19 Apr
2009
19 Apr
'09
10:49 a.m.
Hi,
Just thought I'd ask if anyone has the service info for the standard
IIGS colour monitor (official name seems to be "Applecolor RGB
Monitor"), in this case A2M6014X.
I don't have it, but I am not sure it'd be any more use to you than the
schematics. Very few monitor service manuals (particularly ones for
small, relatively simple montiros like this) have any form of
fault-finding charts. Most of the time you're expected to work from the
schematic [1] .
[1] On the few occasions that I have had fault-finding charts for
something I've been repairing, I've found them to be misleading and
next-to-useless. I find it a lot easier to work from the schematics and
figure out what should be going on.
A bit of background: When I tried to start up my IIGS a few days ago, I
was disappointed to get no life out of the monitor (though a spare
composite monitor was OK for temporary use). Signs pointed to a fairly
easy fix; the power supply seemed OK, it had HV, I could hear it syncing
when I turned the IIGS off and on.
OK, so the bits that normally fail seem to e working.
So, I put it on my workbench, grabbed the schematics
(http://apple2.info/downloads), and realised that I couldn't remember
much about how this works, but it's analog RGB, so how hard can it be? I
wound the screen pot up a little, and was able to see scan on the
screen; when I turned the IIGS on, the scan/retrace lines stabilised, so
at least sync seems OK. Checking test points, I got a nice clean signal
Right.
This is an RGB input monitor, yes?
on the input (TP1R, G, B I think), and also at the
output of the AN5356
interface (TP2R, G, B). But checking the drive at the tube socket, there
was little to no activity, no trace of the almost digital-looking signal
I was seeing on the other test points. However, I don't have a good
understanding of the voltages I should expect to see at those test
points, so the waveform may still not have been right.
OK, what is between the IC and the CRT cathodes (I've yet to see a colour
monitor where hte video signal is applied to anything other than the
cathodes)? Most likely some kind of transistor amplifier.
Start there. Since all 3 gelectron uns are affected, look for a common
cause. Either a missing power rail, or a blanking signal. Look at the
votlage on the transistor pins, do they make sense? Or are the
transistors always saturated or cut off?
-tony