Marshall JCM2000 DSL 401 Volume Problem

[stlcardinals527]

Hello everyone, first post here.

I've frequented this forum a little over the years, always coming looking for information. Now I have a few questions that I cannot find answered anywhere else.

I've owned a Marshall JCM 2000 DSL 401 combo for some time now, maybe 5-6 years. It has been a great companion, with relatively little problems (just a couple tube swaps over the years, and one blown resistor 3 years ago). All of my work, minus pre-amp tube switching, has been done by a certified tech who lives about an hour away. About a year ago, it developed a gradually worsening volume drop. Eventually, the volume has stopped entirely, on all channels. Pre-amp and power tubes are all lit, volume and standby works, and I can hear a slight hum with everything dimed. I've tried different cords, guitars, tubes, etc. Basically all of the standard "what did I forget?" steps that you do when something like this happens. Seemingly no blown fuses. No recent trauma or damage. I don't really know where to turn. I've since let it sit, since I haven't had the time to drive over an hour out of my way to drop it off at the tech.

Any (step-by-step) instructions or advice?

I've built about a dozen guitar pedals from kits and such, only recently learning about more than basic electronic information and theory. Eventually I would love to build a certain amp or two, so anything step-by-step would be great. I'm not afraid to re- or un-solder anything, I've done it before (just don't know how to properly discharge the caps!), and I'm open to trying anything at this point.

Thanks for your time and help.

[The Dude]

Step one troubleshooting any amp with an effects loop exhibiting this symptom: Try running a cable from effects send to return and see if that helps.

[stlcardinals527]

Nope, nothing at all.

[The Dude]

Try either or both:

1) Plug a signal into the effects return. Do you get anything?
2) Run the effects send into another amp. Do you get anything?

The idea here is to determine whether we have a preamp or power amp problem.

[nevetslab]

Where I normally find the problems on the JCM2000's when they stop passing signal is on the rear panel PCB. Small radial solder fractures around the Impedance Switch, the output jacks, as well as the Insert Send/Return jacks are good places to look.

You'll have to carefully remove the chassis from your combo case, as the chassis is upside down. I seem to recall I have the combo amp upside down on my bench when I'm extracting the chassis. Once the chassis is out, you'll probably want to remove the power tubes. I forget if the DSL401 is the 50W or 100W head. If 100W, you'll want to keep your tubes paired up so they go back in to the same sockets, keeping the bias levels the same.

With the tubes out, now you can set the chassis down on the two transformers. I normally discharge power supply voltages with a probe-connected 100 power resistor, though you won't have one. If the amp has been off for, say half an hour or so, the residual charge on the Output Transformer's Primary leads will be pretty low, if not already near 0VDC.

You'll want to remove the rear panel PCB from the back panel of the chassis. There are a group of wires coming from the Output XFMR that are tie-wrapped to metal hoops in the chassis holding them in place. Cut those cable ties. Speaker connector nuts are 15mm hex, Insert Send/Return & footswitch nuts are 14mm hex. Once those are removed, you can back the PCB out off the inside of the rear panel, and flip it over to inspect the solder joints. Very bright light and a magnifying glass is recommended here. You're looking for fractures something liike this:



Granted these two photos are NOT of the rear panel PCB of a Marshall JCM2000, but these are typically what solder fractures look like. If you find solder joints looking like this...having radial fractures around the terminal of your impedance switch, output jacks, etc, the cure is De-soldering each terminal, then re-soldering it with fresh solder...60-40 tin/lead solder preferred. Finished solder joints should look shinny and clean, without any voids. I normally use 0.10" wide desoldering braid in the de-solder step.

if you have a lot of fractures on your rear panel board, you also may have fractures on your AC Mains PCB....I find these on both rear panel boards of JCM2000 amps on a regular basis. Once cured here, you may be ok. Further within your amp, there may well be similar fractures on the main PCB as well as the small bias board. That's better left to your tech, as the disassembly to get down to that level is tedious. Rear panel access is pretty easy.

Best of luck

[The Dude]

The DSL401 is a 40W EL84 combo amp and is a completely different animal than the DSL/TSL 50/100 heads. Not the same PC board layout at all. That said, there's still a good possibility of solder problems, particularly on the jacks.

[stlcardinals527]

Thanks for your help, everyone. I really appreciate it.

So far, nothing. Plugged guitar into the effect send, and also sent the return through another amp. Just to make sure that I'm not crazy, on Wednesday I will try a second set of cables and a different guitar (even though I've done this 100 times) just to make sure.

[The Dude]

Thanks for your help, everyone. I really appreciate it.

So far, nothing. Plugged guitar into the effect send, and also sent the return through another amp. Just to make sure that I'm not crazy, on Wednesday I will try a second set of cables and a different guitar (even though I've done this 100 times) just to make sure.

That is backwards. As I said in post 4 signal goes into the return (not send) and out of the send (not return).

[stlcardinals527]

That is backwards. As I said in post 4 signal goes into the return (not send) and out of the send (not return).

Yeah, I did it exactly as you had mentioned, just typed it backwards. Just doubled-checked now since I had kept everything plugged in. Sorry about that.

[The Dude]

Ok, good.

I doubt it's bad since you said you had some hum, but have you checked the HT fuse?

[stlcardinals527]

Neither the HT nor the Mains is blown.

Before I forget, I did notice upon further inspection (trying the send/return cabling again) that when I change channels on the DSL401, there was a slight pop coming from the second amp. This also happened when the cable jacks in the send/return section touched each other.

[Mick Bailey]

Whenever I see these amps they always have more than their fare share of cracked solder joints. You need to flip the board and closely inspect the joints. The HT fuseholder is often completely loose, contributing to volume drop and eventually cutting out completely. All of the pots, sockets and also anywhere where there's a hot, stressed or heavy component needs to be checked an resoldered. You'll probably find plenty that look like neveslab's examples.

The chassis front separates on these amps so dismantling/reassembling isn't too bad and the board can be worked on with the leads still attached. Just make sure you discharge the HT first.

[g1]

If I understand correctly, connecting pre-out to another amp, no good. Going straight in to FX return, no good. If so you must have a problem common to both sections, so start checking power supply voltages.