Couldn’t resist THAT bad pun 🙂
So I’ve got a couple of days of leave, and now that my wife has plied me with Yum Cha, and I’ve done my first non retro repair around the house, I thought it was time to start working on another one of my retro-computers.
Sorry, not the PET. I’m still waiting for SRAM to arrive for that. Hope that fixes it, otherwise things get more interesting after that. (Hint: I will need to find an EPROM that I can actually plug into a system of that vintage.)
No I thought it was time to work on Microbee number 2.
One of the things that triggered this decision was the arrival of a pair of PCBs for something called a “Core Lifter“. This is an absolutely essential piece of kit for Microbee owners that plug in between the Main Board and the Core Board.
(Cor! Don’t I look fancy using all the “correct” terminology. This is mostly thanks to the people over at MTF and MSFF who are very patient with a doofus like me wandering in and asking strange questions like “What do you call that top board?”)
Now I had one, and had soldered on the various connectors, I was able to set the failing ‘bee up in such a way as to allow me access to all the core chips like the Z80 and the display chip. All ready. Let’s power it on and see if the fault is still the same, to wit: a black screen with no synch.
This is where things get strange.
See instead I got a stable “underscore” cursor. This means something is running.
I decided to get out the logic probe and see if it was clocking. I power cycled the system to see if I got consistency. I did not. This time, I got an error message!
OK so now I’m actually getting something booting through the ROMs. This is great! Pressing ESC even got me to the WORDBEE screen. This is more than booting. This is life. The vast majority of the system is actually functioning.
At this point I went out for a break. When I came back I power cycled it again and got… a BASIC prompt. Whoah. It was stable too. I wrote the obligatory “Hello World” BASIC and it was happily running.
I was a bit confused at this point. All I had done was add the Core Lifter. Was my problem just some dirty contacts between the two boards? DeOxit to the rescue!
Liberally spraying it into the connectors that connect the two boards, and easing them in and out again, followed by cleaning up the mess from applying DeOxit in the first place got me to a system where I was happy that the connectors were probably good. I also inspected all the connectors and checked the underside for dry joints. It all seemed fine.
It wouldn’t boot beyond the underscore.
I reseated all the ROMS carefully and…
OK that seems to be consistent. I have a working Microbee.
The story doesn’t end there, of course. On further inspection I noticed some corrosion around a whole bunch of connector pins. Looks like battery acid, which would not surprise me, considering Microbees had a battery mounted right next to the spot where the corrosion was. I reflowed all the pins on the connectors, desoldering the worst of them. a lot of crud came out which makes me think I was on the right track.
I’ve scratched back as much mask as I can with a glass fibre pen (A nasty piece of kit, perfectly adapted for this task. Any circuit mask that corrosion has crept under tends to come away from the underlying copper with the use of the pen. It does, however, leave evil glass fibers everywhere. These can get into your skin and are irritating. I always work over a piece of paper, clean with a brush at the end and chuck the entire piece of paper out at the end). I have followed up by wiping the exposed copper with white vinegar, followed by distilled water, followed by isopropyl alcohol. I have then covered the exposed metal with nail polish to stop further oxidation.
It’s not perfect. It needs a reboot occasionally on startup to get to the right prompt, but I wonder if that is related to the lack of battery. There’s also no sound. This will require more investigation. Finally, it lacks a case. I’m not quite sure what to do about that.
EDIT: After some more futzing around, it’s less reliable on bootup than I thought. It works most of the time, but not consistently. It sometimes boots to the Menu. It sometimes boots to BASIC. It sometimes boots to WORD-BEE. It sometimes just crashes.
EDIT 2: I’ve since replaced the connectors between the Main board and the Core board, replaced the socket for IC27 (as it was green on the inside), replaced the diode nearby (As it had green growing inside it and was testing rather oddly in my multimeter) and it’s still unreliable. I need to remove and test a couple of key ICs related to line select before I go any further, but 2 trips to Jaycar in one long weekend is enough. I’ll go play with my new toys instead.
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