BBC Micro Model B+ Restoration

Notes on getting RICM's BBC Micro Model B+ working. Contact: bjepson@ricomputermuseum.org

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Will It Boot? 

After cleaning up the case, which had gotten somewhat grimy, the first thing was to figure out if this computer could boot up.  Because it had a 240V power supply, I needed a step-up voltage converter. Fortunately, I keep a Simran AC-300 around for this kind of situation. 

I turned it on, and it booted, but it also immediately blew its notorious RIFA capacitors. The smell was... overpowering. The RIFAs are really only needed to minimize powerline noise generated by the BBC Micro, so it's not a huge problem to run it without them, but I decided against that.

Temporary Power Supply

I connected a PC power supply that I converted into a bench supply with a Dangerous Prototypes ATX Breakout Board. The BBC Micro needs +5V, +12V, -5V, and of course, ground. I had to move the 3V3 terminal to -5V because the breakout board didn't include enough terminals for all the voltages.  


Click image for a larger view.

Click image for a larger view.

With a good power supply, I was able to boot the computer. I couldn't connect it directly to a VGA monitor, because the BBC Micro has only an RGB output. But I found a BBC Micro Computer RGB DIN 6 to VGA adaptor on eBay that did the trick.

 Keyboard Signal Issues

Unfortunately, not all the keyboard keys were working. I spent some time testing, probing, and looking at the keyboard schematic. I went as far as to wire up an Arduino to read the keyboard, and modified Max Chandler's BBC-Micro-USB-Keyboard sketch to give me diagnostic info on which keys were working.

I managed to determine that the non-working keys were all wired to the same line, and figured out that the keyboard PCB had succumbed to some deterioration. I realized a bodge wire was all that was needed, but not before replacing all the logic chips on the keyboard. Well, at least I took the time to socket them, so maybe that will be useful for someone, some day!

Click image for a larger view.

Click image for a larger view.

Click image for a larger view.

Key Replacements

So far so good, but I still had some non-working keys. Turns out that many of the keys were just bad. I attempted to disassemble the offending keys, clean the contacts, and reassemble them, but in the end, I just bought spare BBC Micro keyboard switches off of eBay. This model uses Futaba simplified linear type 1 switches, but different issues of the Model B used different switches, so you'll need to determine what kinds of switches you need if you have a different version.

Power Supply Repair

At last, it was time to re-cap the original power supply. I purchased a repair kit from Cool Novelties on eBay. It included the 3 most important caps to replace (2 of them RIFAs), and some great instructions. I also discovered that there's a handy jumper location for 115V operation, and added a wire across it to allow me to use the computer with US mains voltage. In the end, it did not care that our mains power runs at 60Hz instead of the 50Hz used in the UK.


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