A few weeks ago I picked up a small haul of vintage computers fairly locally from someone who got in touch with me via my website, including a Sharp MZ-80K in its original box – these are pretty cool computers and I’ve been after getting one for a little while.
The computer was in good condition, but required cleaning and servicing due to age – these machines apparently suffer from shorting tantalum electrolytic capacitors on the mainboard (similar to the IBM PC) and PSU as well as unreliable 4116 DRAM, and I also wasn’t sure whether the internal PSU has RIFA mains filter capacitors (which like to go up in smoke) and would need to be removed or replaced.
I first removed the mainboard for servicing before attempting power on.
I cleaned the mainboard up as it was pretty filthy, cleaned all of the IC sockets with contact cleaner, inspected the board for any bad solder joints, and replaced all of the tantalum and aluminium electrolytic capacitors with high-quality modern parts. See my general advice for electrolytic capacitor replacement for more detailed information on the process.
This machine has been upgraded to 48KB RAM at some point in the past, as the 16K low bank is original Sharp parts and the upper two banks are ITT parts with later date codes (there is also a tag on the box which says “upgraded to 48k”, so call me Sherlock). As all the 4116 DRAM ICs were socketed, I took the opportunity to test them in my DRAM tester as these are pretty unreliable parts – sure enough, nine of the ICs had failed, either with short-circuits, bad blocks, or out-of-spec data retention periods.
I also pulled the internal power supply for inspection to see if it has any RIFA mains filter capacitors – it seems to have a polymer XY capacitor soldered directly to the power switch, and all the electrolytic capacitors look physically OK.
I cleaned the PSU up quite thoroughly as it was pretty dirty, then tested it without load and it seemed to work OK – its three outputs (+5Vdc, -5Vdc, +12Vdc) were all within tolerance. It has four 10uF 55V tantalum electrolytic capacitors which are apparently a common failure point, so I will probably go back and replace those in future too – and probably the output capacitors too while I’m at it as these usually have to work pretty hard.
I cleaned the inside of the computer and reinstalled the PSU and mainboard for testing.
Powering the computer up for the first time, it seems to boot OK into ROM! The display works OK, the power LED works OK, all keys work OK, and the tape deck works OK.
