1983 NABU PC – 230V PSU Modification, Replacement Cooling Fan, & Serial Adapter

In case you haven’t heard, the retro computer scene has been recently introduced to the NABU Personal Computer (PC) – produced between 1982 and 1984 by the NABU Network Corporation in Ottawa, Canada – when a large number (1000+) of unsold, new-old-stock, new-in-box systems were found in a warehouse, untouched for decades.

The NABU (Natural Access to Bi-directional Utilities) PC was an early home computer system and a component of the NABU Network which operated from 1982 to 1985 – this was a closed network that operated over the Canadian cable TV system, a precursor to the internet. A “NABU Adaptor” acted as a modem, and connected the NABU PC to the NABU Network via a cable TV provider, allowing the computer to download applications and information from NABU servers.

Unfortunately, the NABU Network failed – not due to the PC’s hardware, but because the cable system back then was not designed for bidirectional data. While the NABU PC did see a limited release in Canada, was never widely successful. When production was shut down, the machines couldn’t be liquidated, as they were useless without the network. So they were left sat in a warehouse for decades, until they were found and listed for sale in November 2022 – as of the time of writing, they are still available to buy on eBay.

The NABU PC is quite a capable machine: it sports a Zilog Z80 CPU @ 3.58MHz, 64KB RAM, a TMS9918 video display chip, a General Instruments AY-3-8910 sound chip, and two UARTs – an 8251 for serial I/O with the keyboard and joysticks, and a TR1863 for communications with the network adapter via RS-422.

There has been a fantastic effort from the retro computing community to revive the NABU Network and give the NABU PC a purpose again, through open-source network emulation software – there has also been a lot of hardware hacking, such as the development of reproduction floppy controller boards.

I decided to buy myself one, to have a play around with – shipping costs were more than the list price for the computer alone at the time ($79.99), and the NABU PC is an NTSC and 110V-only machine (we’ll get to this later on), so it’s not a major surprise that, according to the NABU PC serial registry at least, they’re quite uncommon outside of North America.

When it arrived, it was quite fun to unbox, as everything was pretty much brand new – the outer box still had its polystyrene inserts, accessories box, user manual, and protective coverings, along with the NABU PC base unit and keyboard themselves.

My unit has serial number 007723, which matches across the computer, keyboard, and box.

The composite video output on the NABU PC is NTSC, which my video upscaler can handle, but it has a 110Vac 60Hz internal PSU and mains input, whereas we use 230Vac 50Hz mains here in the UK (somewhere between 220Vac and 240Vac, anyway).

It’s possible to use a step-down transformer to run 110Vac appliances on UK mains, but the sheer lack of quality in modern step-down units makes me seriously distrust them for use in most applications, and I wanted to semi-permanently modify the computer for 230Vac mains anyway, so I decided to do this before testing anything.

230Vac Mains Input Modification & Replacement Cooling Fan

The first step was to disassemble and inspect the computer, which only requires the removal of a handful of cross-head screws.

The power supply has a metal shield covering it, held in place with two cross-head screws.

With the cover removed, this unit seemed to have a rather nice switch-mode PSU, and a very chunky 110Vac mains cooling fan – this had the common problem where the fan jams up due to excess paint causing the fan blades to catch on the housing, but I wasn’t worried about that as I needed to fit a new cooling fan anyway.

I removed all of the parts that I wouldn’t be needing: the PSU itself, which I was going to replace with an integrated SMPS; the cooling fan, which I was going to replace with a modern DC fan; the original mains cable and strain relief grommet, which I was going to replace with a 2m UK mains cable with moulded plug and 3A fuse; the remaining redundant cabling, which I was going to replace too.

The original PSU is held in place with four cross-head screws; the original cooling fan is held in place with three bolts, which are threadlocked and can be quite difficult to remove.

The replacement SMPS that I chose for this application was the MeanWell RT-65B 64.6W PSU, which outputs +5Vdc @5A, +12Vdc @2.8A, and -12Vdc @0.5A, the voltages required by the NABU PC mainboard.

The new SMPS can be fitted to the same standoffs as the original.

I also installed a 5Vdc low-profile low-noise high-flow 80mm cooling fan, which I connected directly to the 5Vdc output on the new PSU using red ring crimps. The cabling was quite fine, so I added some solder for extra connection strength.

