Re: IIgs Monitor Repair

[Polymorph <polymorph69@hotmail.com>]

On Jan 6, 11:48 am, "Michael J. Mahon" <mjma...@aol.com> wrote:

<snip>

> As I noted earlier, you may find it much easier to simply use one
> or two known-good electrolytics to "bridge" across the terminals
> of each suspected capacitor until you find the one that makes a
> (positive ;-) difference.
>
> A 100uF 450v capacitor is a good "high range" test capacitor, and
> a 22uF 450v capacitor would be a good "low range" one.
>
> This saves a lot of unsoldering and soldering and works well in
> the vast majority of cases.

I do have a 100uF 450V cap, but I do not have a 22uF 450V to attempt
testing in parallel. I do have a 47uF 200V cap; would that be
suitable? By the way, what is the best way to discharge a capacitor?
(Told you I'm still learning   :-)  ).

I must admit, apart from the two largest caps (one of which required
the removal of metal shielding), the other caps have been relatively
simple to desolder, so I'm happy enough to keep going that way for
now.

>
> > If the problem is still present after replacing all the high voltage
> > caps, is it worthwhile continuing replacing the lower voltage
> > electrolytics, or should I start suspecting something else as the
> > failure?
>
> Using the bridging technique really lowers the penalty for checking
> out most of the other capacitors, and electrolytics are still the
> most likely failure candidates.
>
> > Michael, you mentioned some non-electrolytic capacitors as
> > possibly being responsible. How would I find the ones you described?
> > Bear in mind the schematics for the IIgs monitor are available (I have
> > downloaded them) from Tony Diaz's info site -http://apple2.info/. So
> > hopefully you (or someone else) can describe what I should be looking
> > for, or better yet, point out on the schematic the likely ones.  ;-)
>
> I'll take a look and see.
>
> What exactly is the symptom, again?


The symtoms are as follows: when the monitor is first switched on, the
contrast / brightness is quite low but the picture is nice and sharp.
By turning the up brightness / contrast a little, you get quite a good
image. However, after about 10 minutes the monitors picture starts
getting noticeably brighter and continues to do so. After about 30
minutes, the image has become really bright and the focus has degraded
to a point that hurts your eyes. Lowering the brightness / contrast
doesn't help as the image is all blurry and very hard on the eyes. If
you look back through the history of this thread, I posted one of the
Apple II FAQ questions and answers which sounds remarkably like what
I'm seeing - this is what started me replacing "filter" capacitors.

Thanks for your help.

Cheers,
Mike