SWR Workingman's Twelve

[x-pro]

Transistors burn-through will be due to either thermal overload or a problem in the bias tracking node. The latter is the most common cause.

[Helmholtz]

But the +ve rail is only 0.25v, meaning something is shorting that voltage to ground.

So the voltage at the positive output of the bridge rectifier BR1 actually is close to zero?

[x-pro]

Oh, I beg your pardon. I looked it up.
If R23 is 48V, then +56 is also there. But there is 0.25V on the collector of Q4. The conclusion is a break in the printed wiring to the +56V source.
P.S.: the rest of what I've described above can be forgotten.

[g1]

Device is drawing 10-12 watts from wall.

If there is so little current draw of the unit, I would think it is more likely that the +HV is missing, rather than something loading it down.
Or a break in the connection between +HV and output transistors, like x-pro said.

[The Dude]

^^^^^^That. If the rail was actually shorted, you'd be drawing lots of current. It's more likely the rail isn't making it from power supply to amp. Possibly a burnt trace?

[xtian]

Haw, I told you guys to rest, but you couldn't help yourselves. Bless you!

This goose chase started because I measured the wrong pin of the output transistor. Corrected voltages as measured:



Amp is passing signal, however very low output before distortion...only way less than one watt into a 2R load.

[The Dude]

What are voltages on the bases of the output transistors. What are voltages on the driver transistors Q3 & Q9?

[g1]

It seems to me that the problem is actually at the 'front' side of R23 and Q2 is probably shorted?

[xtian]

It seems to me that the problem is actually at the 'front' side of R23 and Q2 is probably shorted?

Neither Q1 nor Q2 are shorted.

[xtian]

What are voltages on the bases of the output transistors. What are voltages on the driver transistors Q3 & Q9?

Voltages updated:

[The Dude]

Nothing is biased on and there's hardly any collector voltage on Q5. I'd test Q5. R24 could also have raised in value.

[g1]

Voltages for Q5 and Q8 would be helpful.

[g1]

One thing I noticed about that R23 getting so hot, in the LA15 (basically the same amp) they spec a 2W resistor there. So the heat is probably normal, and if it's a 1W in your unit, I would upgrade it to a 2W resistor for R23.