Audio amp Mystery

Both output transistors, or just one?

An internal short (or short to the center tap) on one side of the output transformer would cause that side to get hot, possibly without totally killing the audio output (did you notice the need to increase volume?)

Is there a capacitor connected in parallel with the output transformer primary?

Hypothesis: you lost connection with the capacitor, transistor radio output transformers have much smaller inductances than say a class B push-pull tube amplifier, output transistors were burning too much power because the current was out of phase with the voltage.

Or an intermittent short in a transformer winding.

Or not.

.... Phil

No, it was at a low volume level, but there was a huge change in current when I put some stress on the output transformer. It was obvious the problem was close to the output transformer. But that doesn't explain why the transistors were getting hot.

If half the primary of the OT were shorted, there would be a massive overload on the transistors, same as shorting the speaker line. But with no drive signal at all, it should make little difference.

Normally, these amps have a centre tapped output tranny with two transistors working in push-pull like a tube amp with two output tubes.

Shorting any winding effectively shorts the whole tranny due to coupling.

.... Phil

I suspect it wasn't really the transformer that changed. Class AB push pull circuits like those used in small radios often have a small thermistor to set the idle current. I suggest you check k that area. M

Yes, I thought of that idea, but I don't have a schematic or a circuit layout picture so it would take a lot of time to trace out the circuit to construct a schematic. It's obviously an intermittent connection, so all I can do is look around and hope to find it. But it's working fine now, so I'm going to leave it alone.

if it ain't broke, don't fix it is good policy.

next time though, look in the area of the bases of the two output xitors.