Well, it's not that the transformer HAS to die....
Adrian should try to listen to the thing: If he clearly hears the
humming under load, throw it away. Or: place small magnetic objects very
to the transformers core. You need a considerably large magnetic field
surface of the core to influence the electrons in the CRT's beam, which
after all, still a few inches away. So, the "small objects" might
Oh, another idea: Adrian, have you tried to place the transformer on the
outside of the casing? I mean: leave as much as possible of the original
in place, just put the transformer out of the way? If this helps, you
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 ...
----- Original Message -----
From: "Michael Schneider" <ms(a)silke.rt.schwaben.de>
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
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
electrons of the CRT beam and causes the wobble.
Tha most probable cause is indeed the transformer, more precisely: the
of the transformer. This core usually is made of a lot of thin sheets of
The purpose of the core is to a) couple the primary and secondary coils
closely and, more important b) to "contain" and keep the electromagnetic
closely within the transformer. Well, ok, that's a bit simplistic, but
the idea. The core is not made out of a compact piece of steel since
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
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
Now, this is the point: With old transformers, this insulation starts to
away, which leads to a significantly larger stray magnetic field around
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,
Even better would be one of these fancy and expensive torroid
they have literally no measurable magnetic field outside the core.
Shielding of the old transformer is no real option. You would need what
call "mu-Metal", which is a special alloy of steel and other things i
forgotten but that make it EXPENSIVE.
It is magnetic, but not a good conductor, which make it ideal for
You could try a shielding out of steel sheets, but they might get hot if
currents induced by the transformers magnetic field are too large. And
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,
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
> all these years. I'd look for a failed
diode in a 4-diode bridge too, if
> what they used. However, what's more
likely is a problem in the wiring,
> the wiring path changes as you open/close
the lid. Since increased
> didn't do much, perhaps you should look
at the power connections, as the
> is perturbed whenever the box is opened. If
you check underneath (on the
> 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
> that moves around when you wiggle the power
wiring. It could even be a
> seated/latched contact in a MOLEX shell if
that's what they used. I don't
> Commodore made the CRT board, and they may
have used a pretty shoddy
> You've apparently got to find something that's different between when the
> open versus when it's closed.
> ----- Original Message -----
> From: "Adrian Vickers" <avickers(a)solutionengineers.com>
> To: <classiccmp(a)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
> > 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
> > 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
> > 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
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?
Be where it's at, B-Racing!
> > >
> > > ---
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> > >
> Michael Schneider email (home): ms(a)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(a)silke.rt.schwaben.de
72766 Reutlingen Phone: +49 7121 492781
Germany FAX: +49 7121 479331
People disagree with me. I just ignore them.