Re: IIgs Monitor Repair

[Polymorph <polymorph69@hotmail.com>]

On Dec 26, 8:52 pm, "Michael J. Mahon" <mjma...@aol.com> 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