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?

[Enzo]

Yes C23 is the input, one would recognize it from the configuration.

You usually cannot measure signal at the input of an op amp. If it shows up on the output, that means it got there.

DIvide and conquer. Look at C11 - the power amp input cap. What does the signal look like there? If it looks good, we then know the preamp works, and the power amp is the problem. If it sucks there, then the preamp is at fault.

Here is a more complete drawing set for the 2004 version. it shows you what they were doing at least. The drawing in the above link is only for what is on the one board.

[Mars Amp Repair]

Hey folks,
I don't want to steal Enzo's thunder here, but I'm wondering what effect you get when you just put your signal generator into the 'power amp in' jack? If your signal is still bad, then you really need to focus on the 2nd page power amp stage.

As for the op amp input U3 p-1 having no signal but the output does...that's totally normal for a closed loop (most amps) opamp config. The output fed back to the input is designed to null the inputs. Don't worry about all that just know that that is the way it is.

My guess is that p1-1 & p1-2 (op amp U3-a feedback path) go to a volume or gain control on the front panel as there really is no real feedback shown for this op-amp other than a cap for frequency trimming purposes.

From that level pot, I'm guessng the signal ends up going to the power amp.

I'd try feeding your signal to the power amp in jack 1st. You might be barking up the wrong tree here. glen

[Don Moose]

These may help a bit.

[Jazz P Bass]

Lowell
Look at the signal on preamp out. If it is ok, move on. Insert an input signal into power amp in. Scope the amp output. If it is bad, then you have narrowed it down. "divide to conquer".
As far as U3, pin 3 goes, sometimes you can't see the input signal clearly. Look on the left side 0f C25. Then look at the output of U3. If it is good, then the opamp is ok.

[Enzo]

I may be confusing terms, but I believe they refer to the op amp inputs as at "virtual ground."

[lowell]

right on... well the amp has effects send and returns but no Pre out and PA in. I guess these terms are semi interchangeable. Anywho the Effects send has clean signal so we're talking a PA issue here.

[lowell]

ok I believe the issue is at the differential input to the PA. Signal is good at the base of Q7 but the top of the wave is clipped at R30. Voltage at the collectors is -58v, at the Emitters it is 660mv, and the Base of Q7 is 150mv while Q6 Base is 135mv. All components between emitters and Q2 including Q2 test fine. I'm thinking the voltage at the emitters should be more around 58v but not really sure if that's correct.

[Mars Amp Repair]

As an aside,
One significant thing to remember on most of these SWR's...the ground on the shielded audio input wire from the preamp board to the power board MUST be connected or you get massive offset. That is the only grounding point for the diff amp input. glen

[lowell]

I don't see anything there as far as ground wires go... all seems connected.

[lowell]

anyone know what voltages SHOULD be on EBC's of the differential pair. And/or what might be causing these symptoms?

[Mars Amp Repair]

Lowell,
What I meant about the shielded audio cable from the preamp to the power board is that sometimes we disconnect that wire to gain better access to the power board & don't realize that the ground (shield of that wire) actually grounds the diffamp of the powr stage. It may not be the case in this one, tho.

As to the voltages in the diff amp, any imbalance in the power stage is going to throw all the voltages off in the diff amp. IMHO, it is really difficult to try & troubleshoot this totally direct coupled power stage measuring voltages & such. Quite often it is easier to just diode check all the transistors & ohm out all the resistors just to save time... particulary on the SWR's.

BTW, any & all bi-polar transistors (barring special purpose transistors like darlingtons) that are connected up in an amplifying mode (barring oscillators & some other weird circuits that only conduct intermittently like a pulse) will ALWAYS have around .6V between the emitter & base. It's basically a forward biased diode junction.

The polarity of that voltage should always follow the emitter & base material ie: NPN transistor...base voltage will be positive compared to the emitter is a good rule of thumb to remember: N-for emitter-neg & P-for base-Positive. BTW NPN = N-emitter P-base N-collector. Sorry if that's a bit basic for you, but I don't know where you are technically.

So without belaboring that any further, your E-B voltage should be around 0.6V (600mv) between the base & emitter. the collector voltage should be reverse biased compared to the emitter. IOW, NPN transistor, the collector should be postive with respect to the emitter...PNP, the collector should be negative with respect to the emitter.

BTW, you need to be very careful in the diff amp ckt as any slipping with the probe can create a catastrohic burn out.

As far as the specific voltages in your circuit, no way to really determine that. My experience with any issues in the power stage of these SWR's (esp if there are any shorted transistors) is to just replace all the transistors...with possible exception of the AC power down muting ones. they ususally don't go bad (in this case, Q-1 & Q-2).

Trying to find the one bad transistor in this sceme is usually an exercise in futility & you still end up replacing all the transistors anyway ;-]

glen

[lowell]

Thanks Glen! That's very helpful regarding the B-E .6v bias voltage. I KINDA knew that but didn't know it's SO common and almost always the case.

[jamoore84]

I just wanted to bump this thread as I'm going through the same situation. I thought it was a blown speaker at first (I picked it up second hand and it was behaving this way), but 80 dollars later, the problem persists.

Any progress?

[lowell]

haven't been able to get back to it yet. will let you know.

[J M Fahey]

Hi Lowell.
To begin with, I fully agree that schematic is cr_p.
It's *not* a functional service schematic but just a PCB program schematic that states what tracks must be traced on the PCB itself, that's why audio "pathways" are cropped and end in connectors.
Many do that same thing, out of laziness and lack of respect for the service technician, what are they thinking of?
One particularly irksome example is Marshall, which does *not* draw tubes on their schematics, because they *are*, after all, socketed. Gosh!!!
Sometimes I print all schematic pieces, glue them, and pencil what I guess are the "right" connections.
There's no signal voltage on U3A-2 because that´s used as a current signal input, invisible to your scope.
If you are still interested in repairing that amp, follow the wiring and "fill in the blanks", otherwise, since it shows no pots, jacks or switches, it's a futile exercise.
Good luck.