Audio amp Mystery

I have a little transistor radio that was drawing excessive power from the battery. It was about 125mA at 6 volts which is 10 times too much. I found I could bend the circuit board to eliminate the problem. So, I studied the thing for an hour or so and found the problem had something to do with the audio output transformer. This radio uses an input and output transformer in a push pull arrangement. So, I used a soldering iron to heat the 5 terminals of the output transformer and the problem went away. Radio works fine now. But the question is, the output transistors were getting warm which indicated the excessive current was flowing through the output transistors. With two transformers, one for input and one for output, how is it possible for the transistors to overheat with only some problem with the output transformer?

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Reply to
billbowden
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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?)

Reply to
Clifford Heath

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

Reply to
bitrex

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.

Reply to
bitrex

Or an intermittent short in a transformer winding.

Reply to
krw

** Is that the idle current with the volume at zero ?

Or not.

.... Phil

Reply to
Phil Allison

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.

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Reply to
billbowden

** I said *zero volume* not some low setting.

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

Reply to
Phil Allison

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

Reply to
mkolber1

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.

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Reply to
billbowden

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.

m
Reply to
makolber

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