Compaq Deskpro 386s

Compaq was a bold and innovative company, producing some of the best computers of the 1980s. Founded by trio of former Texas Instrument employees, the company famously (and legally) reverse-engineered the IBM PC and created the first successful portable PC. After making a name for itself, Compaq pivoted to the desktop. The Deskpro line of computers was known for quality, speed, and a steep purchase price. In 1986, the Deskpro 386 was the first computer with Intel’s groundbreaking 80386 processor, ushering in the 32-bit revolution. The later Deskpro 386s, manufactured in 1988, had an updated form factor and another first–this time with Intel’s new 80386SX processor.

I was visiting a friend’s house in the mid-80s when I was introduced to his father’s Compaq Portable. It had a mysterious suitcase design and “made for business” reputation. My friend’s father worked in the Texas oil business, so I’m sure he spent his days using Lotus 1-2-3, but we took to the skies with Microsoft’s Flight Simulator.

While that Compaq was the first PC to cross my path, I eventually got an XT clone in 1987. However, I had to wait until 1990 to make a homebrew 386SX.

The Deskpro 386s was positioned as an entry-level 386, offering a 32-bit processor but running on a 16-bit data bus. This particular unit is a Model 40, with a 16MHz 80386SX processor, 4MB of RAM (located on the proprietary memory expansion card), built-in VGA graphics, a 40MB Conner hard drive, and both 3.5-inch and 5.25-inch floppies. Along with a single 9-pin serial and parallel port, the computer has two PS/2 ports for connecting a mouse and keyboard. This was an innovation, as the PS/2 interface was created the year before for IBM’s PS/2 line of computers.

The Deskpro 386s sales flyer clearly positions this system as Compaq’s attempt to best IBM’s mid-range PS/2s, such as the PS/2 Model 50. IBM’s newest 286 ran at 10MHz offering 2MB of RAM, VGA graphics, and built on the new Micro Channel Architecture. Given the competition, this Deskpro was a strong play to dominate the mid-level corporate market.

This particular computer was an eBay find. I was looking for a solid 386, and Compaq is a gold standard. The unit arrived untested and in rough condition. After opening the case, I found a number of dead spiders and a fair amount of rust. Though, I was pleased to see a well-populated memory card and a self-contained battery safely attached to the side of the case. I was also pleased to track down the general maintenance and service manual for the Deskpro 386 line and the individual spec sheet for this 386s.

The front plastic was a bit yellowed, but in decent condition. The missing blank plate covered the location of the optional tape backup unit.
After blowing out the case and taking an inventory, it looked good, except for obvious rusting.
Everything except the bottom case laid out for inspection.

The first order of business was to completely tear down the computer, cleaning as I went. I was particularly interested in the custom power supply, as it would be difficult to source a replacement. After cracking open the PSU, I had concerns. One of the electrolytic capacitors was bulging and there was corrosion around it. Also, the PCB had burned near a thick-film metal glaze power resistor.

Bulging electrolytic capacitor on the proprietary power supply.
Scorched circuit board around a power resistor.
More signs of significant heat from the power resistor on the back of the PCB.

The capacitor was easily replaced, though I had to use non-conductive paint to repair the damage caused by the corrosion. The hot resistor was a bit more complicated. After researching the issue, I suspected the resistor might be working as expected, but it was simply too close to the less-heat tolerant circuit board. Therefore, I removed the resistor and re-soldered it a bit higher from the PCB and placed heat-resistant kapton tape under the reinstalled component.

I was finally ready to power it on. The power supply utilizes a proprietary connector, and I was unable to find the connector’s pinout information. Therefore, I plugged it in to the motherboard, added one of the drives so there was a reasonable load on the PSU, and I held my breath. Unfortunately, my test resulted in a periodically flashing of the motherboard’s LED and a simultaneous flash of the floppy drive’s activity light. Checking with my multimeter, I found odd and varying voltages, but the most common reading was 30V–hardly what was expected.

Given the propriety design, and lacking any detailed technical documentation, I was uncertain whether the problem was with the PSU or the motherboard. After setting the project aside for awhile, I took to eBay and found another Compaq Deskpro 386s available for parts. I was watching the item, but the “Buy Now” price was too high for me. After some time, the seller noted my interest and offered to sell me the computer at half the asking price. I jumped on the offer, and now had two questionable Deskpro 386s computers.

The second computer was rustier than the first, but the front plastic was in better condition. It lacked a hard drive and had less RAM, but was otherwise very similar. According to dates on various components, it appeared to be manufactured a few months after the first Deskpro.

