Re: IIgs Monitor Repair

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

Polymorph wrote:
> Hi All,
>
> I have a couple of questions relating to repairing a monitor for my
> daughters IIgs.
>
> For well over a year now, I have had an old Amiga monitor hooked up to
> the IIgs via a SCART adapter (courtesy of VintageWare). Unfortunately,
> 2 days before Christmas something in the monitor died. It was working
> fine, then all of a sudden the picture went out and it started making
> a loud whirring/whining sound (almost like something rotating). I
> thought it might have been the electrolytic capacitors in the power
> supply, so I replaced them all; but the situation did not improve.
> However, I now believe the noise to possibly be coming from the "fly-
> back" transformer. Does this sound likely?

Very likely.

>  If it is, is it worth
> repairing as I could not find any details regarding the make/model of
> this transformer. Also, I could not find a schematic for the monitor.
> I am thinking this might now be a dead-issue, but wanted some experts
> opinions before giving up on it.

Audio frequency sounds coming from a flyback transformer can easily
result from "squegging" of the horizontal oscillator and/or horizontal
output stage.

These circuits can be difficult to diagnose because of the various
feedback loops involved--like switching power supplies.  A schematic
is very useful.

It is probably worth replacing the electrolytic capacitors in the
horizontal output stage, since a radically changed value or an
open or leaking capacitor can cause symptoms similar to what you
describe.

Odds are good that the hard-to-replace flyback transformer and
horizontal output transistor are still OK, and that replacing
capacitors (or any other dubious passive components) will fix
the monitor.

> Also, I have a standard AppleColor RGB (IIgs) monitor that I have now
> replaced the above dead Amiga monitor with, however it has issues of
> its own (which is why I was using the Amiga monitor in the 1st place).
> This monitor starts off clear and sharp but with low contrast/
> brightness; so you need to have both turned up high to get a good
> picture. The picture gradually gets brighter and brighter and the
> image quality deteriorates and loses focus (gets very blurry and hard
> to read). After about 30 minutes, the contrast brightness is very high
> and the image is very blurry (almost to the point of hurting your
> eyes).
>
> Now this problem sounds very much like the problem mentioned in Q.34
> of the Apple II Monitors FAQ which I will now quote the question and
> answer:
>
> "034- I picked up a IIgs RGB monitor at a garage sale. The focus is
>      off and it took several minutes until the display got brighter
>      but it's still not very good. Are there any adjustments that can
>      be made?
>
>      The problem you describe is consistant with low B+ going to the
> flyback
> transformer which results in low brightness level, poor focus, and
> blooming
> when the brightness and/or screen level is turned up. More than likely
> if this
> monitor has been sitting around for awhile, the filter capacitors have
> gone to
> mush. Sometimes they can be revived by leaving the monitor on for a
> long period
> of time <over 24hrs> but, usually, they require replacement. "
>
> Now, as this sounds *very* similar to the problem I'm seeing, I'd like
> to try and replace the main filter capacitors as described in the
> answer. The question I have though is - which are the "filter
> capacitors" in the monitor? I cracked the case open the other day, and
> there is one big 450V cap near the power supply. Is this the one being
> referred to, or is it somewhere else?

That certainly is one.  Any relatively high voltage electrolytics are
likely "filter capacitors" and are also among the lowest reliability
components in a monitor.

Sometimes a non-electrolytic capacitor used as a coupling capacitor
will get "leaky" (meaning that it begins to have a DC resistance
considerably less than infinite ;-), causing slow drift in DC bias--
for example, in the CRT grid, cathode, or focus electrode circuits--
which will also lead to a shift in brightness and focus.

> Any help would be much appreciated as I'd like to get my daughters
> IIgs running perfectly again. Either that or I'll be on the lookout
> for a replacement monitor.

I suspect that in the years ahead, we will have to get good at
repairing these monitors...

-michael

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

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