Last month (Sept '23) an excellent chap called Simon Peacock emailed me about a machine he had that he'd like to give to me. Simon was the software side of Positron who worked with Peter Plinston and Microware to make these machines do useful things. It turns out that after he'd left Positron he'd started a company with Positron's hardware engineer and had also taken this machine with him. It's a Positron 9000/4 board plus 9150 floppy controller in a very Positron like case, coupled with extra hardware to make a phone call logging system for 1980s computerised PABX systems. It was made by Datapulse Limited and is called the Exchequer, there's a similar amount of info out there about these machines than there is about the Positron 9000. In fact I've only found one mention in a book about PABX systems where it shows some printouts of what the Exchequer can do. Unfortunately there's no contact details listed and 'Datapulse Exchequer' is quite a difficult thing to search for.

The machine is complete and contained a notorious VARTA MEMPAK 3.6V clock battery that thankfully HADN'T leaked all over the board. These batteries are notorious for absolutely destroying everything they touch, so I'm hugely grateful that this one only leaked a tiny amount that was easily cleaned up.

Inside the box we have a Positron 9000/4 motherboard, Positron 9150 floppy controller board, a pair of 96TPI DSDD floppy drives and a bespoke board from Datapulse containing a Z80 CPU and some other gubbins, photos yet to be taken. There's also a flying lead terminating in a DB25 plug which contains a tiny circuit board hooking up CTS, GND and floppy pin 16 which is MOTOR B ON, so I'm guessing that this cable plugged into one of the Positron's 4 RS232 sockets and could control the motor action of drive B.

Most importantly of all, Simon gave me the user manual for both the 900 and 9000 which he'd written back in the day. It provides an awful lot of useful information on the switches, memory map and how to connect a serial console. Simon also gave me some excellent Positron literature showing the different products they made, the most interesting of which aside from the machines was a set of ICE modules for chip design and prototyping. I wonder if any of those things are still kicking around in 2023?

For now, enjoy the pics. I haven't dared power up the machine yet, though I've de-RIFA'd the PSU and tested that using the floppy drives as a load, all ok. More soon. These photos are all from Simon as I haven't taken any myself yet.

Did a full investigation on the machine and was slightly puzzled by the switchpacks. My Posi 9000 has 4 8-way and 1 2-way which is what the manual says, but it also says this for the Posi 900 - however this board has 5x 8-way and 1x 2-way. I decided to go for broke and hooked up my FTDI USB TTY to port 1 at 4800 baud. On powerup, my current limiter wasn't happy so I immediately suspected something was up with the PSU and sure enough, the voltage reaching the Positron board was only 3.47V. Further investigation on the PSU showed that the output reservoirs were fine but the actual smoothing caps were largely dead - removing them released a lovely fishy smell. Since I had extra PSU tails from my Positron 9000 I powered up just the board using a venerable Meanwell T60B and you know what? It only worked :D

Downside - it asked for a System Disk. Happily I do have one of those but unhappily it had sat in the machine for all the time it had been in storage so it may have gone mouldy and bad. It's only side B that's got a tiny mouldy spot on it though, so if the disk is actually single sided it should be fine. I'll find out over the weekend.

I've realised I've done loads of work on the Posi 900 in 2023/24 but not updated here! As you saw, back in 2023 the machine worked fine so with some excellent helpers I managed to image both the System Disk and the data disk in drive B. While doing disk tests I noticed that the machine was only reporting 48K RAM and not the full 128 so I got out my 4164 RAM tester and removed all the RAM chips. In doing that the power pin of one of the chips fell off leaving no metal to be able to solder a new leg to.

After chipping off enough of the plastic package to reveal a bit more metal I managed to solder a component leg to what was left and ermanently mounted the chip in its own socket. It tested OK so I went through the rest and repopulated the board. Which didn't work. Huh. It was fine before RAM removal but not after replacing it all? That's where it was left while I tried to source a complete replacement RAM kit of Inmos P2600-12s, which are 4164 compatible. I eventually tracked some 4264 chips which were the right speed but the machine still fails. And that's where we still are in 2026. I discovered that my HP1600A logic analyser can actually debug twin 6809 machines, but I need a pair of 6809 pods for it which I don't have and neither does ebay. Perhaps one day.