Commodore SX64 probs...

From: Jules Richardson <julesrichardsonuk_at_yahoo.co.uk>
Date: Sun Jul 18 15:39:18 2004

On Sat, 2004-07-17 at 22:28, Tony Duell wrote:
> >
> >
> > 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've stripped one machine down. It has three main boards, labelled as
CPU, floppy drive controller, and I/O.

Prominent ICs on the CPU board (all MOS apart from the EPROM):

  6510 / CBM (40 pins)
  6569R3 (40 pins)
  906114-01 (28 pins)
  901226-01 (24 pins)
  6581 (28 pins)
  901225-01 (24 pins)
  2564 EPROM

Prominent ICs on the FDC controller board (again, all MOS apart from RAM
/ ROM):

  6502 (40 pins)
  6522 x 2 (40 pins)
  325572-01 (40 pins)
  325302-01 (24 pins)
  2564 EPROM
  2116 RAM

.. plus the I/O board has a pair of MOS 6526 40-pin ICs.

The reset from the switch on the front of the machine hooks into the FDC
board and appears to reset the 6502 there, which then must in turn reset
whichever is the CPU on the CPU board...

> 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?)

I've read elsewhere that PLA chips in C64's die often and randomly,
however there's nothing labelled 82S100 in this machine. Maybe it's
labelled with a different part number for these machines? Far as I know,
the machines are supposed to be C64 compatible...

> > 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 :-).

:-)

There's no output at all from the supply on the +5V or +12V lines, but
the output from the bridge rectifier within the first parts of the PSU
circuit are giving a good DC reading, so it's not something simple.

These PSU's have a pair of little sub-PCB modules mounted on them, one
for 5V and one for 12V, each with an MB3759 IC on them - however as I'm
getting nothing on either power rail it suggests the fault's elsewhere
in the PSU.

Leave that one with me, as I've at least got the other PSU which works
and I can take readings from. I was hoping that someone would come
forward with schematics, or that there was a known failure point with
the supplies in these machines.

cheers

Jules
Received on Sun Jul 18 2004 - 15:39:18 BST