Soundtech C600 Assistance

[tdlunsfo]

This is amp is giving me fits and I am sure it just my inexperience. Hoping you guys might can find what I am doing wrong here. First, the amp came in with shorted outputs, drivers and some burned up resistors. All bad parts, fuses replaced (Q405, Q407, Q404, Q420, Q419, Q416, R412, R413, R452, R453) and got it to make sound again. It's a design I am not used to seeing. There are two amps. The first amp supplies two full range speakers (this amp is fine). The second amp (that I am repairing) is for a single 18" woofer. The two channels appeared to be summed, sent through low pass filter and then send to the woofer power amp through WCH. As you can see the inputs are XLR. My current test rig is audio generator to DI box, XLR from DI box to Channel A. Sound is distorted. DC offset between left and right channel is around 25mv. Ground lift is lifted. A couple of things I don't understand..

1.When I hook the scope ground to the negative of the speaker (or the left channel in this case) the signal cleans up. Disconnect the scope ground, then distortion?
2.When I ground the base of Q402 the signal cleans up and gets much stronger. NFB issue?

Any thoughts?

C600 Woofer amp.pdf
Pic of scope on left channel and scope ground to chassis. I can here the distortion and sounds the same as when no scope is connected.

Pic of scope across speaker and scope ground connected to left channel negative. Cleans up?

Pic of current setup.

Pic of subwoofer power amp board.

[tdlunsfo]

Looking at this a little further, the distorted hump in the first trace is actually the signal from the positive side when combined together through the speaker. When scoping both sides to chassis the negative side definitely has issues on the top half of the sine wave. Wounded chip syndrome?

[J M Fahey]

Too sleepy now for a full analysis, but I strongly suspect the woofer channel has two *bridged* amplifiers, which is quite usual.
You can NOT hook the scope ground to the negative of the speaker because that point is NOT ground by any means.

a) stop mentioning left and right channels since they seem to work right, concentrate on the faulty woofer one.

b) it is actually made out ow two separate amplifiers, one connected like a conventional one and feeding "Speaker +" terminal, the other one fed same signal but out of phase, and driving the "Speaker -" terminal, which as you see will have full rail to rail signal and is NOT Ground.

c) your load resistor must be floating, connected just to Woofer + and - out terminals, not to ground.

d) your scope ground must NOT be connected to any end of the floating load, period, but to power supply ground.

Some will suggest you need an expensive differential input scope to check floating output ... not *really* ..... just treat it as two amplifiers (which it is ), check/measure + terminal signal to ground, then same with - terminal to ground; when trying to find maximum power output simply add both halves: 20 V RMS per output means 40V RMS across speaker, as simple as that.

EDIT: just saw your schematic
I see they call them right and left channel out of lazyness (they probably use exact same board as the real Right and Left ones), ok, we´ll do the same instead of Top or Bottom amp , but I suggest we add " , as in "Right" or "Left" to make it clearer we´re talking Woofer amp halves, not the actual Left/Right satellite amps.

It´s a relatively simple and conventional amp, very repairable.

[tdlunsfo]

Juan thank you for the reply. I am not a fan of how they labeled this either. From now on most of the convo will be in the “left” woofer amp. Agreed the amp halves are a basic design, it’s just the first time I have seen them combined as such. I still consider myself a noob, so likely me just not having encountered them yet. One thing I did notice last night when comparing the two halves is I have a lot of gain out of the differential amp on the “left” side compared to the “right” side (Q401 and 402). I do see the inverting op-amp but its gain is only 1.2 or so. I retried measurements with the suggested ground scheme. The top half of the “left” output is clipping. I traced back to the drivers and they are still clipping as well. I have already tried swapping and replacing the drivers.

[tdlunsfo]

I had some time this evening and wanted to experiment. Tried swapping the diff amp transistors from "right" side to "left" side and replaced the other driver Q403. No change. I think I will try separating some of the circuits to see if it will help. Will start with opamp output, then move on to diff amp output then drivers. Any suggestions?