DEC Rainbow PC100-A

[tony duell]

IF the PC100-A is like the PC100-B (which I have full schematics for)...

> All LEDs on, means, "replace the system board".
> 
> I'm not sure, he wanted to hear that ;-)

And it's not even right!. You can have all LEDs on with a perfectly good 
system board. A fault in the power-OK circuit in the PSU could cause it!

OK. As has been suggested the LEDs are turned on when the hardware is
reset (they are simply connected between the outputs of TTL latches and +5V,
said latches are reset to 0 by the hardware reset signal) and then controlled by
the processors. So all on suggests the processors are not running. Half of the
LEDs are controlled by the 8088, the other half by the Z80. The 8088 is reset by
the hardware reset signal, one of its output ports controls the Z80 reset line.

So it is likely that the 8088 (at least) is not running. I would start there. Check its
reset and clock inputs, does it seem to be accessing memory, etc.

-tony