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don bridgman (4 posts)
Jul 30, 2010

How Do You Know What A Blown Transformers Ratings Are?

I have a transformer with a bad winding on the secondary. With no schematic or markings on the unit, is there any way to determine the voltage and/or VA rating of the transformer? Didn\'t transformers used to have some sort of identifying features?
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Daniel Snyder (312 posts  +488 likes)
Jul 31, 2010

If you know what the load is you can compute the VA requirements, multiply its rated voltage (Volts) by its rated current (Amps). This information is usually found on the back of the load - on its nameplate. If you cannot locate the labeled voltage and current (to calculate the VA), use the wattage (Watts), if shown. Exceptions are fluorescent lights, neon signs, gas discharge lamps, specialized electronic controls, etc which require a transformer with a VA capacity 1.5 times the wattage rating, if the volts and amps are not shown. Select a transformer with a VA rating equal to or larger than the VA found on the nameplate of the load.

Can you switch to an autotransformer instead of an isolation Xfrm?

Here is a useful link for an electronics forum:

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John Barrett (3 posts  +1 likes)
Aug 4, 2010

 If it does power the digital section, the secondary leads should be soldered to a circuit board, and following the traces should take you to a rectifier bridge. This could be one big square flat device, or four discrete diodes in a straight line or in a diamond pattern. The part numbers for these should show you their input voltages. Google them.

Also know that 12 vac is the most common, brute force, power source for digital electronics. This is stepped down by rectifying, filtered by caps for hum, then pumped through a voltage regulator I.C. typically a 7805, which supplys clean a 5 vdc for the logic system.

But a pre-amp could use heavier input. You may see 7812's or 7824's, and a 7805. Again 5 vdc for the logic, with the higher voltages for the pre-amp.  Use the highest number you find.

A good design rule-of-thumb is 3 volts higher than the output volts, so a 7812 should be powered by at least 15 vdc. Higher voltages are allowed, but this just converts to heat in the device, which will put out it's rated output until it dies.

Without actually seeing the circuit board, the only other thing I could suggest is find the schematic from the model number of the pre-amp. The manufacturer undoubtedly sells parts.

Good luck.


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don bridgman (4 posts  +0 likes)
Aug 4, 2010

Thank you John for an excellent answer. I should have known that, but being new in the field it just went over my head. I very much appreciate!

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don bridgman (4 posts  +0 likes)
Aug 1, 2010

Thanks for the help Daniel. The transformer in question is in a hybrid tube amplifier (hybrid=tube preamp, solid state power amp), with 3 secondary windings. The bad winding appears on one side of a split coil winding, I believe the winding that powers the DC-solid state section of the amp (going from memory here). The bad reading came from a resistance test only and I was wondering if I could bypass the fuse to get a voltage reading from the good side of the winding. I really need to know how to determine the stepped-down voltage of the secondary. I can find no information on this subject anywhere. Thanks again.

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