Fender SPL 9000 noise

[Dave Curtis, dB AudioTech]

As soon as power is applied, channel B makes some bad crackle-popping noises and continues for around 10 seconds after power-off as the voltage bleeds down. I've replaced a bunch of bad components: OP transistors, various resistors, etc. It seems the "muting" circuit on this channel isn't muting at power-up.

Also, if anyone has a (readable) schematic, I'd appreciate it.

TIA,
DC

[Dave Curtis, dB AudioTech]

Here's the schematic.

[Dave Curtis, dB AudioTech]

More info

Here's a better, more unit-accurate schematic, thanks to another MEF member...

OK, I'm not so hot on SS amps, although I have done a pile of PV CSxxxx amps in the past, as well as a number of other SS amps, I'd rather do tube amps.

When I first got this SPL9000, the MJ15023s were shorted on the B channel and I subbed those with 3 15023s and 15016 that I had kicking around (yeah, I know...). I also replaced the two 22 ohm R122 & 123, but all I had looked to be 1/2w. I ran it for a few hours like that, although at startup it made a nasty static/popping noise immediately, as though the delay-on circuit for the speaker output wasn't working. Also at power off the noise would come back and fade out. When the new 15025s came in, I installed them (as well as the Q40 & 52 drivers, although they tested OK). Same noise on power up, but with a bit of smoke. I replaced R156-160, Got the noise at power up, and (unfortunately) let it run long enough to cook the 4 MJ15025s, both drivers and R160. I know, I should have kept it plugged into the limiter...

Would bad insulating micas do this? One of the driver's mica is cracked...

I've swapped various components with the (working) output section of channel A; the triac, bidirectional switch & C40, to name a few. Any help would be appreciated.

Thoughts?

[Jazz P Bass]

Yeah the cracked mica may have been a problem. (grounded collector)
Solid state amps can be a challenge
You really do need to be prepared equipment wise just to test them.
At a minimum I would recommend:
-Mains AC current meter.
-Variac.
-Two DVMs.

[52 Bill]

The output triac circuitry is only there to protect the speakers when there is dc on the output. The amp will run without all of that stuff, so unless the triac is shorted, don't spend much time there.

Did you power up with the speakers when it cooked the second time? Do you read any continuity on the driver case to heatsink with the cracked mica?

I don't see any speaker turn on delay in the schematic. There is a timed turn on circuit to the input section that is built around Q1-Q3. If this is right, the noise can be coming from anywhere in the circuit.

Unfortunately you're back to square one. Does Channel A still work fine?

[Dave Curtis, dB AudioTech]

Yes, I had a speaker on the output when it went tits up. IIRC, there was no continuity from case to Hsink on the drivers before (either with the old or the new ones).
Yes, channel A is good. I was thinking the MBS4993 (bidirectional switch) had something to do with the timing ckt, but I see now it runs the gate of the triac.

AFA equipment goes, I have a large variac with a bulb current limiter, 3 or 4 ammeters & scopes, 6 or 8 DVOMs including Flukes (8020A & 111), Keithley 175, a few Ideals, etc.

This one's a challenge, that's for sure.

[Jazz P Bass]

Yes, I had a speaker on the output when it went tits up.
AFA equipment goes, I have a large variac with a bulb current limiter, 3 or 4 ammeters & scopes, 6 or 8 DVOMs including Flukes (8020A & 111), Keithley 175, a few Ideals, etc..

No No #1: No Load

That is an impressive list if equipment.
The variac would have helped, as well as using an ammeter to measure the mains current. (more than .7 amps means trouble)
I like to take a reading at least one main rail voltage & the bias voltage as I am ramping up a repaired SS amp.
I do agree that they can be a bear.
It seems there is always something waiting to bight you in the a##.
Repairing a SS amp is certainly not a thing that you want to rush.

[Dave Curtis, dB AudioTech]

This thing has a 15A fuse in it, and with the +&- disconnected from the bridge (which checks OK) it draws nearly an amp from the line (.98A @120VAC). That said, I have it back together and with 12VAC it's drawing about .8A (no load this time). I'm going to try taking the DC supply off the B channel and see what it draws with just Ch A working...

[Dave Curtis, dB AudioTech]

The damn triac was shorted. I had checked it before (usually the first thing I check) and even swapped it with the working Ch A... I'd even run it for a few hours before the new parts came in. Oh well. Ramped it up to 120VAC and it's drawing 1.14A I'll hook a speaker up and see if it still has the noise on power-up; if so, it's time to break out the old signal tracer.

Thanks, guys!

[Dave Curtis, dB AudioTech]

So much for that idea... It lasted about 10 seconds (still no speaker); shorted the new triac and more of the same, but new outputs. Not happy, and I'll probably just pack it in and take the (big) loss.