JCM900 chips?

[drewl]

Before I go looking, maybe y'all can save me a little time.
I have a Marshall JCM900 with a bad opamp. It's one of the 5201's in the master volume circuit.
Does it cross to anything common and easy to find?
Thanks...I'll start looking now.

[Twist]

I think www.tubesandmore.com sells those.
Also you can find a datasheet under M5201.

[drewl]

Well luckily the 5201 chip was okay. I swapped it with the one that amplifies the reverb return but I still had the same problem.
The voltages in that preamp stage were all wrong, as a hunch I tried V2b's plate coupling cap which was bad and leaking dc voltage into the following 5201 amp/switching stage causing it to malfunction, so take note if you come across the same problem.

[zw44]

Which cap is the v2b coupling cap? Is it C15 on the marshall schematic found here: http://www.drtube.com/schematics/mar...d0189-iss7.gif ?

What was your original problem with the amp? I am trying to troubleshoot some crosstalk between channels, at the moment I believe it to be a faulty m5201 chip in IC8 but this could be a back up plan (and an easier component to source!)

And yes I know this thread is pretty old...

[bluesfreak]

Yep, should be C15 if I got drewl's posting right...
BTW: if you have the amp already open check BR3 for overheating and C8 for leakage (these part do refer to the premap tube DC heater supply) as they are designed on the limit as the cap is too large for this kind of BR). If not already done change the screen grid resitors to 1.5k and solder them directly to the socket, put a jumper where they where before on the PCB. Change the Gridstopper Rs to 5.6k and solder them also directly to the socket, jumper location on the PCB. This opens the amp and gives more reliability....

just my 2c
bluesfreak