Crackly DSL40C

[mikeydee77]

Nope transformer can't really figure in this. Otherwise fault would occur whether PI was in or out. However it could be the HT dropper resistor could I suppose be suspect. That is R83.

The other thing to consider is the two pkate resistors that you replaced if they are mounted hard on the board then you can sometimes get arcing to pcb traces that traverse underneath. Good practice to raise them a little to prevent this and for ventilation. You might be able to verify this happening with your chopstick.

[Timbo]

Nope transformer can't really figure in this. Otherwise fault would occur whether PI was in or out. However it could be the HT dropper resistor could I suppose be suspect. That is R83.

The other thing to consider is the two pkate resistors that you replaced if they are mounted hard on the board then you can sometimes get arcing to pcb traces that traverse underneath. Good practice to raise them a little to prevent this and for ventilation. You might be able to verify this happening with your chopstick.

V1-4 bases replaced with nice porcelain ones. The fault is still present. Just waiting for new caps to arrive for the PI output, C55/60.
yes, I mounted the new resistors around 5mm above the board, like the originals.
one thing that I don’t much like about this board is how close the ground plane gets to HV connections. Seems like it’s asking for trouble. I imagine it’s done like this to reduce noise? A bit like a screened cable. I’ve found several solder whiskers running across it in places and hoped that I’d found the issue, but so far no joy. See below for an example.

[mikeydee77]

Drat... I was betting on the bases. Very frustrating. I have an all wooden soft haired brush which is good for cleaning up baords in conjunction with a vacuum cleaner. Might be worth a go.

I think we said earlier that the noise was present on the grids earlier on so not sure if the PI output caps can be a factor... I know sometimes it is tempting to just rule things out by repalcement but got to stay logical and systematic on these faults.

[Timbo]

Drat... I was betting on the bases. Very frustrating. I have an all wooden soft haired brush which is good for cleaning up baords in conjunction with a vacuum cleaner. Might be worth a go.

I think we said earlier that the noise was present on the grids earlier on so not sure if the PI output caps can be a factor... I know sometimes it is tempting to just rule things out by repalcement but got to stay logical and systematic on these faults.

Caps replaced, fault still present. I’ve found that it’s particularly sensitive to probing around R83, R84 and R77. When the amp is quiet, often the lightest touch with a scope or dim probe is enough to set it off crackling, so light that it feels like it’s an electrical effect rather than a physical one. The tracks are well spaced in this area though, the solder joints are good and all of these resistors are new. I’m considering pulling out those components and rigging them up on a separate board to eliminate a PCB issue.

[Indyryder02]

I had one doing this a while back. Traced it to the PI. The signal was so low by then I couldn’t trace it any further. Replaced the surrounding components. Still no change.
I was informed on here, that the glue for the capacitors that turns darkish yellowish etc, becomes conductive.
Removed the glue. Cleaned with alcohol. Reinstalled caps. No crackling.
then I secured them with the GE electronics grade silicone is use. If you have some caps in that area with that glue… it’s definitely worth a try!

[Timbo]

I had one doing this a while back. Traced it to the PI. The signal was so low by then I couldn’t trace it any further. Replaced the surrounding components. Still no change.
I was informed on here, that the glue for the capacitors that turns darkish yellowish etc, becomes conductive.
Removed the glue. Cleaned with alcohol. Reinstalled caps. No crackling.
then I secured them with the GE electronics grade silicone is use. If you have some caps in that area with that glue… it’s definitely worth a try!

Thanks for your input. I posted some photos earlier in this thread when someone else made the same suggestion. Although they were quite sparing with the adhesive on this board, I think this will be one of my next jobs as I haven’t managed to find the fault anywhere else yet.

[Indyryder02]

Thanks for your input. I posted some photos earlier in this thread when someone else made the same suggestion. Although they were quite sparing with the adhesive on this board, I think this will be one of my next jobs as I haven’t managed to find the fault anywhere else yet.

I’ve ran into issues with that glue in several amps becoming conductive. Had a Peavey where it was near a power resistor. Melted off the board and into a tube socket. Shorted it out.
it’s referred to as “The Devil’s snot” haha!

[Timbo]

I’ve ran into issues with that glue in several amps becoming conductive. Had a Peavey where it was near a power resistor. Melted off the board and into a tube socket. Shorted it out.
it’s referred to as “The Devil’s snot” haha!

Devil Snot cleared from all of the caps feeding the power and PI sections. So far it’s dead quiet. This issue has gone into hiding before, but this time I have a good feeling.
I’ll post an update if it returns.
Thanks everyone.

[mikeydee77]

Fingers crossed...

[Indyryder02]

Devil Snot cleared from all of the caps feeding the power and PI sections. So far it’s dead quiet. This issue has gone into hiding before, but this time I have a good feeling.
I’ll post an update if it returns.
Thanks everyone.

You’re welcome. That’s prob it. I’ve fixed several that way now.

[Timbo]

You’re welcome. That’s prob it. I’ve fixed several that way now.

Well it’s been a couple of days now and still quiet, so I’m optimistic. I’m not convinced that the snot is actually conductive. I certainly can’t measure anything with a DMM. Maybe it attracts moisture and provides a path for the HV over its surface. Whatever, I’m definitely a new convert to the Anti Devil Snot brotherhood.

[Timbo]

After 3 days on the bench, quiet as a mouse, I put the chassis back in the cab. After about 5 minutes it started to crackle again. It’s not temperature related because I had a board sitting on it while it was on the bench, to simulate being in the cab. Also it happens immediately when turned on from cold.
I now have it back on the bench with C47 and R59 lifted, so the circuit upstream from the PI is isolated. just a really light touch with a DMM probe anywhere on the HT supply around the PI is enough to start it crackling.

[Bryan]

Have you tried looking at it in the dark? That was how someone discovered the corroded resistor problem. Maybe you will see some arcing.

[nevetslab]

Just out of curiosity…..have you removed the LSI pcb that I saw in the chassis photos? I have never serviced one of these and didn’t look at the schematics. If for no other reason but to remove another possible source.

I had to choke for a moment when after three days all had remained quiet with no further outbreaks of the offending crackles and then to have it start all over again putting it back into the cabinet! ARGGGHHH!

I did recently have to replace an output transformer on a JCM900 that was breaking down under drive. With that transformer, it required signal to be fed and pushed hard before the nasty effect of the transformer would arc. Transformer finally failed and tracked down replacement, installed and all better again.

Any chance ambient heat or cold in the room is a factor? Can’t rationalize why but having the presence of the cabinet reintroduced makes me go hmmmmmm…

[g1]

Any kind of foil shielding or conductive paint in the cab?
But I guess it had been happening previously while out of the cab?