H89 #J141KXXX in repair

Last edit Nov 8 2011 (c) Herb Johnson. See my general Web page on Heath-Zenith computers of the 1970's and 80's for more information about my interests. Also see my Web pages on computer restoration. - Herb Johnson


Notes and photos on H89 s/n J141K001. Various dates stamped on boards and cabinet, plus date codes on chips, suggest this computer was built in 1979. I tested the unit in 2009 and it worked. When I retested in Nov 2011, it worked still!

h89_2_1.jpg is photo of H89 from the front with lid up. The paper note on the front says "H89A 64K, H17, DS40T drive, 3 port, flicker-free, set to...". That describes the model, RAM, the hard-sectored floppy controller, the disk drive features, a serial parallel card. "Flicker-free" is a Lee Hart modification to the terminal card that eliminates artifacts when text is passed to the display card.

h89_2_2.jpg is photo of H89 looking in from the top.

h89_2_3.jpg is photo of H89 looking in from the top, at the 16K memory expansion and the ROM and jumper area of the CPU board. The ROM labled "444-61" describes the monitor version indirectly, it's the Heath part number. The lower right of the photo shows a few jumpers which set the memory "size" based on available 16K X 1 DRAM chips. h89_2_7.jpg shows the ROM and jumper area in more detail.

h89_2_4.jpg is photo of H89 CPU board, showing (on the right) the three-port serial and parallel I/O card. There's an empty slot for a third card; the hard-sectored controller is behind the I/O card. The bottom of the photo shows a blue DIP switch, which determines many operating features of the H89.

h89_2_5.jpg is photo of H89 of the top of the power supply. h89_2_9.jpg shows the back of the power supply.

h89_1_6.jpg is photo of H89 keyboard. (There was tape on the area below the keyboard, which I digitally "erased" from the photo.)

h89_2_8.jpg is photo of H89 back panel. Connectors for serial and parallel ports, one 5.25-inch floppy cable. The label on the back says "Z90-81 J141K001 LOT 44370".


I powered up the H89 using a Variac, to monitor DC voltages and check for shorted capacitors. Everything looked OK, and when I gave it full AC voltage, I got the ROM monitor prompt on screen!. To access the memory test program, I had to enter in the ROM location for it. h89_2_10.jpg is photo of the Substitute command for 0000H up. It's oddly positioned because I'm running the H89 at 80 volts AC! It looks better at full voltage.

h89_2_12.jpg shows the memory test program boundary of 377377 (octal, or FFFFH, or 64K). I did not do any "heat tests" to see if RAM performed at elevated temperatures.

floppy testing There is a 360K half-height floppy drive, 40 tracks double sided, on the H89's hard-sectored controller. The ROM monitor has a "Boot" command. Reading the manual tells me "boot" goes to the primary floppy controller, "boot secondary" to the secondary floppy controller. I had a diskette available from my prior work with this machine, and photo h89_2_11.jpg shows that it booted up on the first try!

My next problem, was a lack of bootable diskettes. I could not locate more than a few disks, hard or soft sectored, that were "bootable" and also ones I could "sacrifice". Old drives and older diskettes, have nasty habits. If the head gets dirty from debris (mold, bits of magnetic oxide), it acts as a scraper and actually scrapes magnetic coatings from the diskette. Old diskettes can lose cohesion, and their media coatings get brittle. That's a bad combination that can damge subsequent GOOD diskettes.

So, I'm stalled from further testing, until I can get good boot disks, HDOS or CP/M, hard and soft sectored. I'm working on getting those from my colleages. Since I do have a bootable disk on this machine, however, I'm going to add a second drive and set up a means to copy it. This particualr disk has no software on it, so I'll have to mix and match to create a useable CP/M disk with software tools on it.

- Herb Johnson

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Herb Johnson
New Jersey, USA
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Copyright © 2011 Herb Johnson