From: joe@user.com
"Kallikak" wrote in message
news:1186057585.839822.227320@o61g2000hsh.googlegroups.com...
> I've done a few checks now, but no joy. The interconnect cable checks
> out ok, and the behaviour doesn't alter with wiggling etc, so though I
> still plan to replace it, it doesn't seem to be the source of the
> fault. Since the problem relates to the character scan lines while
> row and column locations are fine, I have checked Z12 with a logic
> probe, and it's definitely pulsing at L1, L2 and L4. Correspondingly
> there is activity at RS1, RS2 and RS3 for Z29 (the character
> generator). The character generator is not a 6670 though - it has
> rather faded markings, but they look like a motorola sign plus the
> numbers 3108001 and 7741 followed by N or A. I also checked Z10, Z26,
> Z27, Z29 and Z30 (the chips suggested in the technical manual) using a
> logic chart from the SAMS service data book for the model 1, and they
> also checked out fine. One other thing I did was piggyback a new
> 74LS93 at Z12, but that also made no difference. I don't have any
> fancier test equipment than my multimeter and logic probe. I guess
> the next thing is to check the traces manually for shorts...
>
> Ken
>
Ok, if EVERYTHING ELSE is working correctly, it's the ADDRESS line
that ties to the specific scan line of the character generator that is
screwed up.
Since you say the first line is doubled, then I would suspect that the
SECOND
significant bit of the scan address is MISSING. Also, since you say you
have
4 scan lines and then blank, I would suspect there is also trouble with the
FOURTH scan line. Check both of these to the CG ROM and work
backwards.
I don't have a schematic handy, and it's been 24 years since I've been in a
Model I, so sorry I can't be more helpful, but really, the system is pretty
simple. You have to identify the problem first, then figure out where to
look
for the problem.
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
|
