The MZ-80K is Sharps first real computer introduced in 1978. This one was purchased in a non working condition from Japanese Yahoo Auction. When it arrived it was not in great physical shape. There was rust on the front of the machine where the paint had rubbed off from many years of hard use

The unit was missing Z80 processor and character ROM, I found English ROM file online and burnt a 2716 eprom. I could not find the Japanese character ROM files, so keyboard graphical characters did not correspond perfectly to display characters. Some time later I purchased a MZ80K2E in much better condition and I was able to dump the contents from the japanese character ROM and use that to burn a new, and correct ROM for this machine.

There is a small hand made board wired to the motherboard. And the motherboard was significantly hacked up with wires running all over the place. I didn’t know what any of this did so I removed it all. This PCB contains 74LS04, 74LS107, 74LS00, resistor, dip switch and LED.

With the hacks removed and some cut traces fixed I was able to get an image. A screen full of random characters.After quite a bit of probing around I found some floating pins on a LS244 & LS245 that were supposed to be grounded. Manually grounding these to the GND line brought the computer to life. However there was still an issue with the display repeating itself every 8 characters!

neoncluster.com

After hack removed

Display repeating every 8 characters

All fixed and running a memory dump

With the hacks removed and some cut traces fixed I was able to get an image. A screen full of random characters.After quite a bit of probing around I found some floating pins on the LS244 and LS245 that were supposed to be grounded. Manually grounding these pins brought the computer to life. However there was still an issue with the display repeating itself every 8 characters!

I probed all the pins around the A3 line and indeed some were low. I eventually found that a tiny piece of solder (so tiny that I had trouble seeing it even with magnifying glasses) was shorting A3 to ground on one of the 2114 RAMs. On power-up the machine was working 100%.

A big thanks to Alex Freed for his help on this one.

Conclusion

I was a little upset that this computer was not in great physical shape, and I believe the seller tried hard to hide the fact in his auction postings. However, I really had fun fixing it. Finding that last tiny bit of solder took some effort, and I could have easily missed it and assumed the problem was elsewhere.

I no longer have this machine as I later purchased a MZ-80K2E in much better condition and the MZ80K was sold.