Marshall Dsl 40c Issue, Need Your Help!

scotheath

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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 undo it.

An explanation in finer detail:
Based on the lack of progress in this thread, I think the cause is firmware related, not hardware.
The DSP daughterboard controls both channel switching and tone/eq logic. However, the designers had a problem with the switching behavior during power on/off cycling, resulting in fun things like the high gain channel switching on at whatever volume it's set to (possibly 11) and then amplifying whatever pops and squeals this circuit makes as the amp turns on or off. Very fun, will blow your ears out.
The DSP engages a muting relay during power up and power down so your don't hear this.
What I think is happening is a programming bug where the DSP randomly locks up in the muted state. This can be fixed by power cycling the amp, but that's no good if you are playing a gig.
Disconnecting the mute relays from the DSP solves this, but reason for the mute circuit still exists. Connecting the standby switch to the mute circuit solves this: engage the standby switch before powering on and off the amp, like a normal standby.
The removal of the standby switch's original function does no harm to the amp. There's no good reason for them and leaving the standby engaged for long periods can be harmful to tubes. This new mute circuit is probably a better design, ironically.

Next post will include photos/ directions.

Glad I could shed some light on this absolute lemon of an amplifier. Please just avoid DSP amps, they aren't worth the trouble!
Well I will be looking forward to that. Can't wait to have this amp usable again
 

Sandon Hess

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Disclaimer: This amp has a plate voltage of over 460 volts DC! Tube amps can kill you, this is lethal voltage! If you don't have experience working with this kind of device, find an experienced professional to do it instead. Before working on a tube amp, disconnect from mains power and check the main power supply capacitors for voltage with a multimeter and discharge them. If you don't know what any of that means, take a step back and do your research.
Schematic:
Marshall DSL40 Mod.jpg
Disconnect jumpers from standby switch and wire transformer directly to board
PXL_20210702_232143132 copy.jpg
Remove DSP daughterboard (carefully!!!) and find R91 and R99 below it.
PXL_20210702_232116136 copy.jpg
Disconnect this end of R91 and R99 from board, and wire to one terminal of STBY switch
PXL_20210702_233957889 copy.jpg
Tie other terminal of STBY to the 5V rail, conveniently brought out to Pin 1 of this unused header
PXL_20210702_234002123 copy.jpg
Replace the DSP daughterboard, add some RTV silicone to your flying leads for strain and vibration relief.
 

scotheath

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Disconnect jumpers from standby switch and wire transformer directly to board

That all seems pretty simple after looking at amp. 1 question, how do you know which wire from transformer goes to s5 and which goes to s6 ?
 

Sandon Hess

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Disconnect jumpers from standby switch and wire transformer directly to board

That all seems pretty simple after looking at amp. 1 question, how do you know which wire from transformer goes to s5 and which goes to s6 ?

Scott, since they are carrying AC from the transformer and going into a full bridge rectifier on the board, it doesn't matter.
 

scotheath

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Disclaimer: This amp has a plate voltage of over 460 volts DC! Tube amps can kill you, this is lethal voltage! If you don't have experience working with this kind of device, find an experienced professional to do it instead. Before working on a tube amp, disconnect from mains power and check the main power supply capacitors for voltage with a multimeter and discharge them. If you don't know what any of that means, take a step back and do your research.
Schematic:
View attachment 91082
Disconnect jumpers from standby switch and wire transformer directly to board
View attachment 91085
Remove DSP daughterboard (carefully!!!) and find R91 and R99 below it.
View attachment 91086
Disconnect this end of R91 and R99 from board, and wire to one terminal of STBY switch
View attachment 91083
Tie other terminal of STBY to the 5V rail, conveniently brought out to Pin 1 of this unused header
View attachment 91084
Replace the DSP daughterboard, add some RTV silicone to your flying leads for strain and vibration relief.

Did this today, So far so good !!!!!
Thank you so much. Now to play the hell outa it and make sure its good to go and sell it !!
 
Last edited:

scotheath

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Disclaimer: This amp has a plate voltage of over 460 volts DC! Tube amps can kill you, this is lethal voltage! If you don't have experience working with this kind of device, find an experienced professional to do it instead. Before working on a tube amp, disconnect from mains power and check the main power supply capacitors for voltage with a multimeter and discharge them. If you don't know what any of that means, take a step back and do your research.
Schematic:
View attachment 91082
Disconnect jumpers from standby switch and wire transformer directly to board
View attachment 91085
Remove DSP daughterboard (carefully!!!) and find R91 and R99 below it.
View attachment 91086
Disconnect this end of R91 and R99 from board, and wire to one terminal of STBY switch
View attachment 91083
Tie other terminal of STBY to the 5V rail, conveniently brought out to Pin 1 of this unused header
View attachment 91084
Replace the DSP daughterboard, add some RTV silicone to your flying leads for strain and vibration relief.
Thank you so much for this. This was the fix for my amp. 6 hrs at band level and no lock up. I hope others find this thread
 

martinsrodrigo

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I know this is an old, old thread, but those amps are still around. Had one of these in the shop awhile back.

IF AND ONLY IF YOU KNOW WHAT YOU ARE DOING: Try changing D9 to a 1N5229B. If I remember right, you don't have to pull the board, you can just clip the diode leads at the body and solder the new diode to the old leads. Make sure you have the polarity right.

Warning: Make a mistake and you'll damage the embedded microprocessor that controls the switching functions and your options will be trash or a factory authorized Marshall service center.

