Re: How to test audio output transformers

powerdoc wrote:

I recently purchased a Fisher 800B receiver and want to test the audio
output transformers for good/bad status and wanted to know if there was
an out of circuit (or in circuit) way to do same before I start to
retube and spend a good bit of dollars.

See if you can smell any burnt order. But proceed anyway with the ohmmeter.

First I'd check with an ohmmeter for continuity of the primary and the secondary. If it's a push-pull amp, each section of the primary should be roughly similar resistance. One side being 10 to 20% higher resistance would not be unexpected. See if both transformers yield very similar resistances.

Next step is to test for shorted turns. Odds of a shorted turn vs it being blown open circuit are fairly low, though. Especially if you don't smell a faint burnt odor when sniffing around the transformer where its wires come out (with the power off of course!) you can probably skip the following test, and assume that the transformers are probably good (however, if the transformer is heavily potted, this lack of burnt odor test won't be valid). You'd need an audio test generator that you can measure voltage and current on. Or a variac *AND* isolation transformer (variacs do not provide isolation from the powerline). And an AC milliamp meter and AC voltmeter. With no load on the transformer under test secondary, you want to measure any large amount of current draw when you crank up the variac voltage. Any shorted turns will cause large current draw. At some point when cranking up the variac voltage you may see the current suddenly go up. Back off immediately. What you are seeing if this happens is the core going into saturation. Normal. Both transformers should behave the same. Try loading the secondary with an 8 ohm resistor, you should see the current draw go up. Both transformers should produce the same current draw when doing that.

If you pass the above, the transformers are probably as good as the day they were made. Of course if the design was poor, this test won't tell you that, but the above test should reveal any abuse that might have happened.
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