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My Heathkit/Zenith Z-90

The Heath Company was an aircraft and electronics manufacturer founded in 1912. As the late seventies rolled around, like many companies, they chose to dive into the home computer industry. It was very common in the late seventies to early eighties to sell kits rather than complete computers. The Heathkit H88 was the first truly user-friendly computer system produced by Heath. With the addition of a floppy disk drive and hard-sectored (100KB) floppy disk controller, the H89 was born. Following the purchase of Heath by Zenith, the system was renamed the Z-89. Yes, the addition of the hyphen in the title was part of the new name. The Z-89 had 48KB of ram, where many systems of the time were being released with 64. My computer, the Z-90, was the same system plus the addition of a 16KB memory expansion card.

History of My Z-90

Back in 1981 the local city government purchased two Z-90's for their power grid control center. I can only guess that the system was used for financial or statistical information because Supercalc, which was a common spreadsheet program for the time, is lightly burnt into the screen. Inside the binders that I received with this system I discovered a few memos from city workers back in the 80's pertaining to modifications they made to the computers. I'm sure they never guessed how helpful those memos would be to a random guy some day! I was lucky that my system came with complete documentation. I have a binder for Z-90 hardware, Microsoft Basic-80, the CP/M operating system, and Supercalc. That includes complete schematics for the circuit boards. I received about twenty diskettes with the system.

Arrival Condition

The dusty, dirty Z-90 showed up with squares of double-stick foam stuck all over it. Not to mention, the power cord was chopped off only three inches from the base. The computer hardware is accessed by sliding latches in the side, as shown on the right. It took me a little bit of trial to understand how they worked. Of course the graphic there I found much later. One of the latches was bent badly where someone had tried to pry it upward with a screwdriver instead of sliding it sideways.

The diskettes I received were so deteriorated that I nearly didn't have a working operating system. Supercalc would boot, but would not warm-boot CP/M to allow me to copy it to a new disk. The other disks that would boot gave me several error messages about bad sectors.

The Z-90 also came with an external diskette drive. The drive was getting very hot, and even though my oscilloscope showed good power output, the capacitors were all bursting!

Making It Work

I started by peeling off adhesives and scrubbing the system down until every speck of dirt had been removed. After an hour of cleaning I still had no clue if the machine even worked... so I cracked it open and removed the top. Before I could test anything the cut power cable had to be replaced, so I unscrewed the AC input box and it dropped out the bottom. I unsoldered the old power cable and replaced it with a new one. After disconnecting the main computer from the power supply, I plugged in the new cord and tested the power supply output voltages and oscillation. To my relief, they were all correct. For the first time in at least fifteen years, the machine turned on. I half expected it to spit dust out the vents or something. This is actually the only 8-bit computer I have seen with a cooling fan.

The BIOS on this machine is unlike any I've seen. It allows me to modify and view memory addresses, can scan the RAM for errors, and as I eventually discovered, can be told to boot from the diskette drive.

I used the focus adjustment to make the display a bit clearer, but I was unable to resolve one issue. The CRT, which is a green phosphor monochrome display, has a strange glow on the right, as shown in the graphic to the right. The glow is very faint and can only be seen in a dark room.

Before reading any disks I cleaned the reader heads inside the floppy drives. The last modification I made to the hardware was to fix the faulty latch component, which I pounded flat with a hammer.

Even though I had quite a struggle with the corrupt floppy disks, I eventually found a solution. I took four system disks and merged the good files onto a new system disk, which I made bootable with the one disk that could still boot. I've made several copies of that disk. Without it this system would be a brick.

A Little Mystery

This Z-90 is like all other Z-90's in every respect except one. Instead of the Zenith Data Systems logo on the front, it features the manufacturer of the soft-sectored floppy disk controller, Magnolia Microsystems. I've made countless searches and I cannot find any record of a Z-90 being released under the Magnolia name. If you or anyone can answer my questions about the logo plate, please, I beg that you contact me and share your knowledge! I don't like unknowns.

System Specifications

The specs of the Z-90 are very similar to other Zilog Z80 based systems. For example, the TRS-80 Model 4 and North Star Advantage have essentially identical specifications. Nearly everything ran on the Z80 processor and sported 32-64 KB of RAM.

Processor 2.048 MHz Zilog Z80 (one for the terminal, and one for the main system)
Memory 64KB total (48KB on-board, and 16KB expansion memory)
Display 12 inch green phosphor monochromatic CRT
Video Output 80 by 25 characters (characters of 8 by 10 dot-matrix)
ROM Size 2KB or 4KB (depending on configuration)
Communication Interfaces 3x RS232 parallel connections, 1x standard external floppy disk connector
Storage Devices A single 5.25 inch double-height single-sided double-density floppy disk drive (support for up to four double-sided double-density drives), support for standard cassette writing, reading, and audible playback
Manufacturer Heath, Zenith Data Systems, and Magnolia Microsystems.


Like most Z80-based systems, this computer was not made for entertainment. Unfortunately, that means the software selection I received is small and boring. The good news is, essentially any CP/M software I can get my hands on will run on this machine. CP/M was a big operating system back in the day.
Supercalc was a standard spreadsheet program for CP/M and many other operating systems. It operates similarly to Visicalc for the TRS-80 Model 4. You navigate the spreadsheet with the arrow keys and fill spreadsheet addresses with values, text, or equations.
Microsoft Basic-80 was the standard on many computers for Basic programming. I was very lucky to have recovered a copy of this program. The only working copy I had was integrated into my Supercalc disk.
Written By: Erik W. Greif
Published: 05/17/2014 21:01PST
Modified: N/A
Article Title: My Heathkit/Zenith Z-90
Article URL:
Website Title: Bit Fracture Online
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Media Type: Blog Post/Technical Article
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