David C. Partridge" wrote in message
.. .
Follow up to tell folks the current status and probable cause of the
problem
(I think):

I replaced the old 10nF ceramic smoothing caps in the grid bias supply as
they were getting distinctly warm after the scope had been on suggesting
that they were getting rather lossy (on a damp day they would steam gently
for a minute or two after turn on).

Didn't solve the brightness problem but new polyesters do NOT get warm, so
I
feel better :-).

Static grid voltage at output of grid cut-off adjustment trim-pot was
drifting up during warm up, while voltage on input side was pretty much
constant. Suggests it might be the trim pot drifting with temp.

The trimpot (R315) is supposed to be a 2M2 linear Cermet 20% 1W according
to
the parts list, but looks to me like a standard Piher PTC15 1/2W part.
Measuring the resistance as the scope cools after power off seems to
confirm
the hypothesis (10% increase in value over 15 minutes, probably as much as
15-20% by the time it reaches room temp).

If I could source a replacement readily I'd just ram a new one in, but I
suspect that the parts list is right and the original WAS a 1W part, and
has
been replaced somewhere along the way with a 1/2W part. Finding a 2.2M
0.5W trimmer of any sort (never mind one that will fit) seems a bit hard,
and as for a 1W part ...!

Before I go totally insane hunting for a suitable part, I'd just like to
get
a sanity check on my diagnosis from folks more experienced with this than
I.

Thanks
Dave


Hi David,

One posible fix (but a bit clunky) is to find out the values of the
resistance is from the ends of the trimpot to the wiper, by measuring the
existing pot, and replacing it with 2 fixed resistors of a suitable power
rating .
At least you could then tell if your diagnosis is correct.

Bye for now,

Ian.