Sunday 24 March 2024

Building lots of Mini PET 40/80s

It all started a couple of weeks ago.

A couple of long weeks ago.

TFW8b found the last of the Mini PET 40/80 PCBs and decided to put those up for sale again.

There were enough for 8 units, with a couple of extra mainboard PCBs without keyboard overlays. (yes, I know there are 9 in the photo, the last one is my Mini PET 40/80, there to make a square number)

It is a lot of work to pack and check (and recheck) these kits, and so it was decided it would be best to sell these as assembled units (which is more work.......).

So I set off to assemble the parts we had and the ones we needed to order to complete these builds. All looked to be available to buy, or from TFW8b stock or my stock, so they were listed for sale again. Initially to my and TFW8bs Patreon supporters, and then to the general public who snapped them up (all apart from the last one that sat there for a few days on it's own).

Whilst waiting for the PCBs to be sent up to me, I got on with the daunting task of assembling the keycaps. If you have ever built a Mini PET or a Minstrel 4D, you will know what is involved in putting these together.

You can imagine how much fun I had with this lot.

As you can tell, I was having so much fun, I made a few extras.

Even though it was only going to be me assembling these, it still made sense to sort out the ICs onto foam pads, although I wasn't as bothered about how neat they were.

That isn't the normal arrangement. This is "things that are going to be soldered to the PCBs", I populated the socketed chips direct from the tubes later on.

I went for soldering the logic chips without sockets, as I think that looks better. I'm a big fan of lining up 14 and 16 pin chips. You have to look quite hard to spot where the 14 pins stop and the 16 pins start.

Right, that's all the soldering done. Time for some ICs.

The microcontroller used on here has been discontinued in the 40 pin DIP package, but I was able to get hold of some form a supplier that still had stock, and miraculously, the EPROMs are back in production.

Lots of chips to program and label.

That's all the chips and the key switches done.

Time for the keycaps.

I use the technique of pickup up a random keycap, fitting it in place, then going back for another, then another etc. until it is done.

By the end of this, I was putting them in without reference to the layout sheet.

All done, time for some testing.

I went through the usual series of tests, self test, memory test, keyboard test, IEEE-488 port (with an SD2PET), userport (using the dual joystick module), and both datasette ports. I also measured the two 5V rails and the two 6V datasette motor supplies.

But this is the Mini PET 40/80, so as well as the usual 40 column favourites,

I also got to test out things like Cheese and Chive

And Colossal Caves in 80 column mode.

And the full built in self test.

So that's one done. Signed and numbered limited edition.

(N.B. in the Patreon preview of this post, I had written "singed" and numbered. So I have to be true to my word. Where did I put my blowtorch?)

Repeat 7 more times to make 8. (the 9th is my own Mini PET 40/80, there to make up the numbers)

That was a lot of work.

But it wasn't all plain sailing. Two of them didn't work first time.

The first started off looking OK, but then rapidly dimmed to a black screen.

After a bit of head scratching, I spotted the deliberate mistake.

I had all the resistors the right way around, but it seems I got one of the diodes reversed.

A quick swap around and that was sorted.

The second board had me puzzled for a bit longer.

Everything seemed to be working, but the picture was a bit rubbish.

It wasn't very stable, and there was coloured fringing all over the place.

I checked around the signals and there was something clearly wrong with the video signal.

There were peaks at the start of the line, and the video seemed to be at two different levels?

I double checked the second 5V rail, as the composite video output is on a different rail to the rest of the logic, but that was fine.

The input signal (below in green) was correct, but somehow when the composite video signal was being generated, it was getting messed up.

The sync input also looked correct, so that narrows it down to only a few components.

When measuring on the transistor base, I was seeing lots of negative pulses, was the transistor playing up?

I decided to cut the transistor out and recheck the signal at the base, to prove it was at fault.

It wasn't.

The negative pulses were still there on the mixed signal?

The diode was the next to go, and with that removed, the signal returned to normal.

I temporarily clipped a 1N4148 in place and it did not show the same negative pulses, so I replaced the transistor, and all three diodes, to be safe.

That sorted the problem, perfect picture again, and all the other tests passed.

So what was going on? well, looking at the diode, I can see a "23" on the marking, which is not part of "1N4148". Looking closer, that is 235B.

Looking that up, it is a 1N5235B, a 6.8V Zener diode used on the 40 column Mini PETs. (N.B. No relation to Kryten 2X4B-523P.)

No idea how that happened, all I can think is that I must have picked up some 6.8V zener diodes by mistake when I was assembling that particular board.

Now I have to pack up all these Mini PET 40/80 and get them down to TFW8b to fit the perspex and the manuals and get them out to the lucky buyers.


That was, as far as I know, the last of the TFW8b Mini PET 40/80s, although I still have the 40 column versions available.

The full range of Minstrel and Mini PET kits and accessories are available form my SellMyRetro store.

All the links can be found here:


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