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Gareth Magennis Gareth Magennis is offline
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Default Marshall JCM 2000, DSL of 2003


This might be the infamous output PCB fault.
Keep metering the bias voltage from the 3 pin connector provided, (Should

be
90mV both sides).
If it starts to climb/runaway in sync with the hum increasing, then
that's
what the problem is.

You will need to buy another PCB from Marshall if so.



Gareth.



I will check that, I could understand with that old lino or whale hide
they
used to use on Fenders.
Marshall fault , some chemical getting capilliary fashion down the glass
fibres of the composite?





It is the board material itself that goes faulty. I do not know the exact
mechanism, but clearly parts of it become conductive, causing absolute havoc
with the bias.
The output tubes often get red hot and are destroyed.

Unfortunately the first thing the owner tends to do is replace the valves.
Which very soon get destroyed again.

(Expensive business when it eventually gets fixed properly - that would be
12 x EL34's, the output PCB, and a fair bit of labour).



Keep a careful eye on the bias as it warms up, adjusting all the time to
keep it less than 90mV each side. The moment you run out of bias adjustment
is the moment you have pretty much demonstrated the faulty PCB, and you want
to turn it off immediately.



Gareth.