CBM8032 - wobbly screen (The saga continues)

From: Richard Erlacher <edick_at_idcomm.com>
Date: Sat Oct 6 15:45:57 2001

I doubt a new transformer will help this very much. If this box ever worked
right, then something's changed, and the filter CAPs are a prime suspect after
all these years. I'd look for a failed diode in a 4-diode bridge too, if that's
what they used. However, what's more likely is a problem in the wiring, since
the wiring path changes as you open/close the lid. Since increased shielding
didn't do much, perhaps you should look at the power connections, as the cabling
is perturbed whenever the box is opened. If you check underneath (on the solder
side) the supply connections on the main board and, more likely, on the CRT
board, perhaps you'll see a deteriorated connection or possible damaged trace
that moves around when you wiggle the power wiring. It could even be a badly
seated/latched contact in a MOLEX shell if that's what they used. I don't think
Commodore made the CRT board, and they may have used a pretty shoddy monitor.

You've apparently got to find something that's different between when the box is
open versus when it's closed.


----- Original Message -----
From: "Adrian Vickers" <avickers_at_solutionengineers.com>
To: <classiccmp_at_classiccmp.org>
Sent: Saturday, October 06, 2001 1:06 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.
> Be where it's at, B-Racing!
> http://b-racing.co.uk


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Received on Sat Oct 06 2001 - 15:45:57 BST

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