PMX2000 opinions / Help

[Techknowman]

I started a new post for this.
The PMX2000 that I needed the linear pot for has been one major pain to repair!
I have lost my ars on this unit with the time I have spent on it.
The parts its needed have been cheap but the time, WOW!
It came into my shop with the VU level meters pegged out and distorted sound.
Also a broken EQ linear pot.
Ordered the pot. (3 month ordeal)
While waiting for the pot, started looking at the other issues.
Found D3,4,5,6 in shorted & open states dumping +V on the audio lines. (PA input jacks)
This was pegging out the VU meters. Replaced them.
Then had 3.5-4V DC at the EQ switch's SW2B and 10B. switching the EQ on and off made very load pop in speakers. After lots of tracking down, found that the DC was coming from the unity gain 339's IC28A,28B, replaced.
No more DC but the VU meters would latch up the first 2 LED's.
Replaced all 339's in the VU section. No change! I spent hours trying to figure this out.
IC28A,and B at a given point would latch up and place -.058 to -.2 V out continuously.
I ended up changing R328 and R329 from 47K to 2.2K and they now work fine.
Had to replace the EQ switch's SW2 and 10 they were intermittent and very scratchy.
So I think I'm done! I do final check out and discover channel 8 and 9 are very noisy!
Bad hum and popping. Ch9 is the worst.
I still haven't nailed it down yet. I know its coming from the high gain amp IC40B,
pulling R549 and R550 makes the channel quiet.
So I scope it and see a lot of popping on collector of T6. Aha! order parts, replace T6, and T5. Nope!
Replaced C161 and C162. Nope!
So I'm left with changing IC40, 4580. I don't hold much hope that this will fix the issue.
I am pulling my hair out, whats left of it! its only channels 8 and 9. and the circuits are identical for 1-7 at the balanced inputs and they only produce a hiss when turned up not rumble, popping and hum. So I'm up for suggestions.

[Techknowman]

I started back on this one today.
Replaced IC 40 no help. Channel 9 is terible with popping and hum when gain is turned up. even worse when phantom power is applied.
I'm really at wits end here.
beh_pmh2000_CH8.pdf

[bkahuna]

I would be looking at the diodes attached to the transistors, and by "looking at", I mean replace...
They'll likely measure fine, but if they aren't absolutely perfect, they'll cause problems.

When working on a noisy mic pre, I'll typically replace the transistors, IC, and those diodes (if present) all in one go. The parts are cheap, and I don't have time to continually reassemble the unit enough to test it after every component swap. The components usually test ok with a meter, but they are in such a critical circuit that it will show up any flaws whatsoever.

[bkahuna]

Does it matter whether Phantom power is on or not?

[Techknowman]

I replaced the 1N4148 diodes,D23,D24, no change. Yes its much worse when phantom power is on but still very noticable.
I have replaced IC40, T5 and T6
Replaced C161 and C162.
Verifyed ground at X23 pin1 to R606, 607 and to rest of system.

[km6xz]

What mechanism of fault did you determine caused all these problems? It is extremely rare to have so many devices in unrelated circuits fail. Something caused it and if a diagnosis is not deduced that accounts for it all, you will never know if it is actually fixed or not. That diagnosis would also lead you to other areas that would be expected to be stressed by the mechanism of fault.
What was the reason for the resistor value change, a field service notice?

[Techknowman]

This unit was and still is a mess.
Its obvious that someone jacked in high power to many of the inputs probably hooked a power amp to inputs.
many of the limiter diodes were shorted some open. components down stream from these inputs were damaged.

The only remaining problem after a lot of work, more than I will probably ever be compensated for, is the CH8 and 9 popping and hum.
If and when I get this unit gone I'll chock it up to education. I may just buy it from the owner so i can have the satisfaction of destroying it with a 12Gauge. LOL

[Jazz P Bass]

I would not rule out a lightning strike.
I had one like this & I could tell which inputs where used when it got hit.
The ones that where blown.