boot problem on Apple iic

[Matthias Söllner <soellner.matthias@t-online.de>]

I have here two Apple iic computers. The first one works fine. The other
doesn't boot from
disk. The diskdrive begins to run, but the sound is not as strong as the
working one.
Then the error "Check Disk Drive" occurs. So i changed the two
diskdrives. The good one ran with
both drives. The bad one with none of them. So I suggest the error is
not in the drive. 
The logic board of the bad one looks good. There's no visible damage. So
I have two questions:

- Is ist possible the error is in the Powersupply ? 
I thought of it because the Sound of the disk drive is a different, not
so loud at all.

- Or is it possible that some bits in the boot-rom have changed?

- What else can it be?

thanks for every reply

Matthias