Jcm 2000 marshall Dls 100

[amptech]

This unit has the the following problem's, (1) crunch/lead channel not working with low distortion plus frying sound through the speaker! how ever using the clean channel things are ok there even when pluging direct into the return so the power amp stage is ok. Has anyone had the same fault with this unit?
any help with this would be most grateful.

[Amp Kat]

My guess would be either a bad preamp tube or the footswitch jack. Does the channel led work when you change channels ?

[amptech]

Jcm 2000 Dls 100

Yes, channnel switching funtion led is ok when you press front buttons and you can clearly see them operating,also all pre/o/p valve's have been replaced. the next thing you can hear is some signal if you trun the gain full up on the crunch/lead channel but it's very very weak. The clean channel is working well with normal signal level's.

[Pedro Vecino]

Check voltages in preamp plates of first two tubes. These plate resistors are of 1/4 of watt and sometimes they fail

[amptech]

Jcm 2000 marshall Dsl 100

Thanks for the tip, I'll check them resistors and see if there o/c.
somtimes you can be looking to long at something and it drive's you nuts but, these things are send to try us!

[amptech]

Jcm 2000 marshall dls 100

This amp is driving me round the bend!
After more checking round the predrive section and changing the op amp ic's it's still not right, the fault is no crunch/lead channel just a hiss distortion sound if you plug a signal in and switch to crunch/lead channel.
any pointer's with this model would be greatful.

[Enzo]

Are ALL four small tubes heaters running? Or have the first two gone dark? If so, that bridge rectifier next to the sockets need resoldering or replacement. I usually find the legs never took solder well. I sand the wire leads clean and remount them. Bridges can fail, but it is a multiamp bridge serving two .3 amp tube heaters. So no stress on it.

If the heaters are all on, then go down the row looking for B+ on each pin 1 and pin 6. Then look for voltage on pins 3 and 8. If pins 3 or 8 are at zero, that tube is not conducting or its plate has no B+.

Of all the voltages make sense, apply a steady signal to the intut and trace it stage by stage through the amp until you find where it is interrupted. A scope is best, but a meter set to AC volts can follow a signal too.

[amptech]

jcm 2000 marshall dls 100

At long last cracked it and now all is well, turned out to be bad predrive valve.

big thanks to all for there input comments to this pain of a problem.