Last year I got my first Viglen-cased Acorn BBC Model B, which was very kindly given to me by a friend – these use most of the original parts from the converted Model B except for the top case, so require servicing before powering on due to ageing RIFA mains filter capacitors which like to fail short-circuit and release clouds of nasty, acrid smoke.
The machine even had a Solidisk Technology Limited (STL) 128K sideways RAM/ROM expansion card fitted, which was a nice surprise – however it had fallen out of its socket and a lot of the cabling that it was connected with had snapped off.
PSU Rebuild
I started off by rebuilding the PSU, which didn’t take too long as I’m pretty used to working on these PSUs by now. At the very least this involves replacing the two RIFA mains filter capacitors before they go up in smoke, but I usually replace all of the aluminium electrolytic capacitors too – see my general advice for electrolytic capacitor replacement for more detailed information on the process.
I’m glad that I did in this case, because the three green Nichicon PX(M) 1000uF 16V parts had started to physically leak electrolyte, it looks like their lower seals were failing. These appeared to be factory parts – the only other Model B PSU that I have seen these capacitors in was from another converted Viglen unit, but this may be coincidence.
You can usually buy capacitor packs for these machines from sellers such as RetroClinic, but I just made up my own by noting the specifications of all of the electrolytic capacitors on the board, and ordering a set of high-quality known-brand parts.
ROM/RAM Expansion Repair & Mainboard Repair / Service
The mainboard needed some work to be able to get the ROM/RAM expansion properly re-instated, as its ROM socket was very loose and some of the cables had snapped off.
Some of the cables were fitted to female jumpers so they could be easily removed or repositioned, however I was struggling to get a good solder connection onto these, so I just removed the pin headers and soldered the cables directly to the mainboard; I also removed the ROM socket that the expansion connected into, and soldered the daughterboard directly to the mainboard too, ensuring a reliable connection. The expansion features a card which slots into this daughterboard, so it is easily removable.
I was able to find a very useful user manual for the STL 128k expansion board which also included the installation information below – also, someone has even reverse-engineered the schematics for it!
I also took this opportunity to replace the handful of aluminium and tantalum electrolytic capacitors on the mainboard, as it made sense to do this while it was accessible.
With this work performed, I attempted to power on the machine – it started up OK with a normal start-up beep and the composite video output worked, however RGB video did not.
Composite video working means that the RGB signals are being correctly generated by the ULA; I checked continuity between the ULA and RGB connector against the schematic, but all seemed OK; I tested the RGB SYNC signal out of IC48 (74LS86) and it was continuously low, so IC48 was likely faulty; I removed, socketed, and replaced IC48, and the RGB video output now worked OK. The ROM/RAM sideways expansion seemed to register OK as ROMS showed 0-F, representing sixteen 8KB blocks, or 128KB RAM.
Disk Drive Service
I pulled both of the internal 5.25″ FDDs out to give them a quick service – general strip and clean, read/write head clean, stepper drive exercising and lubrication, and sensor clean.
DFS Upgrade
The system had an early 8271 DFS fitted because it was shipped with two internal disk drives, but I upgraded to a 1770 disk interface kit from RetroClinic, which includes a DFS ROM upgrade for double-density and ADFS support.
Assembly & Testing
Reassembling the unit was just the opposite of its disassembly.
With everything back together, the system worked great, including read and write on both of the internal FDDs – I even set it up with a Microvitec CUB that I’ve refurbished.
Very nice indeed. I always wanted one of those Viglen cases. There were a couple of companies back then making cases for the Beeb, but Viglen’s ones were the most popular.
A few years ago, I wanted to convert my original very dead childhood BBC Model B into a USB keyboard and I was casting around for a Viglen case to put it in. But what I found was someone selling a metal (sheet steel!) case. I’d never seen them before – at the time or since. I’ve no idea who made them either. There’s a video of the process here:
Thank you ☺️ Wow, how cool – thanks for sharing!