SWR Workingman's 15- Voltages- DC on Speaker

[Enzo]

Q5 has 2v across it, that is its job, to maintain a couple volts space between bases of Q3, Q9. I'd have to say it works.

If we turn on Q8 harder it will pull that whole mess down, but Q5 will continue to make that 2v space. That is how the amp works. Q5 establishes that 2v space to prevent crossover distortion. The Q8 yanks it up and down with the music. The outputs follow whatever Q5 is doing.

In fact you can even short Q5 E to C. That will increase crossover distortion, but the amp will run coolest possible. And I think you would still find 30v offset.

I could be wrong.

[DrGonz78]

Don't know if it helps much, but this thread (https://music-electronics-forum.com/...ad.php?t=34764) has some voltages called out for this model amp that is a lot closer to working than the OPs. See post #9.

Yeah I had pulled that thread up a few days ago and drew all the voltages out on a schematic. I wrote Q4 (61.3v) instead of 66v as the thread said. I kept wondering was why there is 500mv at the base of Q7 and 10v at the base of Q8. When they should be about 100-200mv each.

[Enzo]

Doc, I am confused. Your drawing shows 10v on Q8 collector, not base. 176mv on base of Q7.

The main rails were 61v in one case and 66v in another? NO big. Units vary, and mains volts vary.

One major difference is the other thread only had a 400mv offset, whereas we have a 30v offset here.

The 60v on collector of Q5 looks wrong to me.

[glebert]

Doc, I am confused. Your drawing shows 10v on Q8 collector, not base.

I'm guessing he meant Q6 base of the OPs amp, not Q8.

[Enzo]

OK, I'll buy that.

[DrGonz78]

Yeah I meant 10v on base of Q6. Still I think Enzo is right that Q5 is doing its job and that Q8 is behaving strange.

[loudthud]

Could the pins of Q8 be scrambled ? Could the Base and Emitter be reversed ? What is that note in the lower left corner of the schematic ? Shouldn't the pinout be B C E if you are looking at the top surface of the part with the leads pointing down ?

Edit: Q8 looks OK in the pic at the top of this page, but the B-E Voltage looks too high. Something has to be wrong if Q8 can't pull the output down below ground.

[log1982]

Well, actually, yes, I did, sorta. Enzo, G1, OP, thanks for considering my assertions.

27K is about the right value to bring the output to zero when difference amp q6/q7 is in equilibrium.
At rest, (no signal) we want the output to be zero volts. That means we have to drop 50 something volts across R23 and R24.
To maximize linear dynamic range in the feedback circuit, Q6/7 should share current equally when the output is zero.
*If the schematic is correct*, then the kooky current mirror Q8 will sink about 2mA.
We need something like 25k in R23 to drop 50 volts with 2 ma
** Remember, I said if the schematic was correct.
R37, R39A, R30, and R23/24 each play a role.
The most logical resistor to suspect is R23 because it is listed as a 1W. If it were 2.7k, it would be over-stressed in this application.
Jcon

[BStringThumper]

Good morning,
Thank you everyone for the suggestions and detailed explanation of the circuit.
Yes, I had it on the limiter for these tests. I will take new readings. I did try it direct to mains and then tried plugging in the speaker load and it still had high Vdc on speaker. I'll post new reading asap. Thank you again for all the help!

[Enzo]

ALWAYS check for DC before connecting a load. That 30v offset without one will not go away with one.

[glebert]

If you can I would do direct measurements of Vbe values on Q8 and Q5, since the consensus is that there isn't enough current flowing there. Especially if the voltages are varying I think it is useful to take that directly rather than subtracting the emitter and base voltages that might have been taken with some time separation.

[BStringThumper]

New voltage readings- plugged directly to mains. I'm going back to read and read all the posts. Thank you again.

[glebert]

I know you said you replaced Q6 but now it looks like current is flowing in reverse (out of the emitter) back towards Q7. This may explain where the current going into the base is going, but seems broken to me.

[log1982]

When you back-bias a base-emitter junction, it breaks down at about 6V and conducts reverse current. Look at the data sheet for Vebo. If not limited, it will break the junction, BTW. It's doing what you'd expect.
Enzo, I think Q8 is fine. Working like a current mirror. 2.5mA in, ~7mA out (depending on Vbe which, as OP noted, changes as it warms up) is about right for the Q8/R37/R39A configuration. Hope I'm not missing anything.
Enzo is correct, Q5 is working correctly as a Vbe multiplier. Adjusts for crossover current.
Jcon.

[log1982]

My mistake: Q8 current mirror: 3mA in, 8mA out. (I originally forgot to consider the reverse current from back-biased Q6)
Jcon.