Fender Acoustasonic sfx shorted? Ugh!

[Indyryder02]

Mounted heat sink yes.

the rest, no.

That makes sense though. I was thinking oppositely the load would be safer.. but not the case with transistors.

and no I didn’t check for DC. On outputs like I usually do with HiFi stuff. This solid state guitar amp has really Screwed me up for sure!

what could be causing this emitter voltage fluctuation and ultimately their failure on those same two?

By the time I get everything checked is usually when they blow!

[g1]

The emitter voltage fluctuation is just a byproduct of the current through the output devices. That current can be monitored by measurement of voltage across the emitter resistors (DC milliVolts).
Something in the bias circuit or driver circuits is probably causing the base voltage at the outputs (B to E) to get too large, which increases the idle current of the output transistors. The 2 that keep blowing are on the negative side of the power amp.

First thing now is to locate and replace blown parts. Re-check everything you've had to replace already.
Then you want to bring things up slowly on the variac, with no load, and this may be the time where you want the variac and the lamp limiter.
As you bring up slowly, you want to keep checking several things, and comparing to the good side. Check for DC at the output, check mV across emitter resistors of power transistors. Check B to E voltages of output transistors.
When you find any significant difference, stop raising the variac voltage. You may have a defective part, or something as simple as a bad solder connection.
If you get it to the point where you can bring it all the way up with everything the same as good channel, leave it run like that for awhile, and keep monitoring the idle current of the power transistors (by measuring mV across emitter resistors).

[Jazz P Bass]

At this point you also have to replace Q27 (whether is checks bad or not).
Also the limiting transistor may be at fault, Q23.

[Indyryder02]

Ok.. after much more reading about how these circuits work etc.. and going back in.. removed the two failed outputs...
replaced Q25 and Q27 as a pair for good measure, seems very common for one to be leaking and causing this issue..

Able to bring it up to full voltage via bulb limiter.. 120v pulling .5 amps on variac. Dimly lit bulb holding steady. (Normal)

now afraid to take it off the limiter (haha)
I have the following test readings.

across emitter resistors 12.9 mv on good amp side 8.6mv bad side..
B-E 32.64 and 32.56 respectively.
8.45mv DC at output. Not zero but pretty low.

individually on each of the outputs I have the following
C. For all outputs is running -61.5.
B. For all is -32.92.
E for the following as shown.
Q31 4.22
Q36 6.78
Q35 -36.90
Q31 -19.80 And that seems to be the only real variation..

Ran like this for several minutes or more. Long enough to get all the measurements. And didn’t seem to creep or rise any.. but course it’s still current limited too.. taking off the limiter will be the true test..

but obviously need to make sure it’ll be right before that.

[Indyryder02]

Across emitter resistors specifically...
271 21mv
267 8.3mv
270 9.9mv
266 12.2mv

think I’ll stop here till I hear back with more advice on a safe continuation!

thanks again!!

[g1]

across emitter resistors 12.9 mv on good amp side 8.6mv bad side..
B-E 32.64 and 32.56 respectively.
8.45mv DC at output. Not zero but pretty low.

individually on each of the outputs I have the following
C. For all outputs is running -61.5.
B. For all is -32.92.
E for the following as shown.
Q31 4.22
Q36 6.78
Q35 -36.90
Q31 -19.80 And that seems to be the only real variation..

B-E means one probe at each leg. So it is the voltage between the base and emitter. If the transistor is forward biased, it can not exceed .7Volts. + or - will depend on whether transistor is PNP or NPN.

Something is fundamentally wrong with the other readings. Be careful about whether V or mV.
Emitters of Q32,36,31,35 are millivolts away from the output. If output is approx. zero, how can emitters be at -36.9 or +6.78 or the other numbers you listed?
Either those numbers are wrong, or the output is not near 0V. Are all emitter resistors still ok?
And bases can only be .7V away from emitters, unless reverse biased.
What are you using for a ground reference where you have your black probe?

[Indyryder02]

My apologies. I did measure that incorrectly.. (dyslexia forming must be or something!)
ok
B-E. As follows Vdc
Q32 .625
Q36 .627
Q35 .601
Q31 .620

across emitter resistors mv.
266 12mv
270 9.7
267 7.8
271 23 —- This is Roughly double the rest.

[Indyryder02]

Using chassis for ground.

each leg of each trans.

B for all to ground is roughly .61 vdc

C for all to ground is roughly -32.79vdc

E for 32 is 5.4mv 36 Is 8mv 35 is -36mv 31 is -18.5mv
these are al of course moving about slightly.

I think I have the measurements right now. Hopefully this makes more sense

[Indyryder02]

About 9mv dc at speaker output P9-P10

[g1]

That makes more sense.
It's important now to get absolute voltage readings, rather than referenced to ground. So B-E with one probe on each leg, black probe always on emitter just to be consistent.
And across emitter resistors, one probe at each end.
It was unclear in post #83 how you were checking the emitter resistors, so please re-do. If output wasn't perfectly at zero, voltages with respect to ground don't help much. Measuring across the emitter resistors, and from B-E of transistors takes that possible difference out of the equation.
Also check between TP73 & 75 (one probe on each) and between TP72 & 74.

[Indyryder02]

Ok.
B-E. black on E
Q32 .635
Q36 .638
Q35 .614
Q31 .630

across each emitter resistor
R266 11.95
R270 9.62
R267 7.65
R271 25.05 Still seems to be the one out of order.
I do have the emitter leg of Q35 soldered directly to R271 with a small link of wire. As the pad failed there from the multiple replacements..
Maybe this is causing an issue. I’ll get a pic:
I just followed the trace and seemed I could join them to save the connection.: but maybe not.
not getting anything difference in resistance through it compared to the others. So seems to be a good connection.

[Indyryder02]

Oh yes.
And probe to probe on TP 73-75 is 2.38 And TP 72-74 is 2.42

[Indyryder02]

Was reading in these cases, I should open the collector circuit and place a DC milliamp meter in line with it to measure the E-B junction current? Wasn’t sure of where to “open” it at and attach the meter...

[g1]

The trace repair should not be a problem. I guess you checked that the resistor measures the same as the others.
As the Q35 B-E voltage is the lowest of the bunch, I'd expect lower current, yet it has the highest.
Is it from the same batch as the others? It's insulator is in good shape?

[Indyryder02]

Yes. I tested all emitter resistors.. (actually about every resistor and diode on the particular schematic for that amp!)
all test as expected.
All transistors from same batch. Insulator seems to be fine.