"Phil Allison" wrote in message
...

"Gareth Magennis"

Looks like someone may have tried to sort this problem out before, but
didn't replace the PCB?

http://www.lynx.bc.ca/~jc/TSL122.html


** The PCB material is definitely the cause of the problem.

It is made from UTTER CRAP !!

However - uber clever fixes involving a Dremel or mounting resistors in
mid air or any similar hare brained ideas are all just re-arranging the
deck chairs on the SS Titanic.

If no new PCB is available from Marshall at a reasonable price - just
add a bloody fan !!!!!!!!!!!!!!!!!

A single 120mm, 240VAC fan blowing directly on the OP tubes from below
cures the overheating / bias runaway problem completely.




New PCB's ARE available from Marshall, at about £60 to the customer if I buy
and fit one. Marshall tell me they sell a lot of them.
I understand costs are much higher outside the UK, however.

I totally agree the bodges and drilling malarky - the problem IS the PCB
material.

Charging the customer a whole bunch of labour to bodge a way around this
problem is, IMHO, unethical, as he STILL has a faulty PCB, and has been
charged loads of money not to fix the real problem.

Replacing the PCB is relatively cheap (compared with the alternative labour
costs) quick and easy, and is the only logical solution here.



Incidentally, 2 old faulty boards I have here and have kept, both have 220k
grid stopper resistors instead of the 5k6 the schematic shows.



Cheers,


Gareth.