Swr workingpro 2x10c distorting

[Bigdrums]

Hi all: I have rather nice bass amp that appears to have a problem on the negative swing part of the amp.
Currently I've been tracing the signal from a sine wave generator that seems OK till the amp section where the waveform gets cut and distorted only on the negative swing, the positive swing looks normal.
I need some help trying to find out why.

WorkingPro 2X10CSCHEMATICS.pdf

[Bigdrums]

All the output transistors check OK and its not drawing excessive current

[Bigdrums]

Scoping the output with no load waveform is normal, mostly, but with a load bottom swing is:

[Enzo]

So unloaded the output is a clean full sine wave? And loaded you lose the negative side? That tells me the output can swing the voltage, but can't swing the current. That tells me the likely culprit is the negative side output device either bad or not really connected. And that includes things like open ballast resistor.

[Bigdrums]

Unable to find any bad connections to/or bad output devices.
With the load connected:
Scoping back to R15 there is only a upward swing, same at R24 and R14
Unloaded it cleans up at those points
Wondering why the loaded signal would be affected that early in the signal path when current is our main concern

[Enzo]

The entire power amp is one big fed back loop. Any problems at the output will cause compensating reactions back at the front end.

Those three resistors are all associated with Q16 and are ALL fed by a sampled signal direct from the output, so whatever is on the output is going to show up there.

What does the base of Q44 look like loaded/unloaded? Is R71 open? Bases Q41,43,etc, what is the story there?

[glebert]

SWR combo amps are known to have problems with cracked legs on output transistors. I know you said that they looked OK, but I would check again, remove the heatsink if you haven't already. On some of their combos the heatsink is mounted too rigidly to the chassis and thermal expansion causes stress on the transistor legs.

[Enzo]

These are power amp modules. Two boards mounted to a central heat sink core with a fan on the end. Boards on two opposite sides, transistors on the remaining two sides. Oh, vaguely like HArtke modules, but bigger and frankly better.

I have a pair of the modules in stock.

[Bigdrums]

R71 is good, 100 ohms
Q44 base doesn't look good unloaded, loaded it is worse, distorted like this:


The bases of Q40, Q41 & Q42 unloaded look fine. Loaded they look like this:


Enzo: What do you want for one of the modules?

[Enzo]

Hadn't thought about it, where are you?

[Bigdrums]

Livermore, CA 94551 near San Francisco

[Enzo]

Just making sure you were not in Brazil or something.

Let me get to my storage unit and dig them out, we can continue discussion in PMs. I am sure we can reach a deal.

[Bigdrums]

What kind of waveform would I expect to see at R14/R21?

[Enzo]

I could be wrong, but I think that is where you would see the error signal - the signal the amp uses to correct differences between input and output. Never lose sight of the fact this amp is one great big loop.

And a suggestion, when referring to a random part out of the blue, give a clue where it is on the drawing. I looked all over to find those.

As in: R14/21, the emitter resistors for Q14/16. Or even just left center of drawing.

[Bigdrums]

I have found the likely problem as it was stated earlier in this post
Q44 had all 3 legs cracked inside the case.
Enzo: I think this will fix the problem so I hope you haven't spent any time digging out the module
I'll keep you all posted once I get the new MJE15035 in.
Thank you