Troubleshooting a 5V Apple III

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