Re: IIgs Monitor Repair

["Michael J. Mahon" <mjmahon@aol.com>]

Polymorph wrote:
> On Jan 9, 1:10 am, ferd...@gmx.de wrote:
>
> > Something else: It looks like you have found the problem so stick to
> > it. The problem is extremely likely to be with the focus/screen pots.
> > You should concentrate on this. There is only one cap which could
> > affect brightness, bot it would not affect focus and it would only be
> > able to REDUCE the brightness. The HV divider can usually not be
> > replaced, you'd have to replace the whole flyback assembly, which
> > can't be done at reasonable cost, if at all.
> > But there are other possible fixes:
> > - turn the pots back and forth very rapidly (10 times or so) . After
> > that, turn the monitor on with SCREEN at minimum and readjust.
> > - live with it (perhaps set up the focus with a warm monitor)
> > - leave the case off (not a good idea for your daughter...)
> > - install a cooling fan
> > - drill two holes into the case to allow adjustment from the outside
> > - attach an external divider.
>
>
> Thanks for your tips Ferdinand, I'll most definately give them a try.

I agree with his suggestions.

> It looks likely that I will be able to source another Amiga monitor in
> the next few days, so if I do I will decomission the AppleColor and
> spend some more time trying to get it working 100%.
>
> I think I may send the old Amiga monitor (the one that died
> originally) to the monitor home in the sky, as I don't think its worth
> the effort/cost in fixing, especially if I can get a cheap
> replacement. I may spend a little more time on it looking for anything
> obviously wrong, but other than that, I think it will go.

Don't overlook the advantage of having an identical working version
of a device.  You can measure various points in the circuit on the
good one and compare them with the bad one to localize the problem.

This method does require knowing what circuits can cause other
circuits to behave abnormally, however.  (That is what is complicated
so much by feedback--it couples the whole loop together so a failure
anywhere in it affects the whole thing.)

> By the way, it has become pretty obvious during the course of
> performing this repair that the Apple II Monitor FAQ may require some
> revising. As I stated in an earlier post, one of the suggested
> solutions to a problem sounding very much like mine was to replace the
> filter capacitors. Now, it would seem that performing this action
> would do very little to fixing this problem (as I found out the hard
> way). Does this sound like a fair comment, or is it just that my
> problem was a little different from the "normal" problems associated
> with this monitor?

Many repair FAQs have the problem of being created from sparse anecdotal
evidence, so the true range of causes and their probabilities are not
well described.

In this situation, simply collecting the anecdotes may be the right
way to go.

> Maybe with some agreement from you experts, we could have it revised
> to include some alternative solutions. This will probably be more and
> more relevant as time goes on, as more and more monitors will begin to
> fail.
>
> What do you think?

Good point.  So far, I haven't had any AppleColor RGBs fail, but I don't
use them much, so I'm not a good example.  All of my suggestions come
from general monitor and TV repair experience, which is not nearly as
useful as deep experience with a particular monitor design.  ;-)

-michael

NadaPong: Network game demo for Apple II computers!
Home page:  http://members.aol.com/MJMahon/

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