Heathkit H89 Restoration

A little bit about the Heathkit H89
Heathkit had always been know for its useful and quality electronics kits. Dating back to 1926, they first offered a light airplane in kit form, moving to war surplus components in the 30's, and eventually to more modern audio and electronics test equipment, small projects, and HAM radio communications systems.

Since introducing the first personal computer in 1957, Heathkit continued to develop one of the most complete and innovative lines of personal computers and computer systems anywhere.
Following the success of their Heathkit H8 computer from 1977, Heathkit released the Heathkit H89 "all-in-one" computer in late 1979. The H89 used the popular H19 general-purpose terminal form fracture and a high-quality Z80 based computer board with a six location expansion bus. The H89 came with the MTR-88 ROM monitor program, which allowed operation at machine level without disk drives or cassette tape.
This system was available as both a kit (model H89) or as an assembled and tested system (model WH89) which cost $700 more.
While other computer manufactures were producing some computer systems in kit form, the practice was quickly fading from the market. Heathkit continued to produce computer kits thought the companies final years of operation.

Available: 1979
Original price: $1695 (kit with 16K DRAM)
CPU: Zilog Z80 @ 2.048MHz
RAM: DRAM - 16K, 32K or 48K
64K for CP/M with optional add on card
Interface: 12-inch monochrome CRT
80x24 text, no graphics
Storage: Audio cassette interface
Optional 102K hard-sectored floppy drive
OS: Heath DOS (HDOS)
The H89 I found...
The H89 I found was purchased on Aug 13, 2017 from a company in Florida via eBay.  The following three pictures show the condition of the unit just before shipping.

Unfortunately, like all to many shipments that I receive these days, the H89 arrived at the Astrorat labs with some damaged.

Two of the four CRT mounting posts where broken off close the the case's front bezel. The fourth picture shows the right hand CRT mount broken away. When I opened the H89 cover,  I found the the CRT was hanging from only the two top mounting posts closest to the case top. I have no idea why these two post survived while the two lower ones did not. Given the weight of the CRT, it really needed to be fully supported within the case or better yet removed from the unit and shipped in a separate highly padded box.

Luckily the CRT survived its ordeal and was found to be in good working order.

There was also indications that this H89 was stored in a high moisture location at some point which should be of no surprise given its Florida roots. Having a lot of corrosion and rust, the floppy drive had suffered the most and was not repairable.

There was also a far amount of cigarette smoke  haze on all the case surfaces, keyboard and CRT. Many of the white keyboard key caps where a dull yellow color. If you look closely at the picture bellow, you can see the discoloration on the side and back.

Nothing beats the smell of that damp, smoky stench.

I chose to not power up test the unit until after the tare-down.  I wanted to inspect and test each part of the H89 to verify that each section was good.

H89 system inventory:
DRAM memory size 48K
Hard-sector floppy drive card
Single 5 1/4 inch full height floppy drive
Three port serial interface card
Front view for H89 befor it was shipped
Side view of H89 befor it was shipped
Back view of H89 befor it was shipped
Showing one of the broken lower CRT mounting points on received H89
The Restoration Begins
H89 Disassembly

The first order of business was to do a complete disassembly and tare-down of the H89 to the main components.

First the bracket supporting the floppy disk and three port serial boards was removed. Then both the boards where unplugged for the main Z80 CPU board and removed. The four screws holding both the Z80 CPU and the main terminal controller boards where removed, the wiring harnesses where disconnected and both board assemblies where removed.

The CRT's yoke was disconnected, the CRT's main connection was unplugged from the back of the tube, the high voltage was discharged then disconnected, finally the two remaining upper mounting screws where removed. The CRT was carefully removed for the unit. Both the broken upper mounting studs where removed and set aside for later review.
CRT section. Note broken lower CRT mount
Inside case from top
The keyboard cover assembly was removed and the keyboard separated from the keyboard case cover. The high resolution reference picture was taken bellow for later reassembly. All the key-caps when carefully removed for cleaning.

The floppy drive and related mounting assembly was removed and set aside for further evaluation.

The DC power supply assembly board was unplugged and remove fallowed by the power supply support bracket that contained the three voltage regulators. Next the transformer was removed.
Finally the H89 case was disassembled down to it's individual components.

Cleaning and reassembly

Other than a good cleaning of the H89's enclosure to remove 35 plus years of accumulated dirt and cigarette smoke haze, there was only a few minor dings. Each case component was first washed with automotive soup and warm water using a lite cloth. For the second pass, Mr. Clean MagicErasers where used with only warm water.  This combination removed all the embedded dirt in addition to the yellow cigarette smoke haze.

Sine the H89 case is constructed out of painted high density foam, removing the dirt restored the case to the original Heathkit colors. No Retro-brightening was need.

Like the case above, all the keyboard keys where individually hand cleaned using a MagicEraser and warm water.

to be continued...
Current restoration status
So as you can see from the above picture, there has been some good progress on of my H89 restoration.

Updated the power supply to add three TO-3 heat sinks to both the 5 volt and the one 12 volt DC regulators - This upgrade was done on later releases of the H89 systems to help keep each of the voltage regulators running within their safe operational temperature ranges.

Retrofitted 12-AWG stranded wire between power supply and main CPU board - During my testing, I found that many of the ICs on the main CPU board where only receiving about 4.3 volt DC as measure at their power pins. Looking at the voltages being generated at the power supply showed a good 5.1 volts. It tuned out the wire being used to provided DC power between the CPU board and the DC power supply was only 22-AWG. The CPU board, with all the extra card plugged in to the bus, requires 1.35 amps of current on the 5 volt supply. The measured voltage drop found in the 22-AWG wire was 0.7 volts. I removed the too light 22-AWG wire and upgraded to 12-AWG stranded wire for both the +5 volts and power ground return. With the new wire in place, the measured drop is now only 0.05 volts and the worst supply voltage to an IC on the boards is 4.95 volts.

Retrofitted two 3.5 inch floppy drives along with a Virtual Sector Generator - I retrofitted two new 720K, 3.5-inch floppy drives in place of the very dead, single, full height, 5 1/4 inch floppy that came with the H89. Since 3.5-inch floppy drives are not know to support hard-sector disks some form of sector generator needed to be added to simulate the required hard-sector pulses needed by the H89's floppy controller card. On my H8 restoration project, I tested several designs that generated the needed hard-sector pulses. I settled on Mike Douglas' (not the actor) Virtual Sector Generator (VSG) from to fulfill this role for both my H8 and now my H89 projects.

Repaired the two CRT mountings posts damaged in shipping - I needed to install two 1/4-20 x 3 inch stainless screws through the case front. Unfortunately during the shipping of the H89 from Florida, two of the four CTR mounting posts had broken of close the the case's front bezel. When I opened the H89 case, I found the the CTR was hanging from only the two top mounting posts which seemed to serve for some reason. Since the H89's case was high density foam, I felt that just using glue would not last given the weight of the CRT. While I did not like drilling two hold through the case, I reasoned that having a good stable mounting base for he CRT was more important than the violation to the original case.

Replace one bad 16-pin IC socket in the DRAM area - Along with the normal tarnishing of the IC pins and sockets found on systems that are over 35 years old, there was one flaky SRAM location. This problems even continued are all the IC where removed from the system and the tarnish was cleaned off each IC's pins. Replacing the affected back of DRAM did not fix the problems. In the end it turned out to be a completely corroded pin on one of the IC sockets. The socket was removed and replaced and the original DRAMS re-installed.