17 Jul
2004
17 Jul
'04
10:28 a.m.
I've got a pair of non-working Commodore SX64's here.
One machine powers up, but the floppy drive light remains lit and I can
hear the drive spinning continuously, and there's a 50% grey pattern
filling the screen (i.e. pixels look to be alternating white/black).
Actually, there's also short run of pixels middle-bottom of the screen
which aren't illuminated at all. Pressing reset has no effect. Pressing
caps-lock does result in the caps-lock light illuminating, but I don't
know if that's a simple circuit hardwired to the key and doesn't go via
the CPU...
Is this thing like a C64 + 1541 drive? In other words, does the disk
drive have its own 6502 CPU? If so, then you either have 2 faults, or a
fault in what little circuitry is common to both of them.
I would start by checking the PSUs (voltage and ripple). CBM machines of
this vintage suffer from bad IC sockets, so it's worth at least
re-seating all socketed chips, and maybe replacing the sockets (with
turned-pin ones).
Can the main CPU hold the disk CPU in a reset state? If so, then maybe
the LED and motor-on lines are asserted. Maybe the main CPU is not
initialising the serial bus correctly, and you only have a fault in that
area (RAM, ROM, is there an 82S100 PLA in this machine?)
The second machine is totally dead - no activity whatsoever. There's a
*very faint* humming noise from the monitor area, typical of a display
that's at least getting power though - but on the CPU side of things no
chips seem to be getting remotely warm, suggesting that there's no +5V.
A voltmeter will verify the truth of that last statement, but alas the
voltmeter that does tests over the internet has yet to be invented :-).
It'll be a lot quicker if you do said simple test(s) before I start
thinking of possible causes.
-tony