Re: Problem Using a Commodore 1084s Monitor on a IIgs

[Polymorph <polymorph69@hotmail.com>]

On Feb 21, 8:06 pm, "Michael J. Mahon" <mjma...@aol.com> wrote:
> Polymorph wrote:
> > On Feb 20, 4:01 am, ferd...@gmx.de wrote:
>
> >>If the things you tried didn't help at all, then it doesn't seem to be
> >>bad luck...
> >>You could try replacing C222 or just bridging with your 150µF/400V
> >>cap. At least in theory there cold be a fault elsewhere but this would
> >>be VERY difficult to find, even with an oscilloscope. Using could
> >>spray to cool the parts related to the horizontal output stage one at
> >>a time can help to locate the fault.
>
> > I'll give your suggestions a try. I've used up all of my cooling
> > spray, so I'll need to get some more prior to trying this.
>
> >>BTW does theproblemdisappear if the power is turned off for a short
> >>time (i.e. 1 minute or so)?
>
> > I believe it does go away when turned off for a minute or two, but
> > returns quicker than if the unit was cold. It does appear to be heat
> > related.
>
> (Sorry for not chiming in earlier--I was travelling.  ;-)
>
> I think cooling spray is the correct route to locating theproblem.
> (Do avoid spraying it near the HV components, though.  Components
> outside the "cage" are fine.)
>
> The "singing" component is almost certainly just a symptom of
> theproblem, not its cause, which, as Ferdinand has pointed out,
> is instability in the horizontal output stage.
>
> Almost any component can be temperature sensitive, but capacitors
> and resistors are high on the list.
>
>  From your description of theproblem, it sounds like it will be
> easy to locate with a 10-20 degree temperature drop.
>
> >>Well I'd be interested in fixing these monitors because they have
> >>interesting defects, but this is not an option.
>
> > I'm interested in fixing *this* monitor as I bought it for my
> > daughter, but in future I would prefer to have a working monitor
> > straight away. With 2 young kids and 200+ driving per day, I don't
> > have much spare time up my sleeve to be worrying about these things.
> > Mind you, it has been educational and I am sure I have benefited from
> > the experience imparted from you (Ferdinand) and MJM.
>
> Most people don't relish repair work, but it is inevitably educational,
> and adds to your confidence that you can fix other things.  ;-)
>
> -michael
>
> NadaPong: Network game demo for Apple II computers!
> Home page:  http://members.aol.com/MJMahon/
>
> "The wastebasket is our most important design
> tool--and it's seriously underused."- Hide quoted text -
>
> - Show quoted text -

Well, after spending some time last night with a new bottle of cooling
spray, I think I can confirm that Ferdinand was on the right track
when he mentioned the capacitors C116 and C117. Applying cooling spray
to these caps made the problem disappear temporarily. As these
capacitors are right next to each other, I couldn't tell for certain
which one was at fault; or if both of them were. I'll try and source a
replacement capacitor from my old (now dead) Philips monitor -
hopefully I can find a 100uF cap of high enough voltage (160V+) to use
as a replacement. I do have a 100uF 400V capacitor, but due to its
size and the confined space around C116 and C117, I don't think it
will fit.

Anyway, I thought I'd give a progress update. I'll let you know how I
get on when I start replacing capacitors.

Hopefully once this is done, my monitor will be 100% working.   :-)

Cheers,
Mike