Marshall DSL40C - Weird Oscillating Volume drops once warmed up

[Yamariv]

Hey Gents,

A while back, I tripped on my guitar cord which was left plugged into my DSL40C (D'oh!). Everything seemed to still be ok even after wiggling the input cord around with the guitar plugged in. The input jack looks to be a little crooked now be barely noticeable.

Fast forward a few weeks, and I noticed a weird oscillating volume drop after playing the amp for 10 mins or so. Just like someone is slowing turning the volume up and down as you play. As this amp was rock solid before, I assume something might be slightly damaged on the input circuit board from my tripping on the cord. Anyone had this problem before? Could it just be a tube going out? If I jiggle the input while I play, there's no cuting out at all, just this random oscillating volume on its own once warmed up..

Anyone have any recommendations on where to get a replacement input jack circuit board?

Cheers

 

[mickeydg5]

Start with double checking the input jack, open it up to see and tighten things up if needed.
If that does not help with the oscillation, then next step may be to clean jacks. tube sockets/tube pins with lubricating electronics contact cleaner.
Then if it persists, try rotating or replacing preamp tubes.

 

[Pete Farrington]

Maybe a bad fx this return switching socket. Does plugging a patch cable between fx send and return sockets stop the issue?

 

[wavytech]

If you eventually lose volume after about 10 to 15 minutes, power off and power on, and volume is restored, you may have a mute circuit issue. This is an issue with some DSL40Cs that was fixed on the DLS40CR.

Marshall Dsl 40c Issue, Need Your Help!

I had one of these come across my bench this week, and I think I've found a fix for this problem. TLDR: disconnect the mute relay circuits from the DSP and use the standby switch to manually operate the mute circuit. This is a fully reversible mod, internally and externally, should you wish to...

www.marshallforum.com

 

[Jimi Biscuits]

You may have cracked a solder joint somewhere on the board that expands as the amp warms up. If you're qualified, check the board. If not, take it to a tech.

 

[Yamariv]

You may have cracked a solder joint somewhere on the board that expands as the amp warms up. If you're qualified, check the board. If not, take it to a tech.

This is what I was thinking, I was just going to replace the board if I could find a cheap option but can't seem to find them online Is there a good Marshall parts supplier online?

I've done several amp builds so opening it up is no problem. Just thinking there might be some microscopic damage to the board so replacing might be the best safe option. This is why I build handwired amps! lol

 

[Yamariv]

If you eventually lose volume after about 10 to 15 minutes, power off and power on, and volume is restored, you may have a mute circuit issue. This is an issue with some DSL40Cs that was fixed on the DLS40CR.

Marshall Dsl 40c Issue, Need Your Help!

I had one of these come across my bench this week, and I think I've found a fix for this problem. TLDR: disconnect the mute relay circuits from the DSP and use the standby switch to manually operate the mute circuit. This is a fully reversible mod, internally and externally, should you wish to...

www.marshallforum.com

What the heck is a circuit mute and why does it happen? Never heard of this..

The guitar sounds still comes through, it just gets quiet then back loud then back quiet every second or so when it happens. Very weird

 

[Yamariv]

Went and played for a good 20 mins after reading your comment. Of course the amp barely did it that time but I did plug in a patch cable but inconclusive so far..

Maybe a bad fx this return switching socket. Does plugging a patch cable between fx send and return sockets stop the issue?

 

[TheKman76]

Is the volume modulation very rhythmic? Is it regular and predictable or random and sporadic?

This sounds like bias movement or a bad valve to me. Most of the usual connection faults result in harsh, noisy, static garbage rather than a tremolo effect.

 

[Yamariv]

Is the volume modulation very rhythmic? Is it regular and predictable or random and sporadic?

This sounds like bias movement or a bad valve to me. Most of the usual connection faults result in harsh, noisy, static garbage rather than a tremolo effect.

You may be onto something, theres no popping or crackling like a bad input jack at all. There's no regular or predictable volume drops, just random, some long and short for different amounts of time. So a bad tube can do that?

 

[TheKman76]

I confess to not being an expert on all the ways valve can fail. That said, I understand how they operate and if you move the bias voltage of a valve you get a different amplification factor, all other things being equal. It's the principle behind the tremolo circuit.

It may be a valve, but it may be a cathode resistor or bypass capacitor failing.

 

[TheKman76]

Apologies, I thought I read that you'd checked out the power valves already, but it now seems I imagined it.
This is a classic symptom of power valves getting ready to give their last whisper of musical goodness.

How long have you had the amp and how old are the power valves?

 

[Yamariv]

Apologies, I thought I read that you'd checked out the power valves already, but it now seems I imagined it.
This is a classic symptom of power valves getting ready to give their last whisper of musical goodness.

How long have you had the amp and how old are the power valves?

Interesting, never knew that's what power valves sound like when going out. I've had the amp for about 5 years now. I don't have many hours on it, maybe 50-100hrs on brand new power tubes but I guess with modern power valves reliability can be a problem at any time

Was gonna try and swap preamp tubes but you're thinking to start with the power tubes?

 

[TheKman76]

If you've never replaced them it's a good place to start.
A heavily used amp can go through power valve in a year. Lightly used they car last a decade. Sometimes they just don't last, they are a consumable.

 

[Yamariv]

If you've never replaced them it's a good place to start.
A heavily used amp can go through power valve in a year. Lightly used they car last a decade. Sometimes they just don't last, they are a consumable.

I replaced them when I bought the amp and hoped they'd last longer than this. I've got spares so time to swap them out!