A second Deskpro 386s arrives
Testing with the second power supply

The power supply from the second computer had slightly different markings, but otherwise looked identical. I resumed my testing with the second PSU and found the same results. Feeling frustrated, I pulled the second computer’s motherboard from its case and connected it to the PSU, floppy drive, and speaker. Instead of a blinking LED, I heard long and short beeps of the PC speaker and saw normal activity from the floppy drive. I swapped the second PSU for the repaired unit and got the same results. So, both power supplies operated normally, but the original motherboard had a critical fault. Perhaps the surface-mount tantalum capacitors near the power supply connector were the problem, but that repair can wait.

I was finally ready to reassemble the computer, picking the best parts available from either computer as I went, but first I had to deal with the rust. I had learned of the benefits of fallout remover from Adrian Black’s YouTube video. I stripped the machine down to bare metal, taped off stickers or markings, and applied the smelly Iron Free compound one piece at a time. I watched as the yellow chemical turned rust into a wine color. After a few minutes, I wiped the pieces dry.

After the fallout treatment, if necessary, I sanded the treated spots until I saw clean metal and then prepared to paint. For the inside of the case, I selected Krylon’s Fusion Matte Glacier Gray spray paint. This provided a fresh and clean look to the inside components. For the exterior top and sides of the case, I used Krylon’s Satin Almond, but the color was a bit warmer than I hoped. Matte Clamshell was an alternative I also considered.

I addressed rust on various ports and small components, but with the power supply reinstalled, I added the motherboard and was pleased with the clean and shinny computer coming together on my bench.

Rust gets everywhere, but luckily is fairly ease to remove.
Fine grain sand paper usually does the job and rusty screws get soaked in vinegar.
Factory fresh after cleaning, rust remediation, and painting.

Luckily, I was able to secure the appropriate blank face plate from the second Deskpro, and I cleaned and lubricated the floppy drives. The battery was the last hardware detail. While I was able to find a new replacement Tadiran 3.6V battery, it took me awhile to notice the pins were not the same. With a little fiddling, I was able to move the red wire next to the black, matching the pins on the motherboard.

The drive stack looks good.
Original and replacement 3.7V batteries

While hardware is fun to tinker with, computers are built to run software. Before I could do that I had to configure the system. I was able to find the spec sheet describing the motherboard DIP switches, but kindly Compaq also posts such pertinent information on the inside panel of the computer. While this computer is more sophisticated than earlier XT computers, it does not have a boot-configurable BIOS. Compaq’s early computers are setup through a floppy-based configuration utility. Luckily, this software is still available online along with Compaq’s OEM version of DOS 3.31, complete with customized setup and utility applications.

After some trial and error, I confirmed all 4MBs of the RAM were working; then I moved on to the hard drive. In another first, Compaq was the first to support IDE hard drives. The Conner hard drive in this 386s appeared to be in good condition. After making the appropriate Type 43 selection with the configuration software, I was happy to see “Starting MS-DOS…” on the Model 470A Compaq VGA Color Monitor.

The computer appears to have been used in an elementary school classroom. DOS 6.22 was installed and the hard drive’s well-organized contents consisted exclusively of age-appropriate educational titles, except for a stock version of Windows 3.1 . The hard drive runs fine, but the spinning platters sound a bit odd each time the computer starts. For that reason, I decided to install a Compact Flash adapter with a 256MB card. The provided IDE cable is a custom length, barely long enough to stretch from the motherboard to the single hard drive. I attempted to replace the cable with a longer one capable of connecting both the Conner drive and the Compact Flash adapter, but I was unable to get either drive to work unless I used Compaq’s provided cable. I don’t understand how the cable could be customized, but at the moment, I am only able to use the short cable to connect one drive at a time to the onboard controller.

Thankfully, with the help of an XTIDE card, I could install the Compact Flash card as a second drive and backup the original Conner drive. Installing XTIDE was tricky. After much mucking around, I finally discovered the XTIDE needed the latest IDE_386 binary file flashed to its EEPROM and block mode must be disabled for the legacy drive to reliably copy files.

Compaq Deskpro 386s Model 40 with a Model 470A VGA color monitor and somewhat newer Compaq speakers, keyboard, and mouse.

Compaq was a standout among IBM-clone manufactures. In its early days, the company was rightfully regarded for its innovation and quality. Thankfully, this computer was well-made and is reliable once more. I am proud to own an early Compaq, as it characterizes the spirit and promise of the early PC era.

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