24 Aug
1998
24 Aug
'98
8:13 p.m.
I just bought an Apple color composite monitor that needs some work. It
comes on fine, and the screen goes to a bright white if I turn up the
brightness. When I plug it into the computer the picture doesn't change a
lick. I tried messing with all the settings to no avail. The picture
stays a field of white.
Any tips on how to proceed diagnosing this thing? I've got it opened up,
the digital VOM is on and the soldering iron is hot.
Sam Alternate e-mail: dastar(a)siconic.com
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September 26 & 27...Vintage Computer Festival 2.0
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24 Aug
24 Aug
9:04 p.m.
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25 Aug
25 Aug
12:21 a.m.
On Tue, 25 Aug 1998, Tony Duell wrote:
Excellent. That means (almost certainly) that the CRT
and flyback are
fine. So the fault is likely to be fixable...
BTW, is it the day for finding monitors? I found the original Gemini
monitor (greenscreen) for the Galaxy 2 I got last week. It doesn't work
yet, but I'll fix it.
These monitors are becoming increasingly easier to find, which is a good
thing because they are nice color monitors for any Apple ][ (and ///). I
got this one for $10. If I knew it wasn't working I would've left it
behind, but I plugged it in and saw video so I figured it was fine.
OK, a problem in the video path. Now, it's
composite, so there's an NTSC
decoder, right? And it's likely to be a single chip. Look for a crystal
(3.58MHz, most likely) and find the chip near it. Now look for data on
that (it's probably used in TVs as well - identify who really made the
monitor (FCC id?), and look at some Sams Photofacts or whatever).
FCC ID: BCG966A2M2056
The "A2M2056" leads me to believe this was an Apple manufactured monitor
since that is also the Apple model number for this monitor.
I'm disassembling the monitor now so I can pull the circuit board out and
get a better look. I've located a 42-pin chip (Toshiba TA7644BP) which is
most likely the decoder. I bet I have a Toshiba data book but its buried
in one of the many boxes of books.
Now start tracing signals. Is the chip getting power?
Is the video input
getting to it? Is the oscillator running (check with a 'scope). What are
the outputs doing? What about the video output transistors (on the CRT
baseboard?) Do the voltages there look sensible?
Well, the VOM batteries up and died and I don't have a scope. I'll get
some batteries and check what I can and report back. Luckily I have
another one of these monitors so I have a reference for normal. I've been
looking for a nice scope at one of the many local surplus shops but so far
the ones I've found have all been gutted.
Thanks!
Sam Alternate e-mail: dastar(a)siconic.com
-------------------------------------------------------------------------------
Ever onward.
September 26 & 27...Vintage Computer Festival 2.0
See http://www.siconic.com/vcf for details!
[Last web site update: 08/09/98]
1:06 p.m.
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