Re: IIgs Monitor Repair

On Jan 7, 7:15 am, "Michael J. Mahon" <mjma...@xxxxxxx> wrote:
In fact, using a hair dryer can provide some localization for the
problem--particularly if used in alternation with a cooling spray.
(Careful about high voltages, of course.)

I don't really believe this is a filter capacitor problem.

I agree.  The temperature-dependency argues for either a leaky
non-electrolytic in the HV/focus area, or a faulty resistor in
the same area.

I have two expert opinions pretty much agreeing with each another.
Does this mean I should abandon the electrolytic capacitor replacement
and concentrate on the area's you guys have mentioned? Is the HV/focus
area where the 240V (I'm in Australia) power supply is, or is it
around the flyback transformer? I'm still not up with all of the
terminology being used. I hope its something simple as a non-
electrolytic cap or dodgy resistor. What is the best method for
determining which one is faulty? Replacing the components with those
from the known good monitor? Some more info on where to start looking
would be great.

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.

I agree.  The fault will be something common to all three guns.

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".

Exact replacements for older gear are always a problem.  I haven't
looked at the physical arrangement, but it may be possible to use
discrete high-voltage resistors/pots to bypass part of the divider
or re-create it.  If space is tight, this may require potting.  ;-(


The problem with my monitor is starting to sound more serious than I
had hoped. I don't want to have to fork out a lot of money trying to
get this operational again - particularly considering that I'm so
green at doing this kind of work and probably more than a little out
of my comfort zone. I'll keep trying your suggestions for now, but i
If things can not be resolved after doing this, I may have to give up
and start looking for an alternative monitor. :-(

But I really appreciate all of the effort made in assisting me. :-)

Thanks,
Mike
.

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