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 Post subject: Possibly shorted my audio transformer on RCA portable..
PostPosted: Feb Wed 07, 2018 2:34 am 
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Joined: Dec Wed 23, 2009 8:20 pm
Posts: 113
Location: Ann Arbor,MI
Short version: after recap and revamping the turntable on my 7-es-6h, put everything back in the case but missed that I had 2 wire contacts and pop. Not sure if the speaker transformer is gone, but it raises the question, where can one fine a replacement in this day and age? The primary is 2800 ohm and the secondary is 1.75 ohm. Anyone know of a supplier?


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 Post subject: Re: Possibly shorted my audio transformer on RCA portable..
PostPosted: Feb Wed 07, 2018 3:49 pm 
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Posts: 6354
Basilio wrote:
... where can one fine a replacement in this day and age? The primary is 2800 ohm and the secondary is 1.75 ohm. Anyone know of a supplier?
I looked up that model and noted it is a standard 50L6 output tube. The reality here is that tube will work quite well with anything from 2600 Ohm load impedance up to about 6000 Ohms (nothing particularly critical about it). It's lowest distortion rating is around 3800 ohms, but that also is not critical. See the last page of this... http://www.mif.pg.gda.pl/homepages/fran ... 50L6GT.pdf

So the reality is that you can use ANY audio output transformer that would support a 4 watt tube radio (like any AA5 type radio). A quick search of Ebay looking for "tube type audio output transformer" shows many for under about $15. If you find something that fits physically it will work fine.

I'll deny it if you tell anyone I have done this, but I have even used 6.3 VAC filament transformers in a pinch. Yep, not ideal, but it worked.

Curtis Eickerman

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 Post subject: Re: Possibly shorted my audio transformer on RCA portable..
PostPosted: Feb Wed 07, 2018 3:56 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4043
Location: Boston, MA USA
Why do you think you need a new output transformer?

-David


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 Post subject: Re: Possibly shorted my audio transformer on RCA portable..
PostPosted: Feb Wed 07, 2018 5:47 pm 
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Joined: Sep Tue 30, 2014 6:08 am
Posts: 2269
Location: The Old Dominion VA 23518
Basilio wrote:
Short version: after recap and revamping the turntable on my 7-es-6h, put everything back in the case but missed that I had 2 wire contacts and pop. Not sure if the speaker transformer is gone, but it raises the question, where can one fine a replacement in this day and age? The primary is 2800 ohm and the secondary is 1.75 ohm. Anyone know of a supplier?



What's the part number(s) on the transformer? Without a schematic or parts list, and no physical size given, not much.

Some vintage RCA parts are still out there. I recently got a switch for a 1960's RCA HiFi unit on eBay. I found it by Googling the RCA part number......

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Brian
"Capacitor Cosmetologist since 1979"
USN Retired 1984-2006 (Avionics/Cal)


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 Post subject: Re: Possibly shorted my audio transformer on RCA portable..
PostPosted: Feb Wed 07, 2018 8:03 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4043
Location: Boston, MA USA
Basilio wrote:
Short version: after recap and revamping the turntable on my 7-es-6h, put everything back in the case but missed that I had 2 wire contacts and pop. Not sure if the speaker transformer is gone, but it raises the question, where can one fine a replacement in this day and age? The primary is 2800 ohm and the secondary is 1.75 ohm. Anyone know of a supplier?

The cart is before the horse here. Based on this post there is no reason to suspect that the output transformer has failed, and RCAs are not famous for this. Please give us the long story.

You didn't say anything about the cartridge. You're not trying to use the old one, are you? The original was likely a crystal type and in that case would be undoubtedly dead.

Let's go back to the beginning. What is the problem -- no sound, no B+, what?

-David


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 Post subject: Re: Possibly shorted my audio transformer on RCA portable..
PostPosted: Feb Fri 09, 2018 2:41 am 
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Joined: Dec Wed 23, 2009 8:20 pm
Posts: 113
Location: Ann Arbor,MI
Ok, it's part #100272, with the impedance listed above.
Attachment:
7-es-6h.jpg
7-es-6h.jpg [ 173.82 KiB | Viewed 139 times ]


The longer story. I recapped the the amp, and then put it back together and found that I had used the wrong value leading into the volume and had to run it with the volume knob barely turned up. When I took it back apart, I decided to clean and soak the motor bearings and do some lube. I partially reassembled, excited to try it out, turned it on and heard a pop and a small flash near the audio transformer's bare wires. Unhooked one speaker wire touched the contact and heard no static. Initially I thought it might be the transformer, but now I am leaning toward having burned out the 12sq7 as it is not lighting, or at least compared to the other two tubes.


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 Post subject: Re: Possibly shorted my audio transformer on RCA portable..
PostPosted: Feb Fri 09, 2018 2:45 am 
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Joined: Jan Wed 16, 2013 12:04 am
Posts: 1154
Location: 77001
For the 12SQ7, do a simple continuity test b/t the heater
filaments.
Hotwax


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 Post subject: Re: Possibly shorted my audio transformer on RCA portable..
PostPosted: Feb Fri 09, 2018 2:56 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 11184
Location: North of Mpls, Minnesota
Hotwax wrote:
For the 12SQ7, do a simple continuity test b/t the heater filaments.Hotwax

These tubes are in series, if two of them light up you know the third one has continuity. No need to do a test for continuity.
Some tubes do not get as bright and this is normal.

We still need more information before deciding what direction to take. Pictures of what wires shorted, voltage readings, etc.

Dave


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 Post subject: Re: Possibly shorted my audio transformer on RCA portable..
PostPosted: Feb Fri 09, 2018 2:58 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4043
Location: Boston, MA USA
12SQ7 is normally a metal tube, so you cannot see it light.

I really don't like how the tone control is across the output tube plate. Could short out B+ very easily. I would investigate around there.

The best way to start would be to measure the voltages at the plate, screen, grid and cathode of the 50L6.

-David


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