Leaky AF Out. Transformer

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Leaky AF Out. Transformer

Post by harold wright on Mon Jul 22, 2013 4:53 pm

Simplex 1040 had a post about a Truetone transformer that reminded me of a problem I once found.
Can't remember name of the 1960's radio with P/P output. Sound was somewhat distorted and all the usual tests (leaky coupling cap. to output tubes, etc.) turned up nothing. I finally decided to see if B+ was leaking onto VC winding. This VC winding had both sides above ground and when I metered it to ground I found 250VDC on VC. Glad I did not touch the VC leads beforehand. Weirdest/worst case of leak. I ever encountered in an AF out. transformer.

Harold

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Re: Leaky AF Out. Transformer

Post by Bill Cahill on Mon Jul 22, 2013 7:44 pm

Geez. Sounds more like a short. I had the primary of the output transformer short to chassis on a 1947 Philco radio. Almost caught fire.

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Re: Leaky AF Out. Transformer

Post by harold wright on Mon Jul 22, 2013 8:59 pm

Bill Cahill wrote:Geez. Sounds more like a short. I had the primary of the output transformer short to chassis on a 1947 Philco radio. Almost caught fire.
Bill That reminded me of stories I read about repairmen in the 'thirties that had a radio with crackling/static crashes and they suspected a transformer about ready to open. They would short the output tube plate to ground for a very short time and if audio disappeared they knew for sure the transformer was bad. Seems a good transformer could stand this momentary jolt.
These stories were pub. in Radio Age from Augusta Ga (I think). Don Patterson was ed./publisher. I still miss that periodical; it stopped about 20 yrs ago. Each month they had a story about repairmen--they still had DC mains in some cities. Very interesting stories.

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Re: Leaky AF Out. Transformer

Post by analogdino on Tue Oct 01, 2013 1:16 am

harold wright wrote:(snip)  This VC winding had both sides above ground and when I metered it to ground I found 250VDC on VC. Glad I did not touch the VC leads beforehand. Weirdest/worst case of leak. I ever encountered in an AF out. transformer.
Harold
May be just a high resistance leak if you were measuring voltage with a high resistance voltmeter.  Try a quick grounding of the VC via, say, 1 Kohm, while measuring the B+.  If it sags, you've got a short.  If no effect on B+ the transformer may still be serviceable, but perhaps not for long!  Good luck!
Cheers,
Roger.

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Re: Leaky AF Out. Transformer

Post by Bill Cahill on Tue Oct 01, 2013 5:18 am

You should have seen that audio output transformer go in my 48-1201 Philco go.. When it shorted, it was with a loud pop, and, I instantly heard sizzling. Shut down. Burned my finger on melted wax of audio out. Transformer...

I have an RCA Victor model 110K radio. Help, guys, I need a good case. Mine is shot.
While restoring chassis about thirty years ago, I had distorted sound after rebuilding. This radio used PP parallel 6K6 tubes. Four of them.
Half of primary was open, other shorted..

About 23 years ago I was restoring a cathedral radio for a customer..
I had it playing on the bench, but, was getting a spitting sound through speaker. I had one hand on chassis, and, was going to pick it up on side to check voltages. Spkr. started to move, and, I grabbed it to not let it fall. I got a very strong DC shocki!!!
I let go fast, grabbed my volt meter, and, went from chassis to speaker frame, and, found 350 volts dc. Full B plus!!
Upon taking field coil off, I found a burn hole in cardboard, and, a burn spot on coil. For two weeks, I was pouring super glue in to see if I could save it. Believe it, or, not, it worked. No voltage, no crackling, no shocks, and, radio played great.

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