Hot Power Transformer

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Hot Power Transformer

Post by texasmike325 on Fri Oct 24, 2014 10:47 am

Good morning all,
I had this RCA recapped about a year and a half ago and has been playing great ever since. Recently i was listening to it and I was looking in the back at the tubes (who doesn't do that?  Wink  ) and i touched the transformer to see how warm it was. It was HOT. I immediately turned it off and unplugged it. I let it cool off for a few hours while I went out to run errands. Came home turned it on for about 10 minutes and it did the same thing. I haven't used it since. Can anyone give me some suggestions on the cause from their own past experience? I appreciate any help you can give me.

http://s1361.photobucket.com/user/texasgem1977/library/RCA%203RF91

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Re: Hot Power Transformer

Post by 35Z5 on Fri Oct 24, 2014 11:19 am

At 120+ volts most power transformers run too hot to touch(at least for long)...

Many will use a 12V transformer wired to buck the incoming AC so voltage will be around 110-113v, most of the older transformers will run much cooler...
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Re: Hot Power Transformer

Post by Resistance is Futile on Fri Oct 24, 2014 5:05 pm

Remove the rectifier and see if it runs cooler. If so, then change those filter caps to start with.
Also try and use a dim bulb tester, that's 1 and 1/2 times the wattage, if it is bright then you have a short.
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Re: Hot Power Transformer

Post by jerryhawthorne on Fri Oct 24, 2014 6:49 pm

Two great ideas. I worry a little about the rated values of the e-caps installed. Look like 360V ones. Might be a little low? Perhaps one went south?
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Re: Hot Power Transformer

Post by 75X11 on Fri Oct 24, 2014 7:45 pm

My 8R71's transformer runs relatively hot, also.
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Re: Hot Power Transformer

Post by Bill Cahill on Fri Oct 24, 2014 8:02 pm

My guess is you have a shorted primary on transformer.

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Re: Hot Power Transformer

Post by jerryhawthorne on Fri Oct 24, 2014 8:42 pm

Mike, this is where a variac comes in real handy to set the line voltage down to about 115 to see what is going on. Perhaps you can borrow one if you don't have one from some radio person local? It does help to figure if you need to lower the line voltage or with the other testing indicated to proceed towards that.
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Re: Hot Power Transformer

Post by texasmike325 on Mon Oct 27, 2014 6:04 pm

Thank you all for the suggestions. I did use a variac to lower the voltage to 110-115 and ran it for about 15 minutes or so. It did warm up but not nearly as hot as before. I havent had time to mess with it much besides that but i plan on letting the radio play longer next time to see if the transformer starts acting up. thank you again, i will keep you updated.

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Re: Hot Power Transformer

Post by 75X11 on Mon Oct 27, 2014 6:22 pm

If your variac gives you the current draw of your radio, you might want to watch that also as you operate it. See if there are any changes as it heats up.
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Re: Hot Power Transformer

Post by Dr. Radio on Sat Nov 01, 2014 10:30 pm

Excellent advice above, but I will throw in my 2 cents as well. RCA was notorious in my experience for using borderline transformers. To save $, they usually had transformers that had "just enough" laminations to get the job done. They run warm and with today's line voltages (without circuitry mods) the will run HOT even with no problems in the radio's circuitry.
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Re: Hot Power Transformer

Post by Wildcat445 on Sat Nov 01, 2014 11:03 pm

RCA used more than transformers that was borderline. Zenith did it too. Witness the "6X5" scare/whine. The transformers in Magnavox stereos run really warm, too. If there is nothing amiss in the circuitry of the instrument, I don't worry about it.

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Re: Hot Power Transformer

Post by Dr. Radio on Sat Nov 01, 2014 11:08 pm

WC,

I would still recommend mod'ing the input side of the transformer to make it run cooler. Why put undo stress on something that old?

On my RCA 17K console, I opted not to use a "bucking transformer" due to space, but mounted a military spec grade aluminum housed 50 watt resistor underside of the chassis. It is in series with the primary after the protection fuse I installed.....The transformer only "sees" an input voltage of about 110 volts even with 120+ volt line voltage. Keeps the transformer toasty, not "ouch my ********* hand!" hot! Smile
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