Hi.
This is the trick!
What is happen:
The AT2313 have a pin which is supposed to receive 3.3v across this D9 diode. This pin is an input counter/timer or some other detection function assigned to it within capabilities by design handled by the Micro-Software programmed by Marshall. The AT2313 manufactor tells that the minimum voltage at that pin in order to make it funcional is 3v.
But...
Measuring with multimeter i found 2.12v!!! And nothing was wrong with the diode!
When this input is below 3v, the mute function is enabled, and this is our problem.
Replace D9 with a 4.7V Zener Diode and call it a day. This will bring the Voltage at that pin near to 3.2V.

Edit: remove the AT2313 from the socket before any kind of work!

EDIT 2: some maybe need to change C83 to 10uF too, to improve stability

Best regards from Marília/Brazil
 
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scotheath

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Hi.
This is the trick!
What is happen:
The AT2313 have a pin which is supposed to receive 3.3v across this D9 diode. This pin is an input counter/timer or some other detection function assigned to it within capabilities by design handled by the Micro-Software programmed by Marshall. The AT2313 manufactor tells that the minimum voltage at that pin in order to make it funcional is 3v.
But...
Measuring with multimeter i found 2.12v!!! And nothing was wrong with the diode!
When this input is below 3v, the mute function is enabled, and this is our problem.
Replace D9 with a 4.7V Zener Diode and call it a day. This will bring the Voltage at that pin near to 3.2V.

Edit: remove the AT2313 from the socket before any kind of work!

Best regards from Marília/Brazil
With my amp I was getting 3.1 v at pin 10 of the AT2313 even at failure.
 

martinsrodrigo

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With my amp I was getting 3.1 v at pin 10 of the AT2313 even at failure.
I would try to replace it anyway. 3.1v is just 0.1v above the minimum. And changing C83 to 10uF 100v is another improvement to this voltage line. It would be good to confirm if this mods results more solutions
 

scotheath

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I would try to replace it anyway. 3.1v is just 0.1v above the minimum. And changing C83 to 10uF 100v is another improvement to this voltage line. It would be good to confirm if this mods results more solutions
Hopefully somebody is willing to give it a try I'm going to leave mine as is, the mode I did explained by the member above, Sandon Hess has worked for my amp. We have been jamming twice a week for a month now getting ready for a gig and I've had no issues what so ever. Its just nice to be able to play it w/out waiting for it to silent.
 

wavytech

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Just replaced D9 with IN4731 1W 4.3V zener diode on my DSL 40C amp that would reliably mute after 10 minutes of hard playing. Ran it for over 6 hours playing on ultra gain channel at high volume through a dummy load without error after fix. Not sure I understand the circuit as I measured about 2.1V across diode before the fix and about 2.6V after, still below 3V threshold, but did not scope signal.
 

Smacdown

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Massively incompetent..like releasing amps with firmware issues and then not adressing the issue with a recall and repair..kinda like the 2k dsl/ tsl bias issue. If these amps were cars they would be recalled and fixed at no cost to the current owners, and rightfully so.
they have offered to replace when they go bad. I just talked to usamusiccorp(marshall service for usa) about this very same issue with my DS:L40C. They offered to the entire amp with a repaired/refurbished DSL40CR(newer version)at no cost to me. I am only hesitant to do that because mine is a blonde tolex model with bagrill and gold logo and they can only offer a standard black one. They have offered no other other option, not even to have a tech repair it. They are telling me the problem is the pcb itself and the board is now obsolete and cannot be replaced. Seemsa bit sketchy as i am an electronics tech by trade and pretty much anything that did work at one point can be repaired. But I can't complain since they are willing to replace the entire amp and have acknowledged issue.
 

PelliX

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Seemsa bit sketchy as i am an electronics tech by trade and pretty much anything that did work at one point can be repaired.

Unless it's the board itself. It's cheaper to hand out a new amp than pay a tech to move everything to a new board. I could imagine that they no longer have (stocked or empty) boards for the DSL40C anymore. I'd say it's plausible, but Marshall say all kinds of stuff depending on the time and person involved.
 

scotheath

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Disclaimer: This amp has a plate voltage of over 460 volts DC! Tube amps can kill you, this is lethal voltage! If you don't have experience working with this kind of device, find an experienced professional to do it instead. Before working on a tube amp, disconnect from mains power and check the main power supply capacitors for voltage with a multimeter and discharge them. If you don't know what any of that means, take a step back and do your research.
Schematic:
View attachment 91082
Disconnect jumpers from standby switch and wire transformer directly to board
View attachment 91085
Remove DSP daughterboard (carefully!!!) and find R91 and R99 below it.
View attachment 91086
Disconnect this end of R91 and R99 from board, and wire to one terminal of STBY switch
View attachment 91083
Tie other terminal of STBY to the 5V rail, conveniently brought out to Pin 1 of this unused header
View attachment 91084
Replace the DSP daughterboard, add some RTV silicone to your flying leads for strain and vibration relief.

So after a few months w/out issue my amp locked up again last night, didn't lose sound though but couldn't switch channels.
Before I completed above mod I did not take note or pictures of connection from transformer and switch to board, yeah not smart I know. I believe I'm going to reverse the above mod and replace D9 Diode mentioned later in this thread. I've found some gut shots of the DSL but none with enough detail to reconnect transformer to standby switch and jumpers from standby switch to board.
Or as in doing the mod does..
"since they are carrying AC from the transformer and going into a full bridge rectifier on the board, it doesn't matter."
does the same apply when reversing mod ??
Can anybody help me out with connections please?
 

scotheath

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Lucky for me the tech that tried to fix with new chip labelled all the wires, I forgot. Reversed the initial mod and changed D9 to a 4.7v Zener. See what happens. Not gonna hold my breath though. Would love to just sell this POS but can't knowing its a POS and have a conscience lol.
 


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