Re: IIgs Monitor Repair

On Dec 26, 8:52 pm, "Michael J. Mahon" <mjma...@xxxxxxx> wrote:
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/

"The wastebasket is our most important design
tool--and it's seriously underused."


Thanks Michael, your experience with such matters always proves very
handy when these obstacles present themselves. :-)

I'll crack open the AppleColor RGB monitor tomorrow, take an inventory
of the electrolytic capacitors it has; and head to the local
electronics dealer (Jaycar). Hopefully with a few replaced capacitors,
the monitor will look nice and sharp permanently. I will take
photographs during the procedure, and if it solves my problem then I
will post details/photos online to help out others who may experience
this same problem (as it is mentioned in the FAQ I expect it has
happened more than a few times before). If it doesn't work, I'll be
too busy bashing my head against the wall to post anything. ;-)

Its just a shame that the capacitors that I installed into the
recently deceased Amiga monitor were of different voltages and
capacitances, otherwise I could have re-used them.

Also, upon inspection of the old Amiga monitor circuit board, I
noticed something that looks kinda weird and maybe possible damage. It
looks like an inductor, has come away from 2 of its support posts. See
the following images:

http://www.cirruscomms.com.au/~mike_stephens/Inductor01.jpg
http://www.cirruscomms.com.au/~mike_stephens/Inductor02.jpg

I don't know whether this is normal, but it sure looked suspicious,
especially since it is very close to the flyback transformer. Could
this be a possible cause for the problems I'm having? I may try
desoldering it and taking a closer look. I know very little about
inductors, but it didn't look "normal" to me. I could be barking up
the wrong tree here though.

Thanks,
Mike
.

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