Apple //c Sporadically Crashing into the Monitor

[Stephen Adams <sea4929@earthlink.net>]

Hi folks.

What would cause an Apple //c to sporadically crash into the Monitor? It 
happens at sporadic times; sometimes waiting until after I've been 
working a few minutes or crashing immediately after turning the computer 
on without a disk in the drive. While the Monitor is dumping memory 
register contents to the screen, the disk drive light will occasionally 
come on for varying lengths of time.

Sometimes I can clear this garbage with the usual Monitor exit of 
CTRL-C; other times, only a power recycle will do. If I let the Monitor 
have at it for long enough, it'll display an entire screen full of 
alternating lines of inverse "@" signs and half-toned blocks (with the 
odd inverse/flashing/normal characters scattered randomly about the 
display).

My //c has a 5.25" drive inbuilt, and when I first bought it, the 
motherboard was upgraded so that it would work with the UniDisk 3.5" 
drives. Externally, I have one UniDisk drive and one Laser 5.25" drive, 
although the computer will crash into the Monitor without those connected.

At one time, I had an Applied Engineering Z-RAM Ultra 3 memory 
expansion/Z80A co processor board installed, but that has since been 
removed. This expansion board, when installed, required the CPU and MMU 
chips to be displaced from the motherboard to itself (as the Z-RAM used 
those chip sockets on the motherboard as its data pathway). Upon removal 
of the expansion board, I thought that perhaps I didn't reseat the chips 
back into their proper motherboard sockets correctly, but reseating them 
didn't help.

Could either of the CPU or MMU chips be bad? Could the UniDisk upgrade 
be causing this? Is this a thermal issue (though the problem happens 
even after the top of the case was removed)? Could this be caused by a 
failing power brick?

I would appreciate any insights into this problem. I was given 
approximately 150~200 "Software of the Month" diskettes that I'd like to 
explore. Perhaps, copyrights willing, I could even turn them into disk 
images and upload them to a suitable repository. But I can't if my 
beloved //c is acting up.

Thanks in advance.

Regards,

Steve Adams