Hi all, after a bit of advice please. I've just picked up a used Marshall DSL 50, but I'm getting very loud popping noises when switching between the 2 channels.
It isn't the footswich as it does it when switching via the button on the amp also. Interestingly however, it doesn't do it when the 'tone shift' button is engaged. With that pressed in, it switches perfectly.
The guy who I bought it off said that he hasn't used the amp for a few years, and it hasn't been serviced in a while.
I thought I'd ask for any suggestions on here before booking it in to an amp tech.
Cheers
Comments
If you don’t have a spare valve then just swap V2 and V3.
If that doesn’t help then it could be a faulty cap in the tone stack - which will be bypassed by the tone shift - this could cause a DC voltage offset which will definitely make a loud pop. I’ll have a look at the schematic later and see if I can spot which one it’s likely to be.
It’s going to be a tech job if it’s not a valve anyway though.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Does the amp show any evidence of having been in a damp garage? If so you might have a very slight "leak" across a PCB (I assume the amp have one) .
I had this problem with a Mersey Super 15, EL84s would slowly go to redplate. Fixed it by soaking the PCB in most of a can of WD-40 and leaving it to drip dry for a week. You have to get the PCB out of the chassis so as IC says, tech job.
Dave.
But in this case, I would definitely NOT do that with WD40 - the PCB is so poorly designed and prone to arcing in various places anyway that I would not risk leaving any potentially conductive residue on it.
Since it's stopped by using the Tone Shift switch, I think it's more likely to be a cap. Now I've looked at the schematic, it should be C20 (the treble cap in the tone stack) since engaging the Shift puts C19 in series with it, which would block any DC leak.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
I've just pulled the back off the amp. @ICBM going from left to right, do you suggest I swap round the 2 middle valves then test?
https://imgur.com/mV4eYrK
https://imgur.com/95pS7Ov
Hope the pictures work? Looks like I have a mix of JJ's and also one that just says made in China?
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
I appreciate the help - as you can probably tell i'm not too hot on this stuff!
Thanks
Ah! Bit of misinformation about WD-40 methinks? Since the product has been used on car electrics for decades, especially distributor caps, there is obviously no "conductive residue" left behind otherwise the 25kV+ would surely track? I grew up with crappy British engine electrics and WD was a constant winter companion and before that its predecessor "CRC Rocket fuel".
In the case of the MS 15 the fluid washed off any grease and crap and soaked into the paxolin and drove out any moisture. I was then very careful to let it dry very, very well.
Dave.
Swap these two first and then leave them like that even if it doesn't fix it.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Have a look at this - admittedly this is a MkI board and they did later change it to increase the separation slightly here, but they left a couple of other similar ones...
If you look right in the middle of the picture you can see the point on the standoff leg of the 1K 5W resistor is less than half a millimetre from the large PCB trace. The resistor is a screen resistor at +470V and the trace is the filament supply at 0V. Guess what happens!
It's quite unbelievable when you see that the shape of the trace is totally unnecessary, if they'd just done it at a 45º angle like the bit under the fuseholder the separation would have been about 5mm.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
"Gaps" are not PCB substrate IC. I thought you meant WD-40 would lower the resistance of the actual board? Plainly it does not do that else dis caps would not work after spraying!
Yes, anything in that gap save PTFE could cause an arc which is why I was at great pains to point out the importance of drying the PCB really well.
BTW, PCBs should have an insulating coating that prevent such arcs but, as we both know it can be dangerous to rely on the coating alone! Or indeed a similar insulating coating on resistor bodies.
Dave.
Terrible piece of design and I assume you can now see why I detest them! Presumably so do Marshall now, they must have replaced hundreds of the boards under warranty. This is far from the only design fault, or even board layout fault, in these amps too - some of them are really shocking for such a major manufacturer with a long history of valve amp production, it's just down to basic ignorance in my opinion.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein