MARCH Fix-It, June 2012

Most recent revision of this page Nov 23 2012.


On June 30 and July 1 2012, the MARCH club sponsored a vintage computer repair event, at their site at the Infoage museum and science center in Wall NJ. Vintage computer owners were organized to attend by Bill Degnan, who is also a MARCH officer. Local arrangments were made by MARCH President Evan Koblentz. Several people brought a substantial amount of vintage computers to clean, test, repair and for use. These owners, and their tools and equipment, shared their resources and time and knowledge over the course of the weekend. Also, MARCH computing equipment was inspected and repaired by attendees and MARCH members.

The event coincided with a visit by Lee Felsenstein, a designer and document author of the Processor Technology SOL in 1977, and later a designer of the Osborne 1 computer a few years later.

This document will describe some work and show photos of some systems during this event, which are of interest to me. It's not a comprehensive description: other Web sites will have other descriptions and photos.

I discuss a number of these events on my Web site, as part of my general support for computer restoration. - Herb Johnson

Computers and hardware on site

Other setups are detailed elsewhere in this document. This section are some briefs about other attendee's work. I simply was not able to "cover" everyone's activities: it's not my job.

Ian Primus at one of his AT&T 3B2's , and rust on another 3B2
Ian checks out a terminal, one of several acquired from another MARCH member.
Ian reports Juy 2nd on the MARCH yahoo maillist, that he repaired a few 3B2's including MARCH's; copied some Osborne 1 diskettes; and generally helped others with their repairs.

Matthew Mikolay a student at Rutgers University, with C64 in test
what is that message?

Will Donzelli's 1962 Burroughs B122 card reader clean up Evan is shown in the foreground.

Mike Loewen works on this MARCH owned IBM 029 keypunch

Evan Koblentz, MARCH president repairs an Osborne... but in the process smokes part of an Osborne. (The curl of smoke is just above the front of the Osborne.) The failed component could have been this blown AC capacitor.

David Gesswein of brought a PDP-8 and worked on MARCH's PDP-8; he also looked over my 8/f as I cleaned it up. details are below.

reports from others

The following reports are courtesy of MARCH's Yahoo maillist.

Bill Dromgoole: "My main project for the weekend was to trouble shoot my Cosmac VIP that I bought at the last VCF. The March VIP was on the table with mine and I was able to compare them even though they were different models...[I was] next to Jeff Brace who was working on Commodore 64 boxes."

J. Alexander Jacocks: "I worked on a couple of projects: replacing the A/C filtering capacitors on an Apple IIe power supply, and repairing a Macintosh IIci (1989, 68030), which had gone mute, and started to shutdown after 30 minutes of use."

Jeff Jonas has a Web page on VCF-E 8.0.

Herb's travel and setup

Here's some of what I loaded up.
..and the load in my car before arrival.

Herb Johnson's Northstar Horizon setup, not used that day but available. See my discussion about my SOL system below for relevance.


Saturday dinner break Sunday lunch was much the same.
Sunday morning: clear skies, after a major storm caused distruction in the Balt.-D.C.-South Jersey area

Lee Felsenstein as he gives an informal talk. He discussed the value of learning binary logic with hardware. He advocates simple diode-driven logic for kids to learn on. The inserted image are his notes from later in his talk.

Processor Technology SOL computers

On Sunday, Bill Sudbrink brought his SOL setup. He had a working SOL complete with Northstar brand floppy controller and external floppy drive; and an IMSAI configured with Proc Tech boards to be SOL-hardware compatible. In this photo he discusses it with Lee Felsenstein. Thanks to Bill for copies of his SOL/N* boot disks.

Two SOL's side by side. Bill Degnan's on the right in front; Herb Johnson's in the back on the left.
In this setup, space between is cleared to see Herb's HP180 oscilloscope, Bill's Panasonic video monitor.A schematic of the SOL lies on the keyboard, stripped of the keys. These capacative keyboards can be operated "naked" by touching the keypads, as Bill demonstrates in this photo.

Bill tries an 8080 in circuit emulator on his SOL. It was not able to operate the SOL. All it would display when the ICE halted the processor, was this single line of random characters.

Fortunately for me, my (herb's) SOL came up to a prompt right after full power-up. It had been decades since this computer was last operated. The "prompt" is a little skewed because the monitor is not adjusted to match the SOL's video scan rate.

In this photo, my SOL is operated by Bill and shows a hex dump on-screen. Lee Felsenstein examines the CPU schematic, to determine the operation of the master clock and divider.In the background, David Gesswein is at work on the MARCH PDP-8 serial card, operated from David's 8.

PT co-designer Lee Felsenstein looked over our shoulder to verify the operation of the divide-by-seven circuit that feeds the 8080 with a 2MHz clock from a 14MHz crystal. He read the schematic and worked it out in front of me & Bill. He's "still running on all cylinders" as we old-guys say. He had something to say about making more engineers, later in the afternoon.

