IBM PS/2 Portable Restoration

The IBM PS/2 Model P75 486 (8573-401)

The IBM PS/2 P75 was launched in 1990 as a successor of the P70 from 1989. Both are microchannel based PS/2 machines fully equipped with harddisk, floppy, 10″ monitor and a fullsize keyboard housed in a briefcase sized, portable case.

The main difference between the two is that the P70 had a 386 processor while the P75 featured a 33MHz 486DX.

The P70 was the first portable PS/2 machine and the P75 was the first portable computer ever to sport a 486 chip, with a substantial price tag starting at 15,990 USD for the 8MB / 160MB base configuration.

A really cool feature of these machines is the gas-plasma display with its characteristic red-orange glow. This kind of display technology had a relatively short appearance in the portable computer market at around 1990 – it was used by Toshiba in the T3x00 and T5x00 series too – due its power consumption which rendered battery operation impossible.

Apart from that and the weight – the machine weighs ~11kg – it is a joy to use this computer. The keyboard is full sized and features clicky ALPS switches with tactile feedback (I like it even more than the model M) and the display is made for writing since its self illuminiating and has a CRT like contrast. The red glow is calming like a lava lamp 😉

Restoration

I got the machine at a real bargain from somebody who sold off a mixed bag of vintage electronics. He couldn’t tell me anything about the condition of this box, so I found that out when it arrived: it was dead – the PSU fan would spin, but that’s it. It also wouldn’t output anything on an external monitor, pointing to an issue with either the PSU, the mainboard („planar“ in IBM speech) or the graphics board.

To find out what was going on, I took the thing apart down to its individual electronic boards. Pulled out any expansion cards and removed all but two memory banks. Cleaned all the connectors and plugged things together on the workbench. At some point, I got intermittent output on the monitor however things weren’t stable – maybe in one of ten tries it did something.

In the end, I replaced all the electrolytic capacitors on the mainboard and the graphics board, some of them had leaked, leaving a fishy odor while unsoldering.

The capacitor fluid also had corroded a trace on the graphics PCB which led to the intermittent video output. I found it by „beeping“ out the traces from via to via. Fixed it with a piece of enamel wire soldered in to bridge the faulty connection.

GoTek disk drive emulator hooked up, booting the „Reference Disk“

The only thing I couldn’t get back to proper operation was the floppy disk drive which suffered from bad caps too but in addition also seemed to have mechanical issues. I replaced it with a gotek drive emulator – a genius piece of hardware that behaves like a floppy drive from the computers point of view and which takes disk images stored on a USB stick.

Please note: If you want to hook up a GoTek (or a standard PC floppy drive) to a PS/2 computer, be aware that IBM decided to go with a non-standard pinout on the floppy connector. While most is identical, IBM found it smart to integrate the power lines on the data cable. In particular, pin 6 carries +12V which almost definitely will fry a non IBM drive. The solution is to build an adapter cable that splits out +5V, +12V and GND and connects to the regular power connector.

With that out of the way, it was time to boot up the „Reference Disk“ – PS/2s didn’t have a CMOS setup menu, rather the setup software had to be loaded from disk. While this is annoying it allows for easy field updates of the software by making available new disks. Also, expansion card configuration is integrated into the setup software in way that hardware vendors supply software extensions that are copied to the main reference disk.

Once the machine is configured, it’s time to install software – a period accurate OS would be IBM PC-DOS 3 or 4 along with Windows 3.0 or alternatively, OS/2 2.1. Also MS DOS 6.22 or PC-DOS 2000 with Windows for Workgroups 3.11 will run just fine on this machine.

Since it has a VGA compatible XGA display adapter, it will run most DOS games of the era nicely, in particular DOOM is an interesting experience on the built in monitor 😉

IBM PS/2 P75 working with a HP7475A Pen Plotter

Upgrades

The P75 has four free expansion slots (the P70 has two), so in my opinion a network card is a must have and for retro-gaming a soundcard is a nice addition.

One of the PS/2 oddities is the Microchannel bus which almost no other computer maker used since a) IBM demanded licensing fees for its use and b) the Vesa Local Bus provided a fast alternative for graphics and disks with the PCI bus just appearing on the horizon.

Networking: I went with a 3com Etherlink-III MCA card from ebay which is relatively well available and provides 10MBit TP and AUI connectors. Setup is straight forward, copy the 3com stuff to the Reference Disk and let it configure. Load up the driver in Windows and you’re good to go (configuring the network is another thing though).

Sound: Original MCA – Soundblasters or the IBM MCA soundcard are „unobtainium“ from price and availability perspective, but there are remakes available. I bought a „Resound New Wave MCA“ from Texelec which is essentially a Soundblaster MCV clone with mostly the features of an ISA Soundblaster 2.0. It has the gameport MIDI which allows to hook up external MIDI synths to beef up the OPL-3 sounds.

Resound New Wave MCA in the top expansion slot

CPU: Now this one is a bit more difficult – the CPU sits on a „Processor Complex“ that connects to the „Planar“ – essentially a plugin-card that can be exchanged, IBM just never released any higher spec one…

Also, to further complicate things, the CPU is soldered on the card instead of sitting in a socket which makes the upgrade a fun event…or so.

Processor complex with the 486-DX CPU removed

Somehow I managed to unsolder all 168 pins and to remove the CPU without damaging the processor board. Then I soldered in a new …. SOCKET … and plugged in a 486-DX4 100 Overdrive processor 🙂

With the DX4 100 processor, the machine is really quick and runs Windows 95 very nicely.

Update Nov 2022

Harddisk replacement

PS/2 P75 Technical Data

	This P75	Original IBM Spec
CPU	100MHz 486DX4 Overdrive	Intel 80486DX2 33MHz
Memory	16MB	8MB
Harddisk	SCSI2SD	160MB SCSI
Floppy	GoTek USB Floppy Emulator	3,5″ High Density (IBM specific cabling)
Video	XGA w. mono Plasma Display	same
Other	MCA Soundblaster clone (Texelec) 3Com Etherlink III MCA
Manufactured		1990 – 1992
Price on introduction		USD 15.990 (~USD 37.200 in Dec 2022)