AppleColor RGB Monitor (IIGS) help?

Tony Duell ard at p850ug1.demon.co.uk
Thu Apr 23 13:09:10 CDT 2009


> 
> 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.

Right. My question was really 'There isn't a PAL or NTSC decoder in the 
monitor, is there?'

> 
> > 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 

That's about what I'd expect on a colour CRT video amplifier. FWIW, I'd 
expect about 80V on the power rail for a monochrome one.

> applied to the gun cathodes constantly; only a tiny AC component.

Right. I assume the cathodes are connected to this supply through load 
resistors. And that they also connect to the collectors of NPN 
transsitors. 

Now, the higher the votlage on the cathode, the greate the effective -ve 
voltage on the control grid (g1), and thus the more the CRT is cut off. 
With the cathodes at 200V, the CRT is going to be pretty much cut right 
off, hence the total lack of illumination on the screen. So that makes sense.
 
> 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 

Does the emitter of the second transistor connect to the collector of the 
first? That's quite a normal circuit, and yes the second transistor is 
bart of the blanking circuit (when it's turned off, the cathodes go up to 
the +200V rail, and the CRT is cut off).

> 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. 

OK.

> 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 

I asusme that 12V supply is present and correct (I've learnt to check 
just about everything when looking for a fault...)

> 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.

Right. Could well be the case.

> 
> 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...

What you say sounds logical, and it roughly matches what I'd expect to find..

> 2 - Could I be leaping to the wrong conclusion in blaming that transistor?

You might be right, you might be wrong.

> 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?

Why not test the transistor. Desolder it, and at least make sure the e-b 
and c- junctiuons test as diodes. This doesn't prove it has any gain, 
but it'll find open or shorted transsitors. I can't beleive that 
transistor is very critical, you could probably replace it with just 
about any small-signal transistor of the right polarity (NPN .vs. PNP)

-tony



More information about the cctech mailing list