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From: rubywand@aol.com
Newsgroups: comp.sys.apple2
Subject: Re: IIGS Monitor problem...Help, Please
Date: 18 Nov 1996 04:26:45 GMT
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In article <h1JVYDW.tdon@delphi.com>, Tom O'Neill <tdon@delphi.com>
writes:

>
>While tickling the keys two nights ago and dling my e-mail from Delphi
>using Spectrum, there was a slight "poof" sound.  Immediately, the screnn
>went from text to a checkerboard pattern, multicolored...red, green, and
>blue, I think...and it began flipping about once or twice a
>second...scrolling
>upwards, that is.
>....
>

     Since you still have power for the drives, the "POOF" sounds like a
blowing electrolytic capacitor. (Sometimes, they explode; sometimes, they
poof.)

    The main suspect is one of the caps in your power supply; but, it
won't hurt to look and sniff around your Slot cards and the motherboard.
If you do not find anything, pull your Slot cards, replace the 65C816 uP,
and turn ON the computer.

    Check the +5V line voltage. (You can find +5V on Pin 25 of any Slot
socket.)  If the voltage is at or close to 5V (4.8V-5.1V), there is a
decent chance the 5V line is okay. Also check the other voltages (+12V at
Pin 50, -12V at Pin 33, -5V at Pin 34).

    If your system can boot ProDOS 8 from diskette and the display looks
normal, then, suspicion falls on one of the cards you've pulled. Carefully
examine each. You are looking for a capacitor with scum at its base; but,
if you see a burned resistor, blown diode, ..., this could be your
problem. (Diodes, however, seldom poof; resistors usually either pop or
fry. Globars can poof; but, then, you would be getting no power at all.) 
Coils can poof; but, this is fairly rare.

   Use your nose, too. Lots of times it can zero-in on the problem area. A
blown electrolytic cap smells a bit like burned bakelite.

    If your +5V line is reading low, the power supply should be pulled and
checked. One test is to clip a 10 Watt 10 Ohm resistor between +5V and Gnd
and measure each of the voltages from the output to Gnd. If any voltages
are low, then open the supply and look/sniff around. You should find one
or more caps with scum at the base and a distinctive scent and/or a
charred choke coil or a cracked rectifier diode..

    If the power supply seems okay, you can try increasing the load on the
+5V line by adding another 10 Watt 10 Ohm resistor in parallel with the
first. If it's still okay, suspicion shifts to some kind of short on the
motherboard, probably on the +5V line. 

    Replace any bad components.


Rubywand