CBM8032 - wobbly screen (The saga continues)

From: ajp166 <ajp166_at_bellatlantic.net>
Date: Sat Oct 6 15:40:48 2001

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.


-----Original Message-----
From: Adrian Vickers <avickers_at_solutionengineers.com>
To: classiccmp_at_classiccmp.org <classiccmp_at_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
>of key bounce, non-working keys, etc., and feeling justifiably please
>myself, it occurred to me to try a couple of things WRT the wobbly
>1st attempt: "build" some shielding out of aluminium foil to go around
>transformer. Which I did - all rough & ready stuff, with just a bit of
>insulating tape over the transformer connections to make sure they
>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
>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
>lid down (holding the signal cables so as not to let the tin foil touch
>circuit board - bah! The wobble returned.
>4th attempt: Removed all tinfoil, started up with the lid open, no
>The wobble appears progressively as the monitor approaches the
>(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
>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?
>Cheers, Ade.
>Be where it's at, B-Racing!
Received on Sat Oct 06 2001 - 15:40:48 BST

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