Sunday 29 October 2023

Building a Mini PET B Kit

Mini PET B is the version of the Mini PET designed to act as a drop in upgrade / replacement board for a Commodore PET or CBM.

It provides 32K of RAM, and selectable ROM sets. 40 x 25 monochrome video, compatible with most software other than a few demos which require a 6545 CRTC. Modern WDC W65Cxx chips are used to provide a PET compatible computer, with the same external port - IEEE-488, Userport and two datasette ports. Note the userport is the PET standard, and is not the same as the VIC20 / C64 (most importantly it does not provide external power).

Today I am building one of my kits. Step one is always to knoll all the parts out (or at least it is for me).

Note the parts on the bottom right are optional, if you require composite video output or barrel jack 9V DC power input.

I am going to build this one without those parts initially, but will add them in later so show different use cases.

I start with all the low-height components, the resistors, capacitors, diodes, the crystal, the two power transistors and the voltage regulator.

Next I fit all the IC sockets. If I am building one of these for myself, I normally do not use sockets for the logic chips, but I will in this case.

Next the resistor arrays, noting they are all oriented with their pin 1 dot to the left. Finally the remaining connectors and taller components.

With all that done and cleaned, time to fit the ICs. As ever, they are arranged on the foam pad in the same positions as the PCB.

And we are done. Time to install the board in the PET case. This is my 4032. The two holes on the front of the Mini PET board are for the mounting pillars. They are self adhesive, but you don't actually need to stick them down unless you plan to move your PET around a lot (have you tried to pick of of those it up?)

The top left of the board has an earthed pad which is the connection for chassis ground. Fix that with the original screw.

Top right is also screwed to an original pillar, but is not electrically connected.

Finally the power, monitor and keyboard connectors.

Power is either 9 pin on 2001N/30xx/40xx and 80xx machines, or 5 pin on 2001 PETs, aligned to the right of the connector. The blue wires on the 9 pin connector are for the 18V tap on the transformer, which is not required. The other black is an extra ground.

If you want to check voltages, black is common ground, and the centre tap of the transformer. the two brown wires are the other taps on the transformer. There should be around 9V AC between either brown wire and the black (ground). The two red wires are connected together at the big capacitor. There will be no voltage on the red wires unless they are plugged into the Mini PET. The left hand red wire is 9V DC OUT of the Mini PET to the capacitor, and the right hand is 9V DC back INTO the Mini PET from the capacitor. The grey boxes show the original parts mounted in the PET case, and not on the Mini PET PCB. (there are 100nF capacitors in parallel with each of the diodes, omitted for clarity)

With everything wired up, time to set the DIP switches and power on.

For the 12" 50Hz monitor in the 4032 I set switches 6 and 7 on, and the rest off.

Switch on, and there was the familiar chirp and there is the ready prompt.

I ran various test programs, including the built in self test (SYS 40960), and my old BASIC memtest program.

Maybe I am getting slightly better at taking pictures of CRTs?

To show an alternate use case, I removed the board again and installed the optional composite video out and DC in components. That allows the board to be tested outside of the PET case, or you may have some other use in mind.

Those allowed me to do some further bench testing with an SD2PET and a datasette.

The Mini PET B can be used stand alone like this, although, you probably want the Mini PET A instead.

Here everything again passed all the tests, so time to try a few games, I think I will load one from tape for old times sake.

All looks good. Loaded first time.

Here he comes, good old Rex.

Ooooh, I found the exit this time.

So there is the Mini PET V V1.58


The Mini PET B and A are available in kit and built forms from my SellMyRetro store:

Mini PET A

The Mini PET is a Commodore PET compatible 6502 based computer with 32K of RAM and a choice of PET BASIC 1,2 or 4, or Mini PET BASIC 4.0.

This is the stand alone version that can be used with one of my keyboards or another aftermarket PET keyboard (normal / graphics layout recommended for better compatibility than the business keyboard).

Mini PET A on SellMyRetro

Mini PET B

The B variant is designed as a drop in replacement for a PET motherboard. No keyboard options are provided, as you are expected to use the PETs original keyboard.

In addition to the DC power and composite video output of the A, the Mini PET B can use the PET power supply (5 pin or 9 pin) and drive a PET 9" or 12" monitor.

Mini PET B at SellMyRetro:


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