SWR Workingman's - distorted

[lowell]

This amp came in humming due to a bad filter cap solder joint. I also discovered that the VBE multiplier transistor had a broken leg and then I fixed that. Now the amp hum is gone however the amp is low volume and distorted. I read that the voltage across the VBE mult transistor should be equal to the number of BE junctions in the output. CE voltage is reading 1.3volts and only changes about 10millivolts when I adjust the bias pot. I have a printed schem but cannot locate one online to post here, if anyone can do so please do it. I tried another speaker, that's not the issue.

[Enzo]

What model SWR Workingman is it? What is the drawing name number and date? I'll post a schematic for it if you can;t. Let's talk part numbers and such to avoid confusion. R1, C2, Q3, etc.

[lowell]

It's the Workingman's 12 bass combo.

The date on the schem is 09feb03. The filename says "wm4004_pre_11a.sch."

[lowell]

Hi Enzo,
Any luck w/ the schem on this?

[Enzo]

OK, this looks like your schematic.

[lowell]

Yep that's the one. So any idea in regards to my initial post?

[Enzo]

R1, C2, Q3, remember...

[lowell]

Oh ok I didn't get it... will check those.

[lowell]

Those components are fine Enzo. Both HV rails before D1 and D2 have -+ 60vdc. The -+ 15v rails are good too. I also checked for AC on - and + HV nodes and the + rail right on C1 has 500mvAC but the - rail on C2 has no AC. Is this a problem or clue? Seems odd that one has AC but one doesn't. But yeah Q3, R1, and C2 are all good. Oh and I'm reasoning that C2 is good because the - rail seems fine, however I did not pull and test or replace C2, as I'd only think I need to do that if the - rail was low and/or had lots of AC on it.

[Enzo]

Lowell, I am sorry. What I mean was, let us start using part numbers. Numbers LIKE R1, C2, Q3. I didn;t mean those parts in your amp, they are just example numbers. I want to get to a specific resistor number in the schematic instead of "VBE mult transistor" which doesn't speak to me.

If your high and low voltage rails are all OK - low ripple or very little - then we move on from there.

SO what voltages are at the ends of Q5 for example, and the bases of the drivers.

[lowell]

Ok here goes.

Q5 C 1.4v
Q5 E 80mv

Q3 B 80mv
Q9 B -1.4v

[lowell]

correction, that was dumb of me:

Q5 C 78mv
Q5 E -1.4v

Q3 B 78mv
Q9 B -1.4v

[Enzo]

Now I do feel stupid, of course the ends of Q5 will be the same as the driver bases, they are the same points in the circuit. Oh well, I am getting old.

SO we have low distorted output. That seems reasonable at Q5. Have we scoped a sine wave through the amp?

[lowell]

nope, will do.

[lowell]

Ok so I scoped this the best I could. It's hard to follow the signal path in this schem. There is not even an "input" on there! And what's w/ the W9, W7 etc... I know those are wires but they aren't labeled in the amp as that so what's the point? I'm guessing C23 is the input cap. Ok and while I'm b#$ching why are they labeling the preamp out "power amp in", yes I know they're saying that those points on the schem are connected but sheesh can't we keep things simple and clear SWR?

Anyway, i've got 1khz going into the passive input. I have good signal after C25. No signal on Pin2 of U3-A, however there IS signal on Pin1 of U3-A... how is that possible that there is no signal on the input but there IS signal on the output of that opamp? Is it something to do w/ the feedback and the inverting input? I'm lost as to where to go from there. Through D11?