As for the cable harness to the mainboard, it would be perfectly fine to just modify the original harness, but I decided to build one up instead – this meant removing the original, which meant removing the mainboard, which is held in place with plastic standoffs.

Removing the original cable harness was quite easy using a desoldering station – I built up a new one using 22-gauge solid-core copper cable and some cable ties, taking care to keep the new cable colours as similar to the originals as possible to avoid confusion.

Pin NumberCable ColourPin Assignment
3BlackNot Connected (Ground)
5Grey+12Vdc (Keyboard)
6Red+5Vdc (Logic)
7Orange+12Vdc (Logic)
8Blue-12Vdc (Logic)
NABU PC mainboard factory cable harness pinouts.

I then cut the new cabling to length, fitted the appropriate ring crimps (and added some solder, for good measure), then connected these to the appropriate terminals of the output-side of the new SMPS.

With the output (low-voltage) side of the SMPS connected, it was just a case of connecting up the input (mains-voltage) side, which involved: fitting a new mains cable and strain relief grommet; connecting the mains earth to the computer chassis, then from the computer chassis to the earth input on the SMPS; connecting the mains neutral to the neutral input on the SMPS; connecting the mains live to the computer’s internal fuse, and the switched live from the computer’s power switch to the live input on the SMPS.

The the power supply replaced, it was time for testing – the cooling fan seemed to work and the computer booted up OK, though without a network connection it would just flag up “Adapter Failure” and sit there. The next step was to build a network adapter.

NABU Network Adapter Build

The NABU PC connects to the NABU Network Adapter via a 5-pin female DIN socket with an RS-422 UART, a standard multi-drop serial communications protocol.

The NABU Internet Adapter Software running on a supported computer (Windows, MacOS, or Linux) can be used to emulate the NABU Network via a virtual COM port and a suitable USB to RS-422 adapter.

The USB adapter that I bought also came with a DB9 breakout board, which was easy to make into a DB9 female to DIN5 male adapter for connecting up to the NABU PC via USB.

To save me from having to solder small wires into a bare DIN connector, I just ordered some cheap MIDI cables and cut one in half, then stripped and wired the flying end into the DB9 female breakout board as per the pin assignments below.

5-pin DIN connector pin numbering.
Pin NumberCable ColourPin Assignment (Adapter)Pin Assignment (NABU PC)
1BlueReceive (-)Receive (+)
2N/AGround / ShieldNot Connected
3RedTransmit (+)Transmit (-)
4WhiteReceive (+)Receive (-)
5GreenTransmit (-)Transmit (+)
NABU PC serial adapter pinout.

To make this a bit more professional, I put the serial adapter into a small plastic enclosure with a strain relief for the cable, with the breakout board held in place with hot glue.

Reassembly & Testing

The reassembly of the computer is the opposite procedure to its disassembly.

With the NABU PC connected to the NABU Internet Adapter Software running on my Windows 10 laptop, it was very rewarding to see the computer successfully boot over the network for the first time.

I spent some time playing some games and properly testing the machine.

  • Boots correctly to normal startup screen.
  • All keys register correctly.
  • Power switch works OK.
  • Reset switch works OK.
  • Power, pause, alert, and check LEDs work OK.
  • Dedicated audio output works OK.
  • NTSC composite video output works OK.
  • Network adapter interface works OK.
  • Both joystick ports work OK.
  • Cooling fan works OK.
  • Printer port untested.
  • Expansion ports untested.

Published by themightymadman

My name is Adam Wilson - I'm an electronics engineer based in the North East of England, UK, and I like tinkering with old junk. In my spare time, I collect, repair, refurbish, and (sometimes) sell vintage computer systems and peripherals, typically from the 1980s (the likes of Commodore, Sinclair, Acorn, Apple, Amstrad, and Atari).

One thought on “1983 NABU PC – 230V PSU Modification, Replacement Cooling Fan, & Serial Adapter

  1. Cool stuff!.
    This is what I see in your post
    What a fascinating story! It’s great to see the retro computing community coming together to give the NABU PC a new lease of life. The modifications made to the machine are impressive and it’s fantastic to see it working as intended.
    Ely Shemer

    Liked by 1 person

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