Well, it's not that the transformer HAS to die....
Adrian should try to listen to the thing: If he clearly hears the
transformer
humming under load, throw it away. Or: place small magnetic objects very
close
to the transformers core. You need a considerably large magnetic field
on the
surface of the core to influence the electrons in the CRT's beam, which
are,
after all, still a few inches away. So, the "small objects" might
move...
Oh, another idea: Adrian, have you tried to place the transformer on the
table,
outside of the casing? I mean: leave as much as possible of the original
cabling
in place, just put the transformer out of the way? If this helps, you
know
what to do 8-)
Richard Erlacher wrote:
>
> What Michael says here makes a great deal of sense. However, as I have a number
> of units which, like the Commodore box, have a transformer of considerable age
> in their power supply located near the yoke of the CRT, yet have none of the
> wobble described here, it seems to me that one could probably allow the
> transformer in the Commodore unit to remain at least long enough to check for
> some sort of disruption of the power supply to the sweep circuits. Transformers
> should last longer than this 20 years or so, while the capacitors have reached
> their shelf-life limits, I think.
>
> Partly, I guess, this bias is because I'm quite certain it will be a lot of
> trouble replacing the original transformer, though a kit with appropriate spec's
> might be solution. In any case, one can check the power supply connections to
> the CRT circuitry without leaving the house. This being Saturday, I'd say
> Monday will be soon enough to go searching for a replacement transformer.
>
> Let's keep our fingers crossed ...
>
> Dick
>
> ----- Original Message -----
> From: "Michael Schneider" <ms_at_silke.rt.schwaben.de>
> To: <classiccmp_at_classiccmp.org>
> Sent: Saturday, October 06, 2001 4:15 PM
> Subject: Re: CBM8032 - wobbly screen (The saga continues)
>
> > I have to disagree.
> >
> > What Adrian describes are the "classical" symptoms for electromagnetic
> > inductions,
> > resp. distortions caused by this physical effect. Grounding and
> > shielding with tin
> > foil does not help, since it's the transformers magnetic field (as
> > opposed to an
> > electrostatic field, that would be shielded by the grounded foil) that
> > deflects the
> > electrons of the CRT beam and causes the wobble.
> >
> > Tha most probable cause is indeed the transformer, more precisely: the
> > aging core
> > of the transformer. This core usually is made of a lot of thin sheets of
> > steel.
> > The purpose of the core is to a) couple the primary and secondary coils
> > very
> > closely and, more important b) to "contain" and keep the electromagnetic
> > field
> > closely within the transformer. Well, ok, that's a bit simplistic, but
> > you'll get
> > the idea. The core is not made out of a compact piece of steel since
> > stray currents
> > would be induced within the steel, heating up the core and wasting
> > energy. This stray
> > currents ("eddies" they're called in english, i think...) would be
> > induced perpendicular
> > to the orientation of the sheets the transformer core is made of.
> > Thus, these sheets must be insulated by very thin layers of plastic or
> > resin or something
> > similar.
> > Now, this is the point: With old transformers, this insulation starts to
> > literally crumble
> > away, which leads to a significantly larger stray magnetic field around
> > the transformer.
> > Later, you can usually **HEAR** this effect: the steel sheets start to
> > humm with the
> > line frequency. The transformer is close to dead now...
> >
> > To make this log story short: Yes, a new transformer should help. New,
> > like in
> > "factory fresh"...
> > Even better would be one of these fancy and expensive torroid
> > trasformers, since
> > they have literally no measurable magnetic field outside the core.
> >
> > Shielding of the old transformer is no real option. You would need what
> > we Germans
> > call "mu-Metal", which is a special alloy of steel and other things i
> > have long
> > forgotten but that make it EXPENSIVE.
> > It is magnetic, but not a good conductor, which make it ideal for
> > shielding magnetic
> > fields.
> >
> > You could try a shielding out of steel sheets, but they might get hot if
> > the stray
> > currents induced by the transformers magnetic field are too large. And
> > the shielding
> > effect is not too good...
> >
> >
> > Oh, and by the way: I'm new to this list... 8-)
> > Hi, i'm Michael and i am collecting old VAXen.
> > I live somewhere in southern Germany, which is not that bad. No,
> > really.... 8-)
> >
> > Cheers
> >
> > ms
> >
> >
> > P.S.: sorry for the long post...
> > Richard Erlacher wrote:
> > >
> > > 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.
> > >
> > > Dick
> > >
> > > ----- 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
> > > >
> > >
> >
> > ------------------------------------------------------------------------------
> --
> > >
> > > >
> > > > ---
> > > > Outgoing mail is certified Virus Free.
> > > > Checked by AVG anti-virus system (http://www.grisoft.com).
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> > > >
> >
> > --
> > Michael Schneider email (home): ms_at_silke.rt.schwaben.de
> > Schleestr.8 http://www.vaxcluster.de
> > 72766 Reutlingen Phone: +49 7121 492781
> > Germany FAX: +49 7121 479331
> >
> > People disagree with me. I just ignore them.
> > (Linus Torvalds)
> >
> >
--
Michael Schneider email (home): ms_at_silke.rt.schwaben.de
Schleestr.8 http://www.vaxcluster.de
72766 Reutlingen Phone: +49 7121 492781
Germany FAX: +49 7121 479331
People disagree with me. I just ignore them.
(Linus Torvalds)
Received on Sun Oct 07 2001 - 04:43:01 BST