I recently bought a Dragon 32, yet another 1980s 8-bit computer to add to my collection. I’ve owned several in the past but none of these came with a working power supply, which are difficult to replicate, so I’ve been looking forward to putting this one through its paces.
The computer was basically new in its original box, and judging by its cleanliness and the excellent condition of the packaging, it appears to have hardly been used – I’m frequently amazed by how carefully (or in some cases, how carelessly) people have stored their old computer equipment over the years.
For testing, I installed a UK plug (3A fused) on the PSU as the original was missing, and ordered a suitable composite video cable for connecting up to a display.
Once this was done, I set the machine up with my TV and powered it on – unfortunately, there was no video output displayed on the TV and the only signs of life were a constant buzzing noise coming from the audio output. Oh dear…
The first step of troubleshooting a problematic machine is to check that the power supply is working OK. The Dragon 32 has a relatively simple but very unusual PSU comprising a dual-winding transformer – the input is 240Vac @20VA, and the output is 8.5Vac @1.5A and 28Vac @250mA via a 9-pin D-type connector. This unusual construction makes the original PSU very difficult to replace if damaged or missing.
I didn’t expect any problems with the PSU, as I figured that if the transformer could only really fail open-circuit or short-circuit, the computer wouldn’t do anything at all if the PSU were bad. A simple test to check for a transformer short is to feel the temperature of the PSU when powered on, as it will get very hot very quickly – this one ran cool. I then tested both output rails with my multimeter, and both appeared to be well within specification.
Any further troubleshooting of the computer (i.e. checking the voltages on the internal power supply board) would require opening the case. I was very reluctant to do this, however, as this would mean breaking the factory warranty stickers which, at this point, were still intact after 37 years – I had a funny feeling that the warranty on this machine was probably void in 2021, but it would still be an awful shame.
Before taking this step, I wanted to check the video cable that I’d bought, just in case – this was designed specifically for the Dragon 32/64 and was bought from a reputable supplier which I’ve used many times in the past, so it should be trustworthy, right…?
Wrong.
It appeared that the pin for the composite video output and the pin for the left audio channel output were reversed inside the DIN plug, so the buzzing that I’d heard was actually the composite video signal. After reversing these plugs on the back of the TV, the computer seems to boot up normally – the Dragon was breathing fire!
I contacted the cable supplier, who were very helpful – they reckoned that I’d been sent a video cable for a Commodore 64 instead, and sent me the correct cable free of charge.
It just goes to show how important it is to ensure that you have reliable test equipment, to reduce the likelihood of false diagnosis as much as possible.
In terms of a restoration, there was very little I could do on this machine due to the warranty stickers. I gave the machine and all its accessories a thorough clean and they came up like new, but I couldn’t really do much else – the “Dragon 32” sticker on the upper case had creased, a common problem, but it wasn’t too noticeable so I decided to leave it as it was.
I’ve got my hands on several original tapes as well as an SD card reader for the Dragon 32/64 (and also the Tandy TRS-80 “CoCo”, which is of a very similar design to the Dragon), and I’m looking forward to trying out some games.
Adam's Vintage Computer Restorations 