Category: Dick Smith System 80

The system, as it arrived, looked like this:

I set the monitor aside as a separate project. It had very crude and broken mod that held a sheet of coloured perspex over the screen. I will eventually add that back.

The main unit was in good shape. I just reseated the main RAM.

To judge from the date codes, it was made in 1981 when I was still at high school.

This machine uses an old school linear supply. I inspected it and reformed the power supply capacitors by bring up the supply voltage slowly with a variac.

Two new keys where the second shift key was. The sideways A is the tab key.

Switch added at the top of the pic. Shift changed to backspace. Backspace to clear.

With power (and a kludge for the video cable) it came to life.

The plastic had melted against the soft plastic dust cover. It looked pretty awful.

I had to make some compromises. I used a combination of acetone and wet & dry paper to get it back to an acceptable finish. It may be possible to do better.

I crafted, and then glued on, a replacement door latch. It failed, so i had a second go and that is still holding several years later. To be fair, i don’t use the cassette deck very much.

The tape unit itself was gummed up, but after a clean and lube, it seemed like it might work.

There were two different types of expansion units for the System 80. This is the early one with the S-100 bus.

The first card connects to the System 80 via a 50 pin interface. It also provides the floppy, parallel and RS232 ports. I particularly like the termination networks that have been added to several of the ICs.

The second card provides 32k of RAM. There was one faulty RAM chip.

The power supply unit is the same as the computer. I reformed the caps with a variac.

There is one spare slot for an additional card but the case is not great for adding additional I/O and there’s no obvious mechanism for adding more memory beyond the current RAM/ROM.

The system came with a matching Floppy Drive Unit that connects to the expansion unit. It uses an MPI B51 40 track single sided drive.

I gave it the routine clean and lube, but it would not read disks. Drive speed was good.

Configuration was preset:

I located a service manual on the net and that was very helpful for fault-finding.

I convinced myself that the differential amplifier (CA3054) in the read circuit was cactus and replaced it. With this, the system booted. Alas, it wouldn’t format disks.

Breaking out the CRO and checking the signals on the drive interface showed there were no index pulses. In an unexpected twist, it turned out that the disks i was using were not sufficiently opaque. Is that even possible?

It appears so from the traces; the voltage in the top picture never gets high enough to flip the comparator, whereas in the second picture, with a different disk, it does.

I checked over and over. It’s fine with five other brands, so I’ll call it good enough.

It lasted about a week before it stopped reading from disks.  Perhaps it had an intermittent problem and the original differential amp didn’t fix it. 

With the scope, I was able to see differential signals going into the LM311 comparator, but nothing coming out.  I replaced the comparator and i was back in business.

I made up a few disks and had a play!

I also had no trouble setting up a gotek as a drive. I just powered it off the real drive and connected with a custom cable. It has to be set to drive 1.

The System 80 came with a Thorn 7433 black & white TV that was used as a monitor. It had had a couple of homebrew additions.

The first was a composite video input, so modulation/demodulation is redundant.

The second was a green screen, which was appropriately agricultural. The mount has broken and in any case is too rough even for my low standards.

It seems to be working fine.

I have removed the mount, and rubbed back the glued areas. It’s currently awaiting paint and then i’ll attempt to mount some coloured perspex that looks a little more appealing.