JCM 2000 DSL 50 weird cut in & out & dead issue

[j.walker]

Hi, I've seem some people here have impressive knowledge of amps and could help me with my issue with my Marshall JCM 2000 DSL 50 (2005 make) amp.

The issue: When I turn the amp on, let it warm up for a minute, turn standby off it produces no sound, however after a while (~10-30s) it cuts in (no fade in) playing 100% as usual, but only for ~5-60s until it again cuts off (no fade out) to total silence from which it doesn't recover. This is reproducible.

This might be an aftereffect of a gig I played a while back (a month or so) when I had a faulty power cord which sometimes broke contact, delivering power unreliably, the power switch light flickered on and off (right when I noticed this I changed the cable). However the issue exhibited only just now.

What I tried:
Switching up cables. Different cab. Different channels & turning all the knobs.
Looked on the valves when turned on (except the first pre-amp one which is in cover), they warm up normally and not in excess (no sign of overheating). Checked if the valves are seated well.
Bumping into the amp, if it isn't issue of bad contacts with old solders.

I guess I'll have to bring the amp to a technician, however I would appreciate if anyone has any insight where most likely is the fault (valves, rectifier, transformer?) given the signs, and if I could do the repairs myself.

[Enzo]

Not likely damage from power going on and off.

The whole job of an amp is to make sound, so most everything in it is involved. That means the problem could be many things.

The on/off nature of your complaint points ( in my view) to a contact or connection. Things like impedance switches, FX loop return jacks. Panel switches. Internal connections like the little shielded cables that connect the boards together. and so on.

[Leo_Gnardo]

What Enzo said ^^.

Plus this series of amps has an extra sneaky way of having the signal "get lost" on its way to the 4/8 ohm output jacks. There's a switch built into the 16 ohm output jack, on the ground side of the jack, that is supposed to disconnect the 4/8 ohm jacks when a cable is plugged into the 16 jack. Sometimes this switch contact stays open when nothing's plugged into the 16 ohm jack, and you get no signal to the 4/8 jacks. I just wire a jumper across it then no further problems. If this is where the problem is, you can choose to replace the jack or wire across the switch.

[g1]

Also try with an extra patch cord between FX send and FX return jacks.

[Mick Bailey]

You say it goes to total silence - just let me double check with you that there's no hiss or background hum whatsoever, even with the volume turned right up.

When the amp does this, I suggest you plug your guitar straight into the FX return and check whether you get any sound. Operate the level switch to see if that makes any difference. If you have sound you know the problem is prior to the FX loop. If no sound you know that the power amp or power supply are suspect. You won't get much volume if this works, but it's just a diagnostic check.

[drewl]

Just did a TSL that had an intermittent bridge rectifier for the pre amp tube DC filament supply.

You'll hear slight amount of hiss from power amp but no signal being passed from the preamp.

[j.walker]

Thanks all for the responses! I cleaned the sockets with a contact spray and after second application that completely fixed my issue (at first it was unreliable, volume and I think even frequencies inconsistent, cutting in and out).

I think it really is the 16 ohm output jack causing it. As this issue is bound do reappear sometime in the future, I'd rather fix it properly, than for it to surprise me on a gig. I'm almost completely untouched by electronics theory, but I soldered bunch of headphones etc. Is a jumper (just a connecting cable right?) wired like this going to solve this? Wouldn't this make it so that the 4/8 ohm sockets are still powered when the 16 ohm one is plugged? Would that be a bad thing (assuming the 4/8 ohm ones are unplugged)?


Alternatively is there a better solution to this?

[Leo_Gnardo]

I think it really is the 16 ohm output jack causing it. As this issue is bound do reappear sometime in the future, I'd rather fix it properly, than for it to surprise me on a gig. I'm almost completely untouched by electronics theory, but I soldered bunch of headphones etc. Is a jumper (just a connecting cable right?) wired like this going to solve this? Wouldn't this make it so that the 4/8 ohm sockets are still powered when the 16 ohm one is plugged? Would that be a bad thing (assuming the 4/8 ohm ones are unplugged)?

I remove the output/patch circuit board, access the solder side, and solder a short length of wire over the gap near the 16 jack in order to defeat the exclusive output feature. That tiny switch built into the jack has to handle all the current going to the 4/8 outputs, it's not rated for it, it burns and there goes your connection, phooey who needs that? 5 minutes and a soldering iron, she's-a-feex never gonna break again! Well at least not that way.