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So, I’ve been trying to track down this issue on my DSL40C for many hours. It has a really bad crackle which can occur immediately it’s turned on, or not at all, or after warming up for some time. Quite random in other words. No amount of poking or tapping with a wooden stick can induce it. I’ve checked over the solder joints and tracks connected with the valves and everything appears good. When it happens the volume and gain controls have no influence on the volume of the crackling. I’ve found that I can remove V1-3 and the issue is still present, however if I remove V4 (the PI) and leave in the power tubes, the noise does not occur. I’m not sure if this indicates that the issue is definitely confined to the components connected with the PI valve, or if it could still be one of the power tubes? All tubes have been changed and I’ve checked that there is tension in all of the the valve sockets.. I’ve seen a number of similar threads regarding this amp but no conclusive results or known issues that I can find. It definitely sounds like a bad joint or track somewhere in the HT rail but it’s very elusive.
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My thought process includes....
Sometimes power switches cause a little crackle so give them a tap...
Any sign of cracks on power resistors ?
New valves can be bad... it happpens you could swap them around and see if any variation.
plate resistors although seems like that would be unlikely on a Marshall of this period probably with MF resistors. sometimes you have to replace to find out. -
I've had a few of these where the glue/silicone around the caps in the power section will become corrosive over time and eat through the traces/resistors in the power section causing intermittent arcing. I clean all of it out on sight if it is discolored now. It took forever to find the first time.Comment
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Interesting.. is that acetic acid silicone I wonder? Can we get pics OP?Comment
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Hi Mikey, thanks for your reply. I was initially suspicious of the triode/pentode switch, so I replaced it. I also became suspicious of the standby switch, so I’ve bypassed it. I may investigate the main power switch, but I suspect that if that was the culprit I would still get the crackling when I pull V4. I have tried known good valves in all positions. PI plate resistors have been replaced as have R111and R83. I bought this amp as a non worker with no valves fitted. The initial fault was R111 completely burnt out, but no other obvious damage. With R111 replaced and a set of valves, the amp worked fine, albeit with this horrendous crackle which can sometimes go away for days on end. At the moment it’s doing it all the time, which is helpful for troubleshooting at least!
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There is some adhesive around the filter caps but the tracks for these are on the underside, so I don’t think that’s the problem. I’ll definitely put it on the list though!Originally posted by GoldtoneWorkshop View PostComment
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Looks like there's less in that spot than normal. Maybe a later version of the board/amp? It's def worth poking at though, the traces that were eaten away in the past were under resistors that were flush mounted to the board and I never would have found them without cleaning up the goop and lifting the resistors.
Do you have a picture of the whole power supply section?Comment
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Here are some more.Comment
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Gotcha. Yeah that's a different board revision, or maybe just a different version of the 40C, I know there's a few.Comment
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Here’s the schematicComment
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Later ones have a half and full power in standby switch and older ones have triode pentode on the rear. Or that is my impression.
I guess the option is valve base. Any visual signs of HT tracking?
I guess other possibility is crackling is coming into the PI. Can you look or listen to the signal?
Sorry hard to read schematic just now.Comment
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Valve bases are on order. No signs of tracking. I’m out now but will scope the PI grids tomorrow.Originally posted by mikeydee77 View PostComment
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Check the wires on the speaker - the ones that run from the terminals to the soldering points of the coil itself. They may be cracked.Comment
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Thanks for your input. The fault is the same whether the chassis is in its cabinet connected to the internal speaker or on the bench connected to a test speaker via a load box.Originally posted by x-pro View PostComment
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I would have started resoldering the sockets pins no matter how they looked. Can you put a meter or scope on a few places to see any spikes?Comment
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