Another Marshall JCM2000 issue

[Randall]

I have a JCM2000 TSL100 watt amp in that doesn't make a peep. Owner just got it, says he thinks it has new tubes and a new "motherboard". I'm not so sure, the tubes have the right side pair discolored, which leads me to believe it is from the bias drift problem so common in these amps. What becomes confusing is, someone did number the connectors, evidence of perhaps it was replaced. But it is embossed as JCM2-60-00, which is the board you don't want, and also it has a sticker on it reading TL10-60-02-01-8, which looks like the new and improved board with "01-8" attached to the end. It also has the proper 5K6 grid resistors.

I want to know if this board needs the steady bias mod done, but so far I haven't figured out why the tubes are not conducting. Here's what I know thusfar:

My plates, screens and grids voltages look good, with signal on the grids, but not the plates. And I have 28vdc on pins 1 and 8.

AAAND, I just answered my question, finding both 1 ohm cathode resistors to be open, suggesting back to the original flawed board.

Does anyone have any advice for if I should bias mod this? I have the hollow diamond drill bit to cut around pin 5. I have to pull the board anyway. I do hate all these connectors, however.

https://www.thetubestore.com/lib/the...-Schematic.pdf

[g1]

I think you have a new board in there. Even with a good board, bad power tubes can blow those resistors.

[g1]

Here's a pic of the ones being sold now. I think what you thought was an '8' at the end of the number is actually an 'S'. But more important is the issue/revision number which can be seen is a '20'.

[Randall]

Yes, this one is ISS20. Does that mean I might get away with new tubes and not drill that board out? Replace the cathode resistors, of course. I would just love it if I could do that from topside and not pull that board.

[Randall]

Well, I tacked in new 1 ohm cathode resistors, and now it is running. It was biased very hot. I dropped it down to 62mA per side, and let it cook for a while. It seemed to stay pretty stable, but after about 2.5 hrs I tapped on the discolored pair, and got a noise and a spike in current, so since I don't know the history of this amp, that quad is gettting replaced.

[The Dude]

Yep. If the cathode resistors were taken out, there's a good chance tube(s) are bad.

[g1]

Also, you don't know if the tubes got swapped around or not, so it's possible one of the cathode resistors blew first, the tubes got moved around, and the other cathode resistor got blown. Agree that changing the quad is the best course of action.