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 Testing a Power Transformer in a Radio or Stereo

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FrankB
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Join date : 2010-11-22

PostSubject: Testing a Power Transformer in a Radio or Stereo   June 23rd 2018, 1:05 am

DISCLAIMER:
Use common sense and observe safety precautions. I post this with the warning that you are responsible enough to have common sense in dealing with probable high voltages that can harm you! 
If you get injured, it is not my fault. I am not your nanny or your mommy.
If you can't understand this, or be responsible for your own actions, do not try the following:


Over the years I found that many of the really cheap brands of radio & stereos came up with bad power transformers.

 Most often in the area I serviced, it was from lightning strikes to the power lines, or the power co. "Oops", or a tree knocking a HV line down onto the house feed. 

A device without a surge protector often ended up with an open transformer primary.

Since time was money I came up with a very quick way to test the transformer in circuit. (Besides I was lazy Wink).

 Disconnect the 2 primary wires. BE CAREFUL- During the test they will have 110 volts on them if they are good! Suspect
 
Find a known voltage secondary and back feed it with the correct voltage AC.
 Pilot light windings or filament windings are great ones to use.

Measure the primary voltage. Not there? Open primary.

 All the secondary voltages test OK? The transformer is likely OK. 

Often just back feeding a voltage will cause the device to function like normal too!

Note: Many Asian imported power transformers had fuses or thermal fuses INSIDE the outer insulating covering, and some had them  "buried" deep inside the windings. Sometimes you could remove the insulation and find the primary fuse quickly. I could replace them with the micro fuses they make now or the proper temp. thermal fuse; if the fuse was in the outer area of the winding. I DO NOT recommend trying to unwind the transformer to get to the inner fuse.

 The German made radios often had the transformer fuses in plain sight on top or on the side of the transformers, in fuse clips. Fast & easy to change out!

Sometimes the wire would melt or break at the lug or attachment to the flying leads of the transformer too. Generally that was a real EZ & quick fix. (This can also happen with audio transformers too).

This "Quick & Dirty" method of testing in circuit sure beats unsoldering every lead & pulling the transformer out to test it.

Another note. Some import radios had DUAL PRIMARIES Sometimes they were in parallel for 110V and in series for 220V. Often they had the 2nd primary disconnected or a switch selectable voltage choice.

 If you are lucky enough to get one of these sets where the primaries had to be series for 220V, you could often use the other, unused primary winding as the replacement for the open primary.
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ve1arn
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Join date : 2010-11-23

PostSubject: Re: Testing a Power Transformer in a Radio or Stereo   June 27th 2018, 2:56 am

Good tip on the backfeeding Frank! Thanks.
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