Fading Marshall

[Prairie Dawg]

I've got a JCM2000 DSL401 here-this is a single 12" combo with 4-EL84s, about nine years old, never been out of the house. The guy who owns it says "It's fading out as I play". So before we got a chance to power it up he had to split for a family emergency. So I power it up, flip it off standby, strike a chord and in about five seconds it goes right to zero volume. Tubes are all original and check out well.

Thoughts?

[captntasty]

HOT

As I recall this is a common problem with the 401 (probably the 201 also). I think it has to do with how hot this amp gets in normal operation. I had one and it runs very hot! but never had any problems. What can happen is it gets so hot, marginal solder joints on any number of components (I've seen the bridge rectifier mentioned several times) can lose connection intermittently when the amp heats up. Just a thought to get you started...

[SWEINC]

Bridge rectifier

The bridge rectifier that supplies DC to the filaments of the first two preamp tubes is bad or has cracked solder joints. This is a common failure in this model. The clue is that your sound is fading in and out as the tube heats and cools. If it was a normal loose connection the signal would turn on and off more abruptly.
Replace it with a rectifier that is larger and can handle more current so it will run cooler. Marshall offers an upgraded rectifier as a sub for the original.The part # is RECT00003.

[Prairie Dawg]

You folks have given me a direction to go in. I expect that I can stick my multimeter on the filament string down at the end and see what happens. The sound disappeared in about five seconds....Course, I have a passel of RCA cleartop 12AU7s that'll offer a visual indicator of filament voltage. Something to look at tomorrow am.

[cjlectronics]

I just repaired an exact model. The problem was the reverb was full on and you could not adjust the intensity. The reverb footswitch worked and you could switch it on and off but the reverb was always on full blast.

Turned out to be a bad reverb pot. Fortunately I found an exact drop in Alpha pot and that fixed the problem.

I thought I'd add my experience even though a pot is probably not your problem?

CJ

[Prairie Dawg]

You folks have given me a direction to go in. I expect that I can stick my multimeter on the filament string down at the end and see what happens. The sound disappeared in about five seconds....Course, I have a passel of RCA cleartop 12AU7s that'll offer a visual indicator of filament voltage. Something to look at tomorrow am.

I powered this amp up on the bench and the power to the three preamp tubes that use a DC filament supply dropped off after about 15 seconds. Gotta go shop for a rectifier.

[Prairie Dawg]

The bridge rectifier that supplies DC to the filaments of the first two preamp tubes is bad or has cracked solder joints. This is a common failure in this model. The clue is that your sound is fading in and out as the tube heats and cools. If it was a normal loose connection the signal would turn on and off more abruptly.
Replace it with a rectifier that is larger and can handle more current so it will run cooler. Marshall offers an upgraded rectifier as a sub for the original.The part # is RECT00003.

You were spot on. That's exactly what the problem was. I got a suitable rectifier from Rat Shack. It is the same envelope and pinout, rated for 8A and 400v which is plenty for 3 12AX7s. (part number 276-1181 a/k/a KBPC804m cost about $2.50). I also installed a heat sink while I was in there.

Seems to be working well now, thanks for all the assistance everyone.