I have not tested this yet. An additional character RAM has been added at Z46. The machine seems really designed to have this IC because the modification seems quite simple. This is covered by Technical bulletin I:35.
The lower case version of scripsit “SCRIPSIT/LC” demonstrates it.
The reset switch just parallels the existing reset switch. I added a series connector so that the lid could be separated.
Top cover connections
The mods add an LED (turbo), reset button, and joystick mod. This makes it impossible to remove the cover from the main board. I’ve added in-line connectors so it can be detached.
The system did not come with a floppy disk drive. I did my set-to work using a Dick Smith drive from my System 80 and my generic drive unit.
Ben G from the Australian Vintage Computer Collectors had a genuine one, though, which he passed on at very reasonable cost (not the first time he’s helped me out).
I made up a new cable, as I’d been using the System 80 cable. The Tandy cables used connector pin removal to set the drive numbers – all the selects are connected inside the drives. I really need to pull some pins from the connector but for the short term I taped over DS1 (p12), DS2 (P14), and DS3 (p32) leaving just DS0 (p10) on the drive connector for the Tandy floppy drive.
As is often the case with 40-year-old drives, it did not immediately work. I did the usual clean up, lubrication, and alignment (using Imagedisk) but it had a peculiar issue of losing half the head steps. This turned out to be a timing problem with two monostables that control the stepper motor. One had an adjustment pot, but I had to change the resistor on the other one.
Apparently i couldn’t find a schematic.
Once the timing issue was resolved, the drive could be aligned. Adjustment is by loosening the motor screws and rotating the motor body, but it didn’t have enough play, so i had to move the shaft coupling a little as well.
I found the cable unreliable and after several frustrating hours of checking and rechecking I found that the floppy drive connector on the expansion box had been ever so slightly modified. One side had been filed down – presumably to accommodate a tight floppy cable connector.
The connectors I used on my new cable were just a little wider, so it was possible to be out by half to one pin, resulting in shorts/opens. I need to build up the connector again, but it works fine as long as you know and position correctly!
Why “jmp start ;microcomputers”? Well, ultimately it’s just a name. It sounds like “jump start”, but it’s also a pretty common line of assembly language. It’s a bit technical, but so is this site. I tried some AI names, but this came out of my head.
A microcomputer is a computer that is based on a microprocessor; a highly integrated processing unit that first appeared in the early 1970s. The term “microcomputer” was used a lot during this early adoption period but fell out of fashion by the early 1990s. Microcomputers are now referred to as PCs, notebooks, tablets, phones etc, so i use the term “microcomputer” as a shorthand for “vintage microcomputers” which are the subject of this site.
As i emerged from high school in the early eighties, microcomputers were the tech that everyone wanted, and this prompted me to enter the world of electronics. I got to play with several early machines, but there were so many that it was impossible to experience the gamut.
Over the last 8 years, i’ve had the opportunity to not just reacquaint myself with some familiar machines, including a couple i sequestered from those times, but also to experience many that i missed the first time around.
I’ve purchased many of my treasures through well known online marketplaces, but i have also acquired many from colleagues at the Adelaide Retro Computing Group – always at reasonable cost and often as generous gifts. I’ve tried to acknowledge significant gifts as i’ve added detail, but i apologise in advance if i have missed anyone.
I’m typically interested in workhorse machines but use the occasional game to demonstrate what a computer can do, or in many cases its limitations.
Making machines shine figuratively is my goal, including:
Getting them working
Getting them working again after they stop
Adding software
Adding accessories
Connecting them
My machines are not pristine or mint – i aim for tidy and functional. I like them to be exercised regularly and standing by, ready to save mankind when the current tech collapses in upon itself like an abused CRT.
Generally i will try to make sure that machines are either original or can be readily taken back to original – and that includes modifications from, or typical of, the operating period.
I do compromise on hard disks; they are difficult to source and inherently unreliable. I never bin them, but i don’t think twice about replacing them with hard disk emulators. I also use disk emulators alongside original storage peripherals to make it easier to try, and to demonstrate, a greater range of software.
The purpose of this site is to:
Show off some of my machines
Make what i’ve learnt available to others
Promote related local activities and groups
The info that i’m posting has been extracted from the notes that i have accumulated since 2017. Those notes were pretty rough and not always complete. Mostly i’ve rewritten them here, but their heritage still comes through in places.
My hobby relies on the amazing work that has been done by so many people to archive data – programs, disk images, data sheets, schematics, manuals, articles, photos, problems and solutions. I use these resources extensively, and i am grateful for them. I try to pay this forward by archiving disks and documentation to archive.org or other appropriate sites such as Microbee Technology.
I started attending ARC Group meetups in about 2018 and have been a regular since then. This group has been a game changer for my hobby.
Some might describe retro computer collectors/operators/builders/gamers/hoarders as a little unusual, and numerous stereotypes are routinely rolled out. Many of us wear them as badges of honour.
Retro is a vague but inclusive term, and the group members and meet attendees reflect that diversity. This group is an opportunity for those people to meet in a friendly atmosphere to play games, show off builds, acquisitions, and restorations, and to talk bullshit about computers and life in general.
