Gareth Magennis wrote in message
...


"N_Cook" wrote in message
...
Gareth Magennis wrote in message
...


"N_Cook" wrote in message
...
For those thru-board valve bases it looks as though you would have to
replace with standoff types to retain structural integrity but with
electrical isolation , as well as all that hard wiring and
intermediary
isolation/ mounting points.
Looks like too much of a work up in comparison to paying Mr Marshall
for
a
replacement board.
I'm just wondering if this particular board , only 1 google ref, that
they
will not have a replacement





I think replacements are generic, i.e. many different models share the
same
output PCB.
Its the number of channels and other such stuff that are the variants.


Problem with repair, is that you might repair your V1 symptoms, but 6
months
later it comes back with the massively more disappointing bias
destruction
problem.
Or vice versa.

The whole PCB is possessed, it needs to be exorcised.


It would be interesting if you could establish the physics and
chemistry
behind all this though!



Cheers,

Gareth.





Why did the mod-bods choose to isolate the grid pins rather than the
anode
pins?

Anyone know if they use mineral filler (cheaper) in the epoxy of epoxy
pcb
manufacture. Like the use of calcium carbonate in the epoxy
bulking/fixing
of toroidal transformers or even car body repair filler.
This morning I made a test cell of some calcium carbonate and water to
thick
paste consistency in an inch wide plastic bottle cap. Resistance across
a
diameter about 60K. Waft a low-setting hot air gun over it and
resistance
drops to about 2K, now rising again.
So moisture/condensation can get into the edges and component holes of
such
a pcb and the glass dutifully conduct it capilliary fashion.
Perhaps a cure might be a low oven bake of a day at 105 deg C of a
populated
board , assuming nothing comes to grief at that temp and then some
sprayed
on conformal coatinf along all edges and component leads, but that would
not
get to the prime source of problems , inside /under the valve bases and
those pcb holes





I'd be willing to donate one of my faulty PCBs if you sincerely thought it
might help you to further establish the mechanisms involved here.

My email reply address is valid.




Gareth.


A day on of drying out and my test cell showed 150K, warming easily brought
it down to 10K.
Tried a sample of calcium carbonate as it is , powder straight from the open
bag a decade old at least , and no response to a megger.

My chemistry failed. 30 percent HCl showed no fizzing with the powder.
Warming up and a lighted taper extinguished in the tube.
Ground off a sample from this Marshall board and the same.
But then tried a tube of HCl on its own and same extinguishing of taper.
I'll stick with the day job.