So on Thursday, after receiving a very kind offer, I drove out and collected another system for my collection. It’s another Acorn, which means I now have three different Acorn systems.
Meet the Acorn Electron.
It was Acorn’s attempt to produce a “price reduced” version of then hugely successful “BBC Micro” family of computers. Much of the logic from the BBC has been reduced to a single integrated chip, the ULA. Unfortunately for Acorn, the ULA was delayed, meaning they missed a significant holiday season, and ended up late to the market.
Nevertheless, it’s an interesting creature, and this particular one came with the tape drive, all the cables and even a bunch of books, magazines, and even an example of an assignment that the previous owner had submitted as part of his studies. (He asked if I wanted it and I said “Yes!”. I often find these pieces of ephemera fascinating insights into the real world uses these systems.)
Having got it home and gone through all the bits and pieces, I started investigating the system. First up was some pre-flight checks. The PSU is an external “wall wart” unit with UK prongs. Luckily I had an adapter I’d purchased to go with my BBC Master. I cleaned the prongs and then carefully measured the voltages and got a nice steady 19V AC out of it, which is what I was expecting. I then found a composite cable and plugged everything together.
Unfortunately, on bootup, I was presented with slightly corrupted blocks, rather than characters. I tried both the RGB signal and the straight composite.
A bit of Googling later showed no results, but I had enough experience with systems to know it’d be most likely one of three things. Clearly the CPU was running, otherwise the blocks would not be there at all. This means it’s most likely to be (in order of likeliness):
- A dirty /damaged ULA socket. The sockets for the ULA are rather unusual. The ULA is on a bonded ceramic carrier with many contacts exposed around the edge. The socket consists of a series of spring loaded pins that reach up and press against the matching contact on the ULA. Unfortunately in harsh environments, both the pins and the ULA contacts can oxidise and need cleaning.
- Bad RAM. RAM of systems of this age is notorious. This system uses the same 4164 RAM as the C64C, and I’ve had to replace quite a lot of that in my time.
- Bad ULA. The ULA itself may have failed. I was hoping it wasn’t this one, as this is a relatively “unobtanium” chip. Thankfully it’s also a fairly rare failure mode.
I started tackling the potential issues, one by one.
Starting with the bad or dirty ULA, I popped open the system, carefully removed the clip that holds the ULA in place, and checked the pins and contacts. Both looked fine, but I carefully cleaned the contacts anyway, with an ink eraser, and a tiny amount of Deoxit.
Unsurprisingly, it didn’t fix things. Seeing how clean it was, I didn’t expect this to be the fix. Oddly the only dirty component in the system was the keyboard connector, which showed some strange white corrosion. I removed this with some IPA.
Next I looked at the RAM. I ordered some 4164 RAM chips, as I couldn’t find the 1 or 2 ones that I had left over from C64s. Initially I thought I was going to have to wait for RAM to come, but I thought about how the RAM worked and decided to just see if I could see any activity on the Data line on the RAM.
I dragged out my BitScope Micro, and after some false starts, got it hooked up to the Electron.
Checking the first RAM chip gave me what looked like regular data at about the right voltage. I moved onto the next RAM chip and the third RAM chip with identical output, However, when I got to the fourth (and final) RAM chip, I noticed an immediate difference.
There was simply no data coming from that last chip.
I’ll wait for the RAM to come, get a socket and we’ll see if replacing that RAM chip brings the Electron to life. If that does fix it, I’ll have to get some shots of some games on the system. Down the track I want to see if I can get a disk drive / joystick port expansion for the system.
Leave a Reply