Troubleshooting a 5V Apple III

Mike Maginnis mmaginnis at
Sun Aug 22 21:30:28 CDT 2010

On Sun, Aug 22, 2010 at 1:32 PM, Tony Duell <ard at> wrote:
>> 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

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

More information about the cctech mailing list