AppleColor RGB Monitor (IIGS) help?
Mike van Bokhoven
mike at fenz.net
Sat Apr 25 02:53:27 CDT 2009
Well, the Apple monitor is running fine now. Details below for anyone
> 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.
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