Re: Monitor Problems

[Paul Grammens <pgrammen@sonic.net>]

Supertimer wrote:
>
> I've got one for you, Paul.  My IIGS RGB monitor now only displays
> a horizontal line across the center of the screen.  I've flipped through
> PC Tech books and discovered that this is a common problem and
> that "the vertical deflection is faulty" or something like that.  What
> does that mean and is it expensive to fix?

You are correct, it is the vertical circuit that is the problem. I
jumped up, ran to the garage, zipped the screws out of my spare IIgs RGB
and took a look. I can't tell you exactly what the problem is, but I can
tell you where to look. With the back off and viewed from the rear, note
the 4 wires running from the neck of the tube to a connector on the
board. The red/blue wires are the horizontal deflection signals going to
the coil on the neck, and the yellow/brown wires carry the vertical
deflection signals. Vertical deflection starts with a vertical sync
signal coming from the computer and ends with a powerful sawtooth signal
going up the yellow/brown wires to generate a magnetic field in one of
the coils around the neck. So the problem lies between those two points,
and the most likely culprits are:

1. Cracked solder joints. Use a magnifying glass to look for these,   
following the path of the signal.
2. Blown transistors, diodes, or IC's. Sometimes you can see (or
smell)    which one is gone. Also study the board for burn marks or
discoloration    from heat. That is often the way I find the culprit.
3. Bad capacitors. They can be tough to find, unless they got hot.
4. Open resistors- often will be burned or discolored, but not always.

Looking at the underside of the board, it looks like Q401, Q402, and
IC401 are all involved in generating the vertical sweep, so focus on
that area. The vertical sync signal goes from the external connector at
the rear to the board that plugs onto the end of the tube. It runs
across the board to a connector on the right side (viewed from the rear)
and then by a wire to the main board. IC401 probably generates the sweep
signal, and transistors Q401 and 402 probably amplify the signal to
drive the coil. 

So: Be safe, leave it off for a while before you open it up. Stay away
from the fat red wire going to the tube, it's the high voltage. 
Get some bright light and a good magnifier (I use a small hand lens with
two 5X lenses that can be added for 10X, available at nature stores).
Use your eyes and nose, follow the signal path looking for cracks and
burns. 
A digital multimeter (some cost less than $20) is a good tool for
checking for opens and shorts. Get one with a diode checker, diodes
should measure .5 to .7 volts in one direction, off the scale in the
other. Transistors can be tested like two diodes base to emitter, and
base to collector. Blown transistors and diodes can either be open or
melted into a short. 

That's kind of a thumbnail tutorial on the art of troubleshooting using
only the cheapest tools, and power off.  I also measure the voltages and
signals with power on, but that is beyond the scope of this. 
Good luck, and glad to find some way to help you, considering how often
you have shared your knowledge of Apples with me.
-Paul