As a group, we have a lot of knowledge and skills that are of value to each other. There are many people with eyes on marketplaces, auctions, recycle bins, garage sales etc, so often group members are able to direct gear to the people with matching interests. The group is often a focus for downsizers etc, who are looking to clear cupboards and sheds.
If you’re in Adelaide and you have an interest in the retro computing world and adjacent interests, then come and have a look. The group meetups are the second Friday of each month. Check the facebook page for details.
I have become familiar with tools for recovering files from floppy disks. I’m happy to help people recover important files from the past.
If you have files on 1.44MB 3.5″ floppy disks then just buy a USB drive, plug it into your PC, and copy the files.
If you are in Adelaide or thereabouts, and you have 720k 3.5″ disks, 5.25″ disks or 8″ disks that contain some important files, and you have had no joy with professional recovery services, then contact me and i’ll try my best to help.
Also, contact me if you have 5.25″ or 8″ disks that need a new home!
The Commodore PET (branded CBM in some markets) computer is an icon of early microcomputer history. There are very few in Australia and the only ones I have seen in South Australia was the one in the Adelaide Uni Physics labs back in 1983 and one imported from the UK. I am aware of one other that was for sale on gumtree about 5 years ago.
It was some surprise then that I spotted this image on facebook marketplace. I was further surprised when I contacted the seller and he said he had a second machine covered in pigeon poo.
Presentation is everything, so i was interested of course. We came to an arrangement for the first machine and i drove (with my wife for company) up to Snowtown (a town with a story) where it was located.
The site was an ex Telstra maintenance site, long since abandoned to the pigeons and probably a myriad of other undesirables. Just walking into the building was a health risk and the smell was unbelievable.
I paid a smaller sum for the filthiest computer ever, and another small donation got me a very disreputable 2031 disk drive. The other drive is not related to these machines.
I pondered my sanity as I put them on a drop sheet in the back of the car.
I do have some standards about what comes into my “clean” shed and both machines were well below the threshold, so they went into the “dirty” shed first.
Opening the covers proved confronting. The first was not too bad but very dirty and corroded. The second machine and the drive almost made me gag. They were full of some combination of pigeon fluff and pigeon manure, which I suspect just blew in over a long period. I donned rubber gloves and a face mask and began the ugly task of removing the debris, first in handfuls and then with the vacuum cleaner.
It was clear that I needed to disassemble the second machine and the drive unit if I was to get all of the offending material out – and that was a necessary first step to just being able to stomach and store the machines.
Having got most of the innards clear of crap, I started washing down the cases. This was an unpleasant task, but was not difficult. Pigeon mature seems to wash off fairly easily, and the largely plastic construction seems to have been unaffected by its coating. Some stains remained, and the metal parts had some corrosion, but I was happy with the improvement.
The second keyboard was set aside for the moment.
At this point the machines were allowed into the clean shed, although they were far from clean. Remarkably, though, they both seemed to be complete – not even a missing keycap. The condition of the mainboards was poor, though.
These machines are both model 4016-N which means that originally they would have had 16k of RAM and 40 column text.
They both seem to have been upgraded to 32k of RAM and 80 column text.
I thought initially that they must have been used by Telstra, and perhaps they were, but they clearly once belonged to TAFE.
These machines were produced late in the PET’s product life. They were obviously under cost pressure, eg there are a lot of places where rivets were used instead of screws.
80 columns is handy for things like word processing, but a lot of the games were produced for the 40 column models. It is possible to put the machines into a 40 column mode, but the characters are still narrow and the proportions changed.
These machines use universal boards so they can be reconfigured to 40 columns – this would require several links to be changed and a new edit ROM.
There was a lot to be done before any reconfiguration would be on the agenda.
Both boards have the same set of ROMs. Some of the ROMs on the first board are soldered in but all of the ROMs on the second board are socketed. The Proteus programmer was able to read the EPROMs, but the ROMs are a little more difficult. The program pin on the EPROM is a chip enable on the ROMs so it was necessary to make up an adapter socket for the ROMs to pull this pin high.
Board
Type
Label
Socketed
Location
Address
Condition
Description
2
EPROM
Y
UD12
0x9000
36864
-28672
Word Processor
2
EPROM
Y
UD11
0xA000
40960
-24576
Toolkit / Extramon
2
ROM
901465-23
Y
UD10
0xB000
45056
-20480
Verified
Character ROM
2
ROM
901465-20
Y
UD9
0xC000
44152
-16384
Verified
Basic 4
2
ROM
901465-21
Y
UD8
0xD000
53248
-12288
Verified
Basic 4
2
EPROM
Unlabelled.
Y
UD7
0xE000
55344
-8192
Looks plausible but doesn’t exactly match any candidates
Probably an edit ROM of which there are many.
2
ROM
901465-22
Y
UD6
0xF000
61440
-4096
Verified
Kernal
2
ROM
901447-10
Y
Verified – file size is incorrect but data is good
