I think T1 or 2 may be the problem if the tubes are good.


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 Post subject: Re: Capital Stereo Phonograph balance control.
PostPosted: Feb Sat 17, 2018 1:52 pm 
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Joined: Apr Wed 13, 2016 1:58 pm
Posts: 238
Location: Bartlett, TN
114v DC with signal applied. Interestingly, one of the tubes begins to oscillate when I touch the plate with my probe. Applying signal seems to cure this. Transformer secondaries measure 1.3 ohms, primaries both around 360ohms.


Last edited by Tdlunsfo on Feb Sat 17, 2018 3:50 pm, edited 1 time in total.

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 Post subject: Re: Capital Stereo Phonograph balance control.
PostPosted: Feb Sat 17, 2018 1:58 pm 
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Joined: Apr Wed 13, 2016 1:58 pm
Posts: 238
Location: Bartlett, TN
A few more things I noticed. The balance pot measures 220 ohms but a 500 is called for. Output tubes test good on the emissions tester. Had to replace the 12ax7 as one side was completely dead, the other side tested strong. Swapping output tubes makes no difference, problem stays on the same side.


Last edited by Tdlunsfo on Feb Sat 17, 2018 3:49 pm, edited 1 time in total.

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 Post subject: Re: Capital Stereo Phonograph balance control.
PostPosted: Feb Sat 17, 2018 3:01 pm 
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Joined: Apr Wed 13, 2016 1:58 pm
Posts: 238
Location: Bartlett, TN
I just don't see how this is considered stereo. Both tubes combine into one Transformer then center tapped over to another transformer for the other speaker. It seems you would get the same audio out of both sides? Maybe I am just not as familiar with this type of setup.


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 Post subject: Re: Capital Stereo Phonograph balance control.
PostPosted: Feb Sat 17, 2018 3:58 pm 
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Joined: Aug Sun 23, 2015 6:01 pm
Posts: 683
Location: South Jersey East of Philly
Wow, this is a bizarre circuit that I've never seen before. By the circuit I'll guess is was a low end consumer grade player, so cost saving things might have been done AFA skimping on quality audio design. Tone and balance controls on the secondary audio side(?)..strange. Looks like they are running the 2 channels together into one tranny, like you would for a single channel mono push-pull circuit, then with that center tap feeding the other o/p tranny like a single ended output. Hopefully one of our experts here sill explain this...I'd love to know the logic here.


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 Post subject: Re: Capital Stereo Phonograph balance control.
PostPosted: Feb Sat 17, 2018 4:34 pm 
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Joined: Apr Wed 13, 2016 1:58 pm
Posts: 238
Location: Bartlett, TN
Tbirdkid, this makes me feel a little better. Having never heard this thing in a good working condition, I don’t know what to expect. I am going to try a known good stereo record in a bit just to rule this out. This is the Capitol Phonographs Model 926. It is a suitcase style player with the speakers attached to the ends. Cool little player. The primaries of each audio transformer measure the same. T2 is missing the center tap. I am tempted to rewire these as single ended output tubes for each channel, just not sure how the balance and tone controls will be affected.


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 Post subject: Re: Capital Stereo Phonograph 926 balance control.
PostPosted: Feb Sat 17, 2018 7:17 pm 
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Joined: Jan Wed 16, 2013 12:04 am
Posts: 1219
Location: 77001
Have you checked that the stereo cartridge is good ?

Hotwax


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 Post subject: Re: Capital Stereo Phonograph 926 balance control.
PostPosted: Feb Sat 17, 2018 8:40 pm 
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Joined: Apr Wed 13, 2016 1:58 pm
Posts: 238
Location: Bartlett, TN
Yes. Scoped and have even audio on both sides of the dual gang input pot from the cartridge. In fact, I have even audio on the plates of the 50eh5's. Somehow between there and the transformer secondaries I am losing gain. T1 Transformer that I'm losing volume on is also where the tone controls are. I'm going to try probing in this area for a bit.


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 Post subject: Re: Capital Stereo Phonograph 926 balance control.
PostPosted: Feb Sun 18, 2018 5:21 pm 
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Joined: Aug Sun 23, 2015 6:01 pm
Posts: 683
Location: South Jersey East of Philly
Here's a link to a write up in Audio News magazine when your model was announced. Maybe you should post this circuit in the Electro-Mechanical repair forum here to get more reads/interest in what this circuit does. I can't believe you would get true stereo separation.
https://books.google.com/books?id=2goEA ... 26&f=false


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 Post subject: Re: Capital Stereo Phonograph 926 balance control.
PostPosted: Feb Sun 18, 2018 8:06 pm 
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Joined: Apr Wed 13, 2016 1:58 pm
Posts: 238
Location: Bartlett, TN
Thanks for posting. Listed as a push pull with no phase inversion..just odd.


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 Post subject: Re: Capital Stereo Phonograph 926 balance control.
PostPosted: Feb Sun 18, 2018 8:11 pm 
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Joined: Apr Wed 13, 2016 1:58 pm
Posts: 238
Location: Bartlett, TN
Thanks for posting. Listed as a push pull with no phase inversion..just odd.


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 Post subject: Re: Capital Stereo Phonograph 926 balance control.
PostPosted: Feb Sun 18, 2018 11:17 pm 
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Joined: Apr Wed 13, 2016 1:58 pm
Posts: 238
Location: Bartlett, TN
Looking at this further, ended up one of the channels was quieter than the other even at the 12AX7 grid. I ended up adding a resistor on the front end to open up that quiet side a little bit. At this point I have lost money with the amount of troubleshooting time that I have done so I decided to just compensate and move on. Good learning experience but it's a weird circuit for sure.


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 Post subject: Re: Capital Stereo Phonograph 926 balance control.
PostPosted: Feb Mon 19, 2018 5:25 pm 
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Joined: Nov Wed 27, 2013 5:59 am
Posts: 498
Location: Metzger Oregon
I've seen mention of this type of circuit before, the cartridge is wired with a channel out of phase, then the sum goes to one speaker, the difference to the other speaker. Aside from that weirdness, it looks like your tone and balance are just part of a feedback circuit where a secondary coil on the output transformers provides audio that loops back to the amp's input.

Something was posted on here a while back about an article in a period magazine showing how this type of stereo amp was built. At the time I was hard pressed to see any real advantage of this scheme.


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