AppleColor RGB Monitor (IIGS) help?

[Mike van Bokhoven]

Hi again!

I've spent a some more time with the monitor, following the CRT driver 
board schematic, with some good results (I hope). Thanks to Tony, I had 
a better idea of what to look for.

Tony Duell wrote:
> 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.
>   
Fair enough - I've given it a go.

> Right.
>
> This is an RGB input monitor, yes?
>   
Yes - analog RGB, as far as I can tell.

> 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?
>   
Done. The power supply to the gun drivers seemed OK - very close to 
200V. Sounds about right I hope? Of that, nearly the whole lot is 
applied to the gun cathodes constantly; only a tiny AC component. 
Between each output of the AN5356 and the CRT itself are two 
transistors. The first looks like an amplifier (the base is driven from 
the AN5356's colour output). For the second transistor on all three 
colours, the base is driven by the same line, which is derived from an 
output on the AN5356. I'm guessing this is the blanking part of the CRT 
driver. Looking at the output of that pin (#1) on the AN5356, I get a 
very regular waveform, which makes sense in the context of blanking. 
This drives something on the main board (I'm guessing the scan 
electronics...), plus a small two-stage amplifier driven from a 12V 
supply, which then drives the bases of all three of the final CRT drive 
transistors. Things look pretty good after the first stage of the small 
amp (a nearly square waveform), but the second transistor appears to be 
doing nothing, the output of the amp sitting lowish despite the good 
signal into the base. So, my suspect is that transistor.

The way this normally works is that there is a voltage divider (5.6k / 
330 ohms) that keeps the output low. The output transistor is placed 
across the 5.6k resistor; when it is driven, I expect its resistance is 
a lot lower than 5.6k, pulling the voltage high.What I'm actually seeing 
is the output voltage sitting at about 4V, with a very little AC 
component. So perhaps that transistor has failed, sitting at a constant 
resistance, and holding blanking on.

So, three simple questions for anyone who knows more than me!
1 - Does my understanding of the way this works sound correct? I'm 
guessing at a lot of this stuff...
2 - Could I be leaping to the wrong conclusion in blaming that transistor?
3 - How can I check? Could I pull the 5.6K resistor and the suspect 
transistor, replace the resistor with a 5K pot set to 5k initially, and 
wind the pot down a little, see if the picture comes back in some form? 
Good idea, or misguided?

Mike.