Marshall JCM2000 DSL 100 biasing?

[odv]

Hey, I'm sam and I'm new to the forum. I'm starting to turn into all my local bands tech, including mine. I've found a lot of useful info here so figured I'd join up.
Anyways, I have a DSL100 that belongs to one of my friends and it was starting to get a lot of craackling once it got warmed up so he had me look at it and I noticed that the right side was running hot. I didn't know the exact settings and procedure for marshall's biasing so I took it home with the intentions of a quick fix, found the recommended bias setting of about 80mV and went to go do it.
I adjusted the left side and went to do the right and my multi meter went to protect and popped up high voltage. I had to put it on the 500V scale to get a reading which was +350V. I was wondering if anyone had an idea why its reading what appears to probably be the full plate voltage? Does that sound like the correct diagnoses? Any help would be greatly appreciated.

[Enzo]

You had 350v on the little pin sticking out the back?

Oddly enough, looking at the rear of the amp, there are four power tubes in a row, with the little bias adjust centered. The little control on the left and the test pin next to it control the two tubes on the RIGHT. The test pin and control on the right control the two tubes on the LEFT. Damned odd.

If you get 350v there, you have at least a bad tube, and most likely a burned out 1 ohm sensing resistor.

ANy chance one of the tubes has its littel central post broken off? And now the tube is not oriented properly in the socket?

Always make sure the mute button is not engaged - it shorts the two sides together.

[odv]

Yes it was showing 350-400V depending on how the bias pot was adjusted. The tubes are all oriented properly and are matched pairs. The thing is it works fine when you play through it which is contradictive of the bad tube theory. It just has a crackle which gets louder the longer its on. That and switching the tube sides didn't move the issue with it. I'm going to check the resistor and a few other things tonight. Thanks for the input. I'll let you know what I find out.

[amptech]

marshall jcm2000 Dsl 100

I was given jcm dls super triple lead head unit and it's main fault is poor lead/crunch channel and distored clean channe lwhat a mess! Also the bias was set to high on the right side valve's the followings readings are .98ma and the left side was set to .78ma. I've reset them to .78ma's so that part is done next part, has anyone had the same problem with this amp ie channel fault's any help with this problem would be most greatfull. thank's

[Mars Amp Repair]

hello Amp tech,
Better to start your own thread on a new issue. You'll get more responses. As to the issue you mentioned, I would first try to drive a signal to the power stage via the return jack & see if it is distorted there. you need to eliminate the power stage as a possible problem causing both clean & crunch channels sounding bad or distorted.

You problem could of course just be a bad preamp or PI tube.

glen

[Enzo]

Look on the board next to the end power tube - the tube closest to the power transformer. There is a small disc ceramic cap - C46 - near pins 3 and 4. Is it blackened or funny looking? Or are pins 3 and 4 shorted together on that socket only? If so, snip that cap out and see if the amp wakes up and works better.