In this photo, Herb (left) gets an autograph for his SOL, from the designer Lee (right). In the background is Will Donzelli.

More discussion about my Sol computer, can be found on this linked Web page. I continued to work on my Sol's keyboard on this linked Web page.

floppy controller for Sol

I was prepared to add a working Northstar floppy controller. I brought a known-working original Northstar controller and single-sided drive, made for the SOL. That's why I brought the NorthStar Horizon system, if I had to construct a boot disk. But Bill's Sudbrink's SOL disks and drives were double-sided sided; and he did not arrive with his SOL and IMSAI until Sunday. So there was only time to ask him for copies of his SOL/N* disks. Thanks!

SOL keyboard repairs

Here's a series of photos about my (HErb's) work on restoring a SOL keyboard. Removing the keycap assembly, you can see the PC board. The keyswitches include a foam insert with Mylar/metalized pad (shown as brown). Here's the keycaps in closeup. One of the keys pads has been replaced with a new foam-rubber and Mylar-capped insert. This photo shows a bag full of those inserts. It's a tedious process to scrape out the old foam, add a dab of cement, and insert the new foam insert. This picture shows that effort in process, keycaps in various stages. In a later photo, I've removed all the degraded old foam and started to add new foam/Mylar inserts. I was not able to complete this process.

More discussion about my Sol keyboard and floppy work, can be found on this Web page.

PDP 8's on site

MARCH's 8/E and David Gesswein's Educomputer 8/M for side-by-side cleanup and testing. David's educomputer is an 8/M and fully working.. The MARCH 8 was in storage for many years and unpowered. Also, I brought my 8/F, also in storage and not powered in some time; it was only cleaned during the event.

Differences: David's educomputer 8 (8/M) and my (Herb's) 8/F are physically alike to boards and power supply. The MARCH club 8/E is somewhat different because it has slightly older boards, and two backplanes as is possible with the /E's power supply. The 8/E supports a front panel with a voltage line for lamps; versus the LED front panel as in David's 8/M system and my 8/F, which need no extra voltage. DEC labeled their 8's as either /M for OEM's to modify for resale, or 8/F for direct sales. Then there was the earlier 8/E model as owned by MARCH, with it's various differences as noted. David and I discussed these differences after the event.


After opening the MARCH 8, he decided to take it outside to work on. Discovery of this mouse nest of packing material, led to this discovery of the prior user of this PDP-8, and his last meals.

Additionally there was a lot of crud flaked off from the interior foam padding. David pulled all the cards to vacuum out most of the debris. After a through vacuuming, David disassembled the chassis down to the backplanes.

This PDP-8 consists of two identical backplanes, connected by a pair of Ommnbus jumper boards. David is holding one of the backplanes in this photo. A close inspection of one appears to be clean, but look for specks of green. In this closeup, you can see spots of corrosion, blue-green copper oxide. The back of one backplane shows oxidation. again, here's a closeup.

after cleaning MARCH's case, David reassembled the backplanes in the chassis and reinserted the boards. The bulk of his work past that, was to debug the M8655 teletype (serial) control card. He got it operating in his educomp 8 - an incredible effort of chip-level and software diagnostics.

In this photo, my SOL is operated by Bill and shows a hex dump on-screen. In the background of this photo, David Gesswein is at work on the serial card. He had to leave early to return to his home, which was without power (and likely will be through the following weekend).

David reported later on the MARCH maillist: "The backplanes were dated 1970 and 1974. It may have been a single backplane machine expanded later. The processor cards were M833, M8310, M8300. The M833 indicates its an early machine since it was replaced by the M8330 early in the 8/E life."

"The machine also has:
M837 Memory extension and time share
M8655 Teletype control (late model)
M882A Line frequency real time clock
M865 Teletype control (very early model, also later models are M8650)
M835 Positive I/O bus interface (early model, replaced by M8350)
M849 RFI shield
M8320 Bus loads
G227, G104, ?? core board (two sets) 4K core" - David Gesswein

Herb Johnson's PDP-8/F

There was not time or opportunity, to do more than clean up my PDP-8/F, and invite some comments by David Gesswein. As he did with the MARCH PDP-8, I removed cards and vacuumed my machine. David inpsected my system, noted it was in pretty good condition, with little of the rust and debris of MARCH's 8/E. I removed the foam pad under the top, which David noted as a potential source for corrosion, gunk and shorts.

Here's my 8/F after cleaning and here's a front view. For details of the cleanup, check my PDP-8/F Web page.

Herb Johnson
New Jersey, USA
follow this link to email me @ my email address

Copyright © 2012 Herb Johnson