Troubleshooting a 5V Apple III

[Mike Maginnis]

I have a 5V Apple III that won't boot and could use some troubleshooting 
advice.  When I power it up, the LED at CR7 on the motherboard and the 
keyboard lamp both light up and I get a brief flash on the internal 
floppy light and nothing else.  No video or activity of any kind.  When 
I hit CTRL-RESET, I do get the expected clicks from the internal 
speaker, so I know there's some life there.

I've swapped out the floppy drive and cables, and the power supply with 
known good units, and reseated all the socketed ICs and get the same 
results.

There are no expansion cards or other peripherals attached to the machine.

Can anyone suggest a starting point for tracking this down?

Thanks.

- Mike