21 Jul
2004
21 Jul
'04
8:56 a.m.
One trick that should work on LS type logic chips is
to bend the pins in so
you end up with a chip that will sit on top of another one and make
reasonable contact with the chip below. Bend the output pins up, then hook up
a scope. Use one channel to check the chip that's in circuit, use another to
check the known-good chip on top. Solder the chips together if you want
(probably a good idea) but DO NOT connect the output pins together.
HP actually made an instrument that worked like this. It was called a
'Logic Comparator' IiRC. You put a little PCB inside the instrument
containing a chip identical to the one under test, said PCB also had
links to define pins as inputs or outputs. There was a test clip that you
fitted over the chip under test, and the instrument displayed any pins
that were difference between that chip and the known-good one inside the
instrument.
I've never used one, but it seems to br rather lucky-dip servicing rather
than actually undertanding the problem. I find it easier to look at
signals and work out if they're incorrect and if so, why.
Obviously, if the logic differs, then the chip on the
board is suspect.
I'd also be tempted to desolder the video RAM, fit some turned-pin sockets to
the board (Augat sockets are fairly cheap if you buy them by the tube,
they're also pretty good quality - Farnell sell them), then replace the RAM
chips one by one (or two by two, whatever it takes to get a full byte). I
Dragons have 8 DRAM chips (either 32K bit or 64K bit) which provide the
main memory and video memory. It's one chip per bit, all 8 for the byte.
don't know how the video generation in the Dragon
works, but I'd check the
video RAM, main RAM and character generator ROM first, then move onto the
logic circuitry.
The Dragon is based round 3 chips. The 6809E processor, the 6883 SAM,
which handles address decoding, DRAM control, video address generation,
etc, and the 6847 VDG which contains the character generator, video shift
registers, etc. Since you're getting stable video, I guess the SAM and
VDG are doing something (maybe not the right things, but let's assume
they're OK for the moment). It sounds like the SAM and RAM are not being
initialised properly, which sounds like a CPU, ROM, or RAM fault. Or PSU,
reset circuit, or...
-tony