JCM2000 Blowing Fuses

[dpm309]

Have a JCM2000 dual super lead that keeps blowing fuses. Pulled all the tubes and powered on (using a light limiter) and it still showed a short. Disconnected the secondaries and the light dimmed after a couple of seconds. I am now suspecting that the rectifier might be bad. This is a SS rectifier and I checked the 4 diodes (D7, 8, 9, and 10) and they are all showing ~.5 VDC in the forward position and OL in the reverse position. I tested these while still connected to the circuit. I have not pulled the boards to see if there are any issues. I do not see any indication of burning/overheating. What else should I be looking at?Thanks, Dan

[dpm309]

Here is the correct schematic.

[AlNewman]

You only unhooked the HT leads?  Look for shorts across your filter caps, also across the output transformer primary.

[shooter]

which fuses, there's like 6


break the circuit at the X's do they still blow??

[stratomaster]

Oh boy ...

This is likely the end stage of the conductive board syndrome.

There are a few 3rd party aftermarket boards you can install, but they may be cost prohibitive.


Barring that:
Sometimes the small 22pf cap from plate to cathode on the power tubes shorts.  Check for continuity between pins 8 and 3 on the socket. If you get anything, clip that cap out and retry.

[dpm309]

It was the HT fuse that blew. It was the HT leads that I disconnected when testing with the light limiter. Disconnected the 2 red leads also and am not getting the light limiter to light up at all. Reconnected the red leads and still not getting the limiter to light up at all.  I am getting 370 VAC from the HT connections, 7.2 VAC from the red leads (heater?), and 22VAC from the black and white leads (bias?). Cannot find a Marshall transformer diagram that matches the one I am working on. It looks like the PT was replaced at some time in the past.

[dpm309]

 Stratomaster, I am not finding the 22pf cap for the power tubes. Not shown in the schematic. I have replaced a new board on a DSL100 a couple of years ago that had a bunch of burnt traces near the power tubes. Antique Electronics sells them for about $200 and it brought the amp back to like. Took about one hour to replace. Don't know if I want to go that route yet.

[dpm309]

Here is a picture of the PT.

[stratomaster]

Stratomaster, I am not finding the 22pf cap for the power tubes. Not shown in the schematic. I have replaced a new board on a DSL100 a couple of years ago that had a bunch of burnt traces near the power tubes. Antique Electronics sells them for about $200 and it brought the amp back to like. Took about one hour to replace. Don't know if I want to go that route yet.

Did you check for continuity between 3 and 8? If there's nothing, then don't worry about the cap.

Regarding the board, try to find one now.  They have been discontinued and sold out for a few years now.

[AlNewman]

Seems like an awful lot of rust in that amp.  If it was me, I'd spend some time cleaning everything before even plugging it in.