1987 Compaq Portable III 386

This project was a pleasant surprise in more ways than one. For a long time I’ve only thought of Compaq branded computers as exceptionally cheaply made, mass produced commodity appliances. While this was certainly true during the later Pentium-era years as Compaq, Dell and Gateway raced to bottom of the retail market, I’ve been pleased to learn this wasn’t the case with the Compaq Portable line.

This 1987 Compaq Portable III 386 is well documented online and its hardware has been extensively explored by others. One fan has even created an entire website devoted to the Compaq Portable family. In this article I will just be showing a few specific aspects of this repair and restoration which I found interesting, and sharing some more tips with you.

Disassembly, Inspection & Cleaning

I acquired this computer from the local Goodwill through a shopgoodwill.com auction (which is where they send most of the good donations in order to get more money). It was remarkably clean and un-yellowed. It would not need any retrobrighting and thankfully only needed minor cleaning.

A toothbrush is excellent for getting in-between ventilation slats and molding lines. A quick brushing and vacuuming takes care of all the loose dust and debris. A magic eraser often makes quick work of stubborn black marks and smudges, but they are abrasive and can noticeably remove surface texturing from molded plastics, so use sparingly.

Disassembly was also easy, despite Compaq using two sizes of torx headed screws. I was surprised to find large labels on the inside of the back cover with all the system board jumper settings and locations clearly printed on them. This is something you normally only see on servers, and was the first indication that this machine was much more well engineered that many of it’s descendants. This was also evident by the extensive amount of RF shielding covering almost every circuit board in the machine.

The keyboard was also unique in that it’s screws were located on the front underneath the keys. There was even a screw underneath the Compaq Portable III name badge, which came off easily with a little IPA.

This 386/20 unit turned out to be fully loaded, with a math co-processor, 2 megabytes of onboard RAM, a 4 megabyte expansion card and a 2400 baud modem card. Both of the expansion cards were connected to the main board with a PCB that Compaq labeled the “32 bit option I/F”, kind of an upside-down ISA riser card. The cable seen connected over the cards is for the front mounted keyboard.

As with all vintage electronics, the power supply was also inspected, cleaned and tested prior to turning it on. For 1987, this Compaq power supply was one of the more advanced I’ve seen, with multiple riser cards, surface mount components, beefy resistors and thankfully no RIFA capacitors in sight! You’ll notice the main power cable which provides all the working voltages to the mainboard as well as a second red molex style connector. This provides 200 volts to the gas plasma display, so extra caution is definitely recommended when working on this unit.

(Mostly) Minor Repairs

Minor repairs included replacing the CMOS battery, which thankfully on this model was one of the larger 3.6v rectangular ones, instead of an integrated Dallas chip battery or soldered-on Varta style. Also easily replaced thanks to 3D printing was the one missing riser foot from the keyboard. Versions of this part had already been designed by other people and uploaded both to Thingiverse and Printables.

While removing the handle for cleaning, I removed the end caps with little resistance, but found they had been heat-staked to the mounting bracket. A bit of double sided tape will hold mine back on, but if you decide to remove the handle on one of these computers, don’t try to remove the plastic end caps!

Another repair, common to Compaq portables is replacing the keyboard cable. These cables degrade over time in a way I haven’t seen anywhere else. The rubberized outer insulation develops a fine white powder on it, hardens and eventually cracks into pieces. Luckily XT/AT coiled keyboard cable extensions can still be found on ebay fairly cheaply. Due to the length of my replacement cable I attached it to the keyboard circuit board with an easily serviceable 5-pin connector and ribbon cable extension.

Finally, the most important (and lucky) repair. When I received this computer it would not fully power up. When turned on the power supply would make a rising high-pitched whine for about 2-3 seconds, reset and start over in a continuous loop. All of the components inside the power supply appeared intact, and for a fraction of a second before it reset, I did measure all the correct output voltages from it.

Working backwards from the power supply onto the motherboard I found a blown tantalum capacitor (C47) across pin eight of the power supply connector. This turned out to be the ground return wire for the “power good” signal, which the power supply uses to verify that it’s connected to a working motherboard that can safely receive all its working voltages. After replacing this one capacitor, the power supply started up fully and both the computer and aged gas plasma display came on good as new!

That this was the only fault in the entire machine and that all the ICs were still fully functional was a welcome surprise and relief. It’s always gratifying when a repair story has a happy ending.

The Unrepairables

While the overwhelming majority of this venerable Compaq Portable III 386 was now in working order, there are still a few parts that show their age. There are two dark marks on the edges of the screen that may be thermal damage. These gas plasma screens have somewhat fragile circuitry in them and once they start to fail and develop light or dark lines on them, they cannot be rescued and can only be replaced. Surprisingly, this particular display, while having some obvious burn-in, doesn’t have any horizontal or vertical lines or dead sections and is nice and bright. These displays have a very wide angle of view and great contrast for a digital monochrome display from this time period.

The other casualty of age is the historic Conner CP-341 IDE hard drive. This was one of the earliest IDE interface drives and its use in the Compaq Portable series did much to make IDE the new defacto standard over the older MFM interface used in most XT class computers. Unfortunately, this particular drive was completely silent and immobile when power was applied to it. I even attempted to connect to the undocumented serial interface as described by the OS/2 Museum website without success.

Thermal imaging showed hot spots at several of the ICs when powered was applied, the hottest of which was the ML8464C Micro Linear brand pulse detector chip. This chip is directly connected to the read/write heads of the disk and reads the amplified magnetic flux data they send it. A failure of this chip would certainly make the disks unreadable. In addition, the fact that the drive no longer spun, offered evidence of additional failures elsewhere on the control board. Further disassembly and autopsy of the disk revealed a solidly frozen head actuator motor and visible circular lines on the platters, possibly indicative of a head crash some time in the past. The exact failure mode of this drive may ultimately be impossible to determine. With its historical importance and unique tray-style assembly though, I’m happy to add it to my display collection.

Upgrades & Final Thoughts

With the cleaning and repair of this machine essentially done, there are a few upgrades that can make it more useful these 36 years later. The 5.25″ HD 1.2 megabyte floppy drive can be replaced either with a 3.5″ 1.44 megabyte floppy drive or a Gotek/HxC style virtual floppy drive emulator. The spinning rust hard drive can be replaced with an SD or CF card hard drive replacement as demonstrated by Todd Vernon of OldSilicon.com who even created a 3D printed mounting bracket that fits in the old hard drive bay.

As I mentioned earlier, I love a repair that comes together in the end. The Compaq Portable 386 is a well-engineered computer, in a convenient form factor, and of historic note. Its biggest limitation is the monochrome display and CGA graphics, but it will be a great addition to my collection, and the perfect option next time I want to use Windows 3.1 or play some Gato.

Total Restoration Cost $350