Crackly DSL40C

[Timbo]

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

Yes I have, unfortunately no visible arcing.

[Timbo]

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

There is foil in the top of the cab, but it’s all secure. As you say, the issue is still present with the chassis outside the cab.
btw, is there a way to upload a video? I got an error when I tried.

[Timbo]

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…

C47 and R59 lifted so everything before PI is isolated.
I don’t want to go down the transformer route until I’ve eliminated everything else. I don’t have spare ones on hand to check with.
fault occurs hot or cold. Humidity is stable between 50-60%.

[Timbo]

A very interesting discovery, but I can’t figure out where it leads me - if I pull one of the yellow wires from the power transformer which feed the 12V supply to the op amps etc, the noise instantly disappears. Bear in mind that C47 and R59 are lifted an only the PI and power output valves are fitted.

[nevetslab]

Do you have an external bipolar power supply that could provide power to the amps’ supply requirements as a check to isolate where this crackling noise source is coming from?

[Helmholtz]

Could be a problem with the Resonance circuit (IC3).

[Timbo]

Could be a problem with the Resonance circuit (IC3).

Looks like it. It goes away when I lift C62. Suspect dry joint on R78, so that’s next on the list.

[Timbo]

Still noisey after resoldering C62/R78. Replaced IC3 (with a TL072 in a socket as I don’t have an NE5532 on hand). Now very quiet. If I hadn’t accidentally left the 12v circuit disconnected, I don’t think I would ever have found it.
but only time will tell!…….