I got the PC85 Microbee working 😀
First up, I am, once again in debt to the Retro Communities to which I belong. The people over at both Microbee Technologies and MSPP (Especially Brad) who provided advice, pointers and general assistance as I got the second of my Microbees working.
Now, the fault I was seeing was instability on boot. There seemed to be an issue where some times the ‘bee would boot into the wrong mode. Most times it would boot into the Menu. Sometimes into BASIC. Sometimes into WordBee. Sometimes it would just “hang” in a strange corrupted mode. The Menu was also only showing two items, not the half dozen it should be showing.
Once it was in BASIC, I could happily boot games and run them without a hitch. I was confused. Usually if a system was unstable, it would crash randomly. This meant it was something very specific that was only involved in choosing which ROM to boot from.
I started troubleshooting things, firstly by reading in the ROM images and comparing them to “known” images. All the ROMS came up clean. (There was a brief detour when I thought the Menu ROM was bad, but this turned out to be not the case. There’s two versions of that motherboard and I was looking at the wrong set of ROMs)
I then cleaned all the pins on all the ROMs. The socket the Menu ROM sat in was green with battery corrosion so I removed it and replaced it with a Machined Pin socket. This improved reliability somewhat, as now the system tended to boot into the Menu.
Around this point I removed the connectors completely between the base board and the core board to see if that helped. The system seemed no better.
I also discovered there was a diode in the area that was growing green inside its tiny glass tube. That came out and was replaced, but I was still unable to boot reliably.
At this point I put it away for a few days.
Brad over at MSPP started asking some leading questions about symptoms, and, based on my replies, was able to narrow the likely fault down to a handful of chips, most likely IC26, a 74LS74. He suggested that if the system was stable without the D ROM (and it was) then the problem was with the control logic, and IC26 was the one closest to the corrosion.
To test this, I dug out my logic probe and started checking logic states on the IC. I could see logic states coming in on the inputs (High, Low or Pulsing) but the outputs were just showing as floating. This was highly suspicious behavior, so I zipped down to my local Jaycar (I’m not sponsored. They’re just conveniently close) and purchased another Machined Pin socket and a replacement 74LS74.
Removing the 74LS74 was not easy. It really didn’t want to come out. While I could clear one row of pins, the second row just didn’t want to play at all. I tried all sorts of tricks. Extra solder, wicking solder away from the top. Extra flux. Resoldering. they just wouldn’t budge.
In the end I was worried I’d start damaging pads, so I cut the legs free of the chip and was able to remove the legs with a normal soldering iron. Even with this, I’d lifted one pad and damaged a second. Thankfully this was an easy repair, but still. I cleaned up the pads with braid and attached the new socket.
I dropped in the new 74LS74 and checked the logic again. I could see “sane” logic on the outputs, so that was an excellent sign. In went the D ROM and… it booted without a hitch. More importantly, my Menu was now showing all the options. Ooh!
I spent the next few minutes testing as many options as I could, including the Self Test. Everything just ran. The self test also confirmed that the keyboard was fine.
While I had everything open, I also put a proper heatsink on the 5v regulator, for longer life. Next time I go to Jaycar, I’ll probably build a little circuit with a diode inline and a switch, to drop the voltage from 12v to 10.8v, to take some load off the regulators.
My final step will be to make a nice clear acrylic case for it. Something so you can see all the internal “gubbins”.