6 Oct
2001
6 Oct
'01
8:40 a.m.
What you need is Mumetal which is a MAGNETIC alloy.
Lacking that distance helps.
Having built numrous O'scopes over the years the problem of
transformers radiating magnetic fields are well known to me.
Allison
-----Original Message-----
From: Adrian Vickers <avickers(a)solutionengineers.com>
To: classiccmp(a)classiccmp.org <classiccmp(a)classiccmp.org>
Date: Saturday, October 06, 2001 3:39 PM
Subject: Re: CBM8032 - wobbly screen (The saga continues)
Curiouser and curiouser...
Today, I happened to have the machine opened up to effect some keyboard
repairs (i.e. a jolly good clean up). Having successfuly removed all
traces
of key bounce, non-working keys, etc., and feeling
justifiably please
with
myself, it occurred to me to try a couple of things WRT
the wobbly
screen.
1st attempt: "build" some shielding out of aluminium foil to go around
the
transformer. Which I did - all rough & ready stuff,
with just a bit of
insulating tape over the transformer connections to make sure they
didn't
short out. No effect - the screen wobbled its way along
as ususal.
2nd attempt: Same as above, but this time I made absolutely sure the tin
foil was earthed, by screwing it onto the chassis via one of the
transformer screws. Same effect as 1st attempt: FA.
3rd attempt: Getting desparate now, I wrapped a load of tin foil around
the
signal cables (having removed it from the transformer).
Decided to leave
the lid up when I switched on and bingo! Steady as a rock (literally no
movement whatsoever. Refusing to get too excited, I carefully brought
the
lid down (holding the signal cables so as not to let
the tin foil touch
the
circuit board - bah! The wobble returned.
4th attempt: Removed all tinfoil, started up with the lid open, no
wobble.
The wobble appears progressively as the monitor
approaches the
transformer
(the lid down position).
5th attempt: Put a sheet of tinfoil over the whole of the bottom of the
monitor section. No effect - wobble remained.
Clearly, therefore, the wobble is induced by the monitor. There must be
a
fairly monster field coming off the transformer
interfering with the
electron gun aiming circuit(s), or something like that.
The question is: What *would* shield it? Do I need some thin steel or
copper, for example? Presumably, the shielding needs to be earthed (not
difficult, there's plenty of earth points around there. Or, would it be
easier to simply replace the big transformer with a smaller modern one?
TIA!
--
Cheers, Ade.
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