AppleColor RGB Monitor (IIGS) help?

Tony Duell ard at p850ug1.demon.co.uk
Sun Apr 19 12:49:12 CDT 2009


> 
> 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




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