Next issue: amp still works fine, can play guitar through but when I power on the amp makes this nasty loud honking squealing sound for a good second, almost two then goes quiet and runs fine. When I turn off the amp it makes a very loud clicking sound.

Yahoo! Amp works again! After replacing resistor and diode I found proper voltage on op amps, voltage regs 1 and 2, and the analog switch.

Thanks for everyone's help so far.... :tu:

Looks like R108, the 2.2 K 1W resistor is charred, showing .34 VAC to ground on the circuit side, whereas the other one, R109, shows 11.34 VAC to ground. I measured R108 in circuit and it showed a read of 44K...will have to wait until tuesday to pick up resistor...hopefully this explains why I'm getting such a low voltate output at Reg 1 and therefore all the ICS.

Checked all the IC's and yes -15V on pin 4 and 1.3 v on pin 8, the analog switch has -15v on pin 4 and 1.3 on pin 13.

Update: found 1.3 V output on Reg. 1, -15.36 V Output on Reg. 2....D11 had been burned out so I replaced it and it is reading normal. The amp was working but maybe when I pulled out the circuit board (I was careful) I loosened a solder joint? Thanks.

Newb question: I measured 1.3v DC on pin 8 (V+),  and -15.36 DC on pin 4 of NJM4558. I checked others in the circuit  as well, same readings. Now, shouldn't V- and V+ have the same voltages? I thought they would both be -15.36. An equivalent circuit usually shows the same voltage on the rails, neg. and pos. Also this is a dual op amp, so....hmm, what am I missing? Thanks.

Might as well ask. How does one remove the connectors from the board without causing damage? The top section of each with all the wires: Con 1, Con 2, Con 3, Con 4. thanks! (see attachment)

Thanks Gents! Easy is right, as it turns out. Used a pair of needle nose pliers, squeezed arrowhead and pushed up simultaneously and it slid through. It was easy, but glad I asked, better than cracking a pcb! :cheesy:

Thanks Dr. Gonz I'll give it a try!

Here are the pics inside and out of chassis. What is the best way to remove? Thanks!

Advice needed: I'm about to remove the circuit board from the amp but it has plastic poles/standoffs. I've tried a small amount of prying but the board starts to bend. I don't want to crack the circuit board so can anyone suggest a good way to remove the board from these plastic standoffs?  Also, would you unplug all the connectors in advance?  I have to replace a diode, need to discharge caps, and I want to trace out the circuit board for learning purposes. Thanks!

Thanks JM and Enzo :tu:!

Thanks for these posts. I just figured I was missing something with regards to schematics. Good to know I'm not alone and I'm not completely out to lunch! Is it worth contacting Marshall/SWR to ask them why they don't supply proper schematics? Or is it a case of not wanting techs to fix their amps? Would they rather the customer chuck the amp in the garbage and buy another one? Seems that way to me...any thoughts?

Thanks JM! Off to work now but will study up on your info later today. Thanks for taking the time. (This is a fantastic forum!) :tu:

D11 is charred and reading O ohms both ways. Any comments on how that would effect the circuit/amp? Hoping to replace it tonight or Monday

Thanks for the last comment regarding voltages fed from. But, silly question I'm sure, where exactly is the -15/+15 volts picked off from? On page 2 of schem. where it shows V+, V-, I measure +34 volts to ground, and -34 volts to ground, both from R10 and R11. Thanks.