Marshall DSL 40C - clean channel dropping volume once amp is heated

[Tone Meister]

The CLASSIC GAIN clean channel drops volume significantly once the amp is heated. The ULTRA GAIN channel works fine. Preamp tubes are known good, all jacks have been cleaned and exercised. The combo uses the 16-ohm jack to a single 16-ohm speaker. Volume drop is the same through an external speaker. I've re-soldered all the tube sockets but haven't dug any further.

No schematic at the moment, but I do have a request in over at the schematics sub forum

[Tone Meister]

Still haven't located the schematic, but I'm hoping to pinpoint the cause of the clean channel dropout without it. Any ideas from anyone who has experience with this amp?

[Jazz P Bass]

As far as I know, that schematic has not been released to the public.

There is a DSL 40C forum (I do not have the link at hand. Google it.)

I would suggest that you measure the plate & cathode voltages of the preamp tubes.
Something may standout as odd.

Short of that, send a signal through, document the gain at each preamp tube & then remeasure when it fails.

[Mark Black]

If you can access the component side of the PC boards while operating in failure mode this might be a good candidate for testing with some freeze spray while monitoring the output for changes. One component at a time or blocks of components to narrow it down. Just stay away from the hot tubes!

[Tone Meister]

Thanks JPB and Mark, good points. The problem is, I can't get it to fail with the amp chassis outside of the cabinet. It'll work perfectly for hours on the bench, but once I reassemble the amp, the clean channel volume will drop dramatically after about 20 minutes.

I'll find a way to trap the heat somehow while it's on the bench and then use some freeze spray to try and isolate it.

[52 Bill]

It'll work perfectly for hours on the bench, but once I reassemble the amp, the clean channel volume will drop dramatically after about 20 minutes.

I use a cardboard box to cover the amp when I need to heat test them.

For your problem, it may be easier to use a heat gun/hair dryer to try and make the problem happen.

[Tone Meister]

C16 (.001/630) is intermittent during the heat gun/freeze spray treatment. It appears to be a coupling cap, so I'm gonna pull the preamp board and swap in a Mallory 150 and will report back in the morning.

Thanks fellas.

[Tone Meister]

Well, C16 wasn't the issue. Clean channel volume still drops out when heat is applied to the area around the CHANNEL SELECT switch. I've re-flowed every solder joint in the vicinity to no avail.

To further complicate matters, now I've blown a couple 500 mA HT fuses (FS1) with no tubes installed. Clearly something happened when I pulled the PCB, but I can't spot it. Checked for shorted caps and obvious mistakes, but can't pinpoint the cause of the short.

[Tone Meister]

Sorted out the FS1 short, but still have the clean channel dropout. I want to suspect the CHANNEL SELECT switch, but the way the PCB is built and without a schematic it's tough to trace. Here is a look at the area where the heat gun causes the dropout to occur.

[g1]

Maybe just try resoldering the switch and the pots on either side of it.

[Tone Meister]

Maybe just try resoldering the switch and the pots on either side of it.

I did that already, but I'll make doubly sure I didn't miss anything in the entire area there. The switch works fine to switch channels manually, and it isn't dirty and doesn't seem loose internally. It's got me scratching my head, that's for sure.

[Mark Black]

C16 looks like it doesn't have a good solder fillet on the component side based on the photo - might be a break in the 'thru-hole' connection plating. Also might be worth solder-filling the other two thru-hole via connections nearby, preferably with a bit of component lead or buss wire incorporated.

[Mick Bailey]

I had one of these in last week and almost every joint had no solder fillet on the component side - no through-flow whatsoever.

That one dropped volume, too and sometimes wouldn't kick in off standby. Turned out the terminals on the back of the mains socket were very loose.

[Tone Meister]

Made sure solder fillets were solid all throughout the amp, and now everything points to a bad CHANNEL SELECT switch. Now when heat is applied to the problem area the switch will not switch channels, which coincides with the CLASSIC GAIN channel dropout. That wasn't happening before.

Once the area is cooled either naturally or using freeze spray, the switch functions normally and the CLASSIC GAIN clean channel volume is restored.

[okcrum]

This sounds like a hairline trace crack or a plated-through hole that isn't. An ohmmeter along the traces will tell you whether it's the board or the switch. It doesn't take much flex from heating to open a hairline up.