AppleColor RGB Monitor (IIGS) help?

[Mike van Bokhoven]

Well, the Apple monitor is running fine now. Details below for anyone 
who's interested.

> The most basic test is to remembner that a transistor has 2 diode 
> junctions, one between base and emitter, the other between base and 
> collector (no, you _can't_ make a transistor from 2 diodes!). So you can 
> test each of those junctions as a diode using either an analogue 
> ohmmeter, or more likely these days the diode-test range of a DMM. 
>
> I'll assume you're using the latter. Desolder the transistor form the 
> circuit and connect the meter, on the diode test range, between the base 
> and emitter. Then try it again with the proes the other way round. One 
> way should read about 0.7V (forward biased Si junction), the other should 
> be 'overrange'. Then do the same tests with the meter conencted both ways 
> between the base and collector terminals of the transistor. Again, one 
> way should be 0.7V, the other way 'overrange'.
>
> If either junction measures 'overrange' both ways round, it's open. If it 
> reads 0V (or 0.7V) both ways round it's shorted.
>
> This will actually find a lot of defective transistors. Most failures end 
> up with one of the junctions open-circuit. 
>   
First thing I did was remove the transistor I initially suspected and do 
the test as above. My multimeter proved a bit strange - it read 
approximately 1000 (unspecified units...) in one direction for both 
'diodes', and out of range for the other. That seemed like it might be 
reasonable, but I figured I'd try replacing it anyway. The replacement 
showed exactly the same behaviour - no great surprise. So, I decided to 
look further upstream, and actually take note of the waveform voltages. 
There was something very odd; the input from the blanking pulse 
generator was about 10V peak to peak, fed to the input of the amplifier 
via a voltage divider, but the signal at the base of the first 
transistor was only about 50mV peak to peak (not what I expected given 
the divider's values). That made no sense - I checked the resistors, and 
then pulled that transistor and checked it. It looked exactly the same 
as the first. Next, I checked the waveform at the amplifier's input 
without the transistor fitted, and it was much higher voltage. And more 
surprisingly, I had what appeared to be a perfect picture. I figured I 
just couldn't see the re-scan lines due to the fairly bright picture. 
Loading different software to get a mostly black screen, the lines were 
obvious. So I replaced that transistor, the lines disappeared, and the 
monitor once again has a very nice picture. I'm very pleased with that 
success - thanks to everyone who offered suggestions, especially Tony!

> If a transistor passed this test, you need more complex equipment to test 
> it. Basically you bpass a small current trhough the base-emitter juction 
> and see how it affects the collector current -- in other words you use 
> the transisorr as am amplifier. Real enthuisats have a thing called a 
> 'curve tracer' which does this automatically and plots one of a range of 
> voltage-current or current-current traces on a CRT. But I find my Tekky 
> 575 (such an instrument) rarely gets used when I'm faultfinding, the 
> defective transsitor can be found with simpler tests.
>   
In my case, perhaps a tool like that would have found the bad transistor?
> I don;t recognise that sort of number at all. US bipolar transistors tend 
> to have numbers starting 2N, European ones are things like 'BCxxx or 
> BFxxx' and Japanese ones start 2SA..2SD (often the '2S' is omitted, but 
> you still start with a letter). 
>
> My suggestions is to look at the maximum voltage in this part of the 
> circuit (is it just 12V'? and to pick a small-signal transistor of the 
> right polarity (NPN or PNP). The well-known 2N3904 (NPN) and 2N3906 (PNP) 
> would proalby do. 
>   
In this case, I had some random BC547s and BC557s floating around - 
these are what I used to replace the odd transistors in this monitor.

Now, if only I could get one of my three faulty Apple II Pluses running 
again... I've tried that before with no success.

Mike.