Re: IIgs Monitor Repair

[ferdimh@gmx.de]

On 6 Jan., 02:42, Polymorph <polymorp...@hotmail.com> wrote:
> 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   :-)  ).
>
a 47µF cap should be ok. The best way to discharge a HV cap is to
connect a ~5kΩ across it. Don't use a small 1/4Watt resistor, you will
just set it on fire! I use 5W ceramic resistors, but a 1W resistor is
ok too. Don't hold it with you fingers, because there may voltages you
don't want touch.
Screwdriver discharge is NOT a good idea at all. It can damage the cap
and it can blow other devices (I used a screwdriver some time ago and
blew a TV set's control logic beyond repair).
> 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.
>
>
This is better anyway. I've had many electrolytics which became leaky
(electrically speaking...)
> > > 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?
>
I'd suspect C514, C601 and C607 (which could be left out for testing).

> > 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.
>
But you'll have for a warm-up every time.
> > > 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.
>
I don't really believe this is a filter capacitor problem.
It sounds more like a problem in the HV divider section. This can be
verified with a simple test. You should see the fat wire coming from
the top of the picture tube. This wire leads to the Flyback
transformer, which also has the HV divider in it (the part with the
two or three trim pots). The lowest pot (usually labeled SCREEN) is
sort of a master brightness control. With it, the brightness can be
varied from a completely black screen to an extremely bright white
screen. The other controls are focus controls.
Fire up the monitor with a picture and first find out what the pots do
by turning them until something happens. If you know how they work,
leave the monitor on and try to correct the picture while the monitor
warms up. If it is possible to get a good picture or if they don't
have enough range to correct (might happen to the focus, unlikely with
screen) then the divider is propably bad.
Another idea would be to measure the voltage on the G2 terminals on
the CRT socket. It shouldn't change at all. But note that there might
be no picture as long as the meter is connected.
If the picture can't be improved using these controls you might either
have
a) a fault in the G1 supply (test voltage on G1. It shouldn't change
from cold to warm)
b) a fault in the video amplifier or supply (measure voltage on the
CRT cathodes (usually labeled R,G,B). This should not change as long
as the Picture is the same.)
Because all three colors are affected I'd expect a supply problem.
BTW: Many monitors have voltages printed next to some points
(especially connectors). Check these!
If you know where to look for the fault then you can trace the wires
and replace the caps you find on the way.

If the divider is bad, you're in trouble. These are not easily
obtainable and very expensive. The only sensible repair would be to
get another flyback transformer and transplant the HV divider.
This procedure is decribed in the Repairfaq as "Focus/Screen divider
bypass surgery".


---
Ferdinand Meyer-Hermann