23 Aug
2010
23 Aug
'10
2:30 a.m.
On Sun, Aug 22, 2010 at 1:32 PM, Tony Duell <ard at p850ug1.demon.co.uk> wrote:
The PSU was borrowed from another, working 5V Apple III, so I'm
confident it's good.
Thanks for the suggestions. I'll pick up a logic probe and report my findings.
- Mike
The motherboard has both composite and digital XRGB video outputs. ?I've
been working with a composite monitor, as I don't have a compatible RGB
model. ?Unfortunately, I don't have a scope or anything more advanced
than a multimeter to troubleshoot with.
I don't know the Apple /// at all, so I can only give very geenral tips..
The first thing I woudl is check all the supply rails at the mainboard
with your voltmeter. I know you say you've swapped out the PSU, but
excessivel load on on rail could be pulling it down. And do you know that
the replacement PSU is good?
Is there any way you can get a cheap logic probe? A 'scope or logic
analyser would be great, but you can do a suprising amount with just a
logic probe and it's a lot cheaper.
If you can get a logic probe, I owuld look at the sync signal(s) on the
RGB socket. At least see if there are there (which would indicate the
video timing circuitry was doing something). And then see if there is any
activity on the CPU addres and data pins, the address and data pins of
the ROMs and RAMs, etc.
-tony