Three Bee, Three Bee!

For anyone trying to follow along at home, last we saw the Microbee computers, I’d managed to make the cable for the Microbees, and proven the base 32k “Communicator” model was working fine. I’d resurrected the PC85 and upgraded it to an 85b model complete with a bunch of embedded games. Now back to the last of the Microbees, the 16k early model.

Straight up this was missing the “A” ROM, so I needed to arrange to get one. This unit used the rarer 2532 ROMs. I played around with the idea of getting some 2732s and making adapters, but in the end I found an eBay seller selling them so I ordered and waited… and waited and waited. Approximately 2 months after my order, they finally arrived. They looked a bit scabby but that was OK as long as they worked. Unfortunately I had no way of burning these models, but I had a friend who could help. I dropped them off to him and waited. Turns out both ROMs were duds. Neither would take an image. D’oh! Thankfully the seller refunded without argument (which makes me suspicious). The friend was able to find a 2532 in their parts bin and was able to burn me the ROM.

Now, at this point I have one good confirmed working ROM but that’s it. I plugged it in and turned it on and… Nothing.
Not even video synch.

Hmm. Not the end of the world. I was kind of expecting this. Next step is to clean all the ROMs and the legs from the top “Core” board to the lower “Main” board. While doing this I noticed that the last row of pins on the “C” ROM were completely missing. I can only assume at some point in the past, someone had inserted the ROM incorrectly and accidently sheared off the two pins, and then hadn’t noticed when they put it in correctly. This was easily fixed by simply soldering on some replacement legs made from offcuts from a resistor.

What’s missing from this picture?

Aaand this time we get a loud “BEEP!” from the speaker. Alright! Still no picture though.

I checked up with the various Microbee forums and someone confirmed that a beep means the CPU has started and is running code. Excellent! Now to look at the picture.

At about this point I bumped the board and suddenly got an inverted picture with a barely visible cursor. That’s promising! I’d occasionally get a screen full of garbage. At this point I knew what to do next. I replaced all the ROM sockets, the socket for the 6545 video chip and the connectors between the base board and the top board. I also cleaned the legs of every single ROM and the 6545.

I only got this a few times

No picture.

I try swapping the 6545 for a known good one and still no picture. Hmm.

Oh wait! I haven’t plugged in the video cable.

Now I’m getting a picture! A little grey but a picture nonetheless! Keyboard is rubbish but not unexpected. Let’s put it in the case and… No picture… What?

OK so something was obviously loose or failing here. I knew it wasn’t the chips as I’d swapped them over. I knew it wasn’t the sockets as they had been replaced. I pulled out the schematic and had a look at likely candidates. Next in the chain was a resistor (R20, an 82Ω resistor) or the socket the video plugged into. Both were a bit “crusty” so out with the desoldering gun and out they come. Replacements went in and… no improvement. Oho! It’s going to be one of those kind of jobs. By this point I was having to go to Jaycar or Altronics more than once a week, just to keep the forward momentum going.

The schematic.

I actually broke out my BitScope at this point and started looking for video signals, starting at the composite connector and working backward. At the composite connector there was absolutely no signal. At the connector side of R20, there was no signal. At the other side of R20, nearest the transistor (TR2, not labelled properly here) I could see a video signal.

See those dips? Every line of a composite signal starts with those.
See those squiggles sticking up? Individual pixels on the screen in white.

It took me a while to realise the signal was at a way too low voltage of about 0.3v before it went into R20. It was supposed to be closer to 1v peak to peak on the other side of R20 and it’s not going to go up at all. If I jumpered around R20 I’d actually get a picture. Could it be the amplifier transistor at TR2?

The offending video circuit area. At this point I’d already swapped out R18, R20 and was about to swap the transistor

OK so on the forums everyone thinks that TR2 is the likely suspect so back to Altronics I go.
And that makes no difference at all. Gah! I’d also swapped R18 out as it was out of spec. I’d also reflowed every single solder joint in the area and gone over the board with a magnifying glass, just in case there was a short.

Could it be the Capacitor? Hmm.

At this point, someone on the forums mentions that I must have a very old Microbee as it sounds like the video fix has never been applied. Their Microbee didn’t even have a capacitor at C20.

Video fix?

OK it turns out that MicroWorld had issued various fixes, and one of the BIG fixes was updating the video circuit with a bunch of changes. Diving into the “Microbee Hardware Notebook with Updates”, on page 63 was a complete change to the video circuit.
What the hey? I have all the components on hand. Let’s give it a go.

The revised video circuit.

Guess what? It worked!

It’s possibly crisper in “real life” than through the camera lens.

Nice stable picture, no issues.

Huzzah! Interestingly, once I got C20 (the Capacitor I was suspicious of) out of the circuit, I tested it and it was showing only a few nanoFarads. I’m curious if the circuit would have worked “as is” with that replaced, but I’d much prefer a “fixed” circuit. The picture is wonderfully crisp.

Fixed at last

The next step will be to work on the keyboard. It’s an interesting mix of three different styles of keyswitch. Desoldering has commenced.

My long term plan is to take the nicest looking of the three and see if someone is interested in trading for a “premium” disk based model, with me paying the difference. I’d really like a disk based model. Stick a Gotek in and I’d be right to load all the software I could want, without having to rely on tapes!

A huge thanks to everyone on both MSPP and the MicrobeeTechnology forum for all their help and support. You folks rock!

Published by ilike8bits

I collect old computers and consoles

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