DSL 100 noise

[hellvis]

I have a Marshall DSL100 that has developed a crackling/static-type noise as soon as the stand-by switch is turned on. It makes no difference whether or not
I have a guitar plugged in or not. It will make this noise even if the volume
is turned down to "0" and it does it on both channels. I have also tried using
all three speaker jacks with the various ohm ratings through multiple cabinets
and several different cables, both the speaker and the guitar cables.
While the amp is being played through the intensity of the noise can increase and decrease with no discernable pattern. I have tapped on all of the preamp tubes and have removed all of them one at a time and the thing still makes this noise. Any suggestions.

[Alex R]

Try swapping in a new power valve. Or it's actually ok to remove the power valves one at a time and run the amp - you'll get a bit of hum, but you'll be able to hear if the noise goes away.

[hellvis]

I tried pulling the power tubes one at a time and it still crackles. This thing sounds like a CB radio
with the squelch control wide open. Apparently this amp was left on for 24 hours straight which
blew the power tubes. They were replaced and the amp re-biased and now it is making this noise.
I am wondering if it is a cap.

[Alex R]

If it definitely still makes the noise with the phase inverter tube pulled then I would next suspect that tube's plate resistors. These are much more likely to make hiss & crackles than are caps. With the PI out there's not too much in there that is capable of making crackles. Next suspect for me after that would be bias supply components.

You can 'test' these resistors by tapping them with a wooden chopstick to see if this worsens the noise.

[Enzo]

Leaving the amp on for 24 hours will not harm it in any way. Once it has been on for a half hour, it is as warm as it is going to get. Your power tube went because it was at the end of its life.

[Twist]

I know capacitor C46 can cause hum on these amps. I wonder if it could also cause a static noise if it goes bad. You didn't really say if the static was gone with all the power tubes out.

[hellvis]

Sorry, the static was still there with the power tubes out, although I didn't have them all out at once. I took them out one at a time. I've got a schematic for the amp and I found C46 on paper but I cannot locate it on the board.

[Enzo]

C46 is the tiny disc ceramic cap next to the last power tube socket next to the power transformer. It should not look black. Measure across it for short. or just measure between pins 3 and 4 of the socket. SHould not be shorted together.

Did you do what ALex suggested and pull the phase inverter tube?

[hellvis]

Thanks for the reply. Yes, i pulled the phase inverter tube and it did not fix the problem.
I will check C46 and see if it is shorted.

[Alex R]

...if C46 is ok my suspects are R2 and R3, the PI plate resistors, which will still make some noise with the PI out. If not them, check the bias circuit.

An 'audio probe' can be handy for tracing this kind of thing. Just a probe connected via a cap (value not too critical - 0.1 to 0.01uF) and a screened lead to a second amp, screen grounded normally at the input socket. Lets you listen in at any point on the circuit. Get a chopstick, tape a cap to it, inner of a screened lead to one lead, the other lead is the probe, insulation and heatshrink all around, just leave the tip showing.

With such a probe you could quickly find the source of the crackling.

[chris101]

Hi folks,

I bet its C46, both my DSL's have had bad C46s at one time or another over the years. The result was a loud hum that didnt change regardless of how high the volume knobs were turned etc.

Chris

[milosch]

I have one of these in my shop now. HT fuse was blown, which was why the guy brought it in. That and tubes getting very hot. C46 wasn't shorted but was darker than normal. R76 was open.

They had 4 GT E34LS tubes in here and on one pair the labeling had turned brown. After replacing parts, I tried the bias. The brown pair's bias is creeping. I'll be retubing it this week.

[milosch]

Still creeping, but better, with the new tubes. I touched up some solder connections and replaced R6 and R9. Will try again tomorrow.

[spy]

I'm bringing the thread back in life because I have the same problem with a TSL100. C46 was fried and I found it unnecesary to put back a new one.
I put some new tubes on it and the amp worked fine until yesterday that it started to do crackles and noises. Now a pair of EL34s are fried, too!

Which is the purpose of this cap? Why is it there? To prevent what? And with what kind of cap is best to replace it so it won't get fried again.

[Robert M. Martinelli]

Which is the purpose of this cap? Why is it there? To prevent what? And with what kind of cap is best to replace it so it won't get fried again.

C46 is there for stability reasons, to prevent the amp from oscillating. To replace it, you'd better switch to a better quality-higher voltage rating one (you could also use two 47 pF, 400 V caps in series, this way you'd get a cap with half the capacitance but with twice the voltage rating).

Hope this helps

Best regards

Bob

edit: I forgot to say that the amp MIGHT work just as well without that cap.