Zenith Z-100 Tips

ROM Upgrade

The Z-100 ROM came in several verisons. My machine orginally came with ROM version V1.2 (which fitted in a 8KB 2764 EPROM). I found that it could boot from an MSDOS disks formatted as 320KB (ie 8 sector per track). I had some disk images which were 360KB (ie 9 sectors per track) which the machine just ignored. I found that I needed to update the ROM, so I obtained a copy of the V2.5 ROM (16KB) and burned it to a 27128 EPROM. I had to change some jumpers (to change ROM size from 8KB to 16KB) and modify one of the decoding TTL chips (as per instructions here) which involved bending pins and adding wire links. Once the machine had the V2.5 ROM it could boot from either 320KB or 360KB formatted floppies making much more software available. The newer monitor also has a self-test facility.

Repairs

I had a few issues with my Z-100 for the years I had it, here are the various repairs performed.

The power supply is a unique switch-mode design, circuit diagram is in the hardware reference manual. Mine needed one electrolytic capacitor replacing, and re-soldering of the wires connecting the daughter board to the main PSU PCB.

The floppy drives are Tandon TM100A. Once a drive failed with a shorted tantalum capactor.

The machine once failed to power up. Removing drives and boards led to the failure being on the Z-207 floppy disk controller. The board schematic showed a capacitor in parallel with the +12V regulator input, C26 1.0uF tantalum. It doesn't look like a capacitor, it looks like a diode (see photos). I removed it and sure enough it was short circuit. I looked carefully with a magnifying glass and it said 25V rating, I replaced it with a 35V 1.0uF capacitor.

I had issues initially using the built in serial port (machine failed to transfer files using the ZIMP utility). I loaded the ZDOS diagnostics, the serial port internal loop back was fine but the external loopback (with a straight 25-way cable from J1 to J2) failed. The diagnostics gave a list of 4 buffer chips to replace, either 75189 (MC1489) or 75188 (MC1488 ). I started with U245 75188, which I replaced with a MC1488 equivalent (had to remove the S-100 card cage to get to it). The diagnostics then passed the external loopback, and the ZIMP transfers to and from a PC worked fine.

This page was last revised on: 16/11/14