Fixing a disk corruption / reboot / no-boot problem on a Dick Smith System 80 (aka Video Genie or PMC-80)

The problem

The Dick Smith System 80 was my first computer and as such is a treasured member of my collection. Actually I should say "they are treasured members" as I have three of these models. One is a 48k "Blue Label". This is the best in the fleet and works just fine. Another is a rather beat up 16k "Black Label". I consider this a parts-machine. The third unit is my original Black Label 48k System 80. It's this latter computer that's the subject of this article.

The machine had no issues when working as a non-disk unit. Plug it up to the expansion interface and drives however and strange things happened. If you were LUCKY, the machine booted successfully. If you did manage to make it through to the DOS prompt, it was just a matter of time before things would turn to custard. The machine might freeze, or suddenly return to non-disk mode. Sometimes it would just access disks for no reason at all, often corrupting them in the process.

One of the advantages in having another 48k machine (the Blue Label) is that I could nail down the problem to the computer itself rather than the expansion unit or drives. The Blue Label manifested none of these problems. Disk operations were smooth and stable. Something in this Black Label unit just did not like being connected to an expansion unit and disk drives. I set out to find out why.

Diagnosis

My first thought was oxidisation on the expansion card-edge connector. With the TRS-80 Model 1, this problem was legendary and it caused symptoms much like those described above. I set to with a pencil eraser and gave the edge connector a good clean both on the top and bottom. No change!

With the most likely cause now known not to be the issue, I decided to take a systematic approach to the problem. Having two identical machines is helpful in this regard. Given that the 48k Black Label appeared fine without the expansion unit and drives, I figured the problem had to be caused by whatever was going (or not going) across the expansion bus. I decided to take measurements with a voltmeter and logic probe on the expansion edge pins (Figure 1) of the grumpy 48k Black Label model and compare readings with those found on the well-behaved Blue Label.

Figure 1. 50-pin expansion card edge in the Dick Smith System 80

The address and data lines all gave similar reading between the machines but there was one obvious difference. The power. The voltage coming off the power pin was 5V on the Blue Label but only 4V in the misbehaving machine. I tried other parts of the circuitry. They too only showed 4V when the Blue Label showed 5V. As a further check I dragged out the 16k Black Label and tested its voltage on the edge connector. It was five volts, the same as the Blue Label.

Could it be that the cause of the problem be this low voltage? Perhaps at 4V the machine can get by in a non-disk state, but it needs its proper 5 volts to handle disk access? There was one way to find out. Swap the power supplies and see what happens!

I decided to replace the 4V 48k Black Label's PSU with the 5V 16k Black Label's one. If this was indeed the problem, I could then leave the donated power supply in place. The 16k machine was a parts unit anyway so it didn't matter if it carried a marginal PSU.

Changing the PSU in a System 80 is easy. The pack comes away as a module as seen in Figure 2. Figure 3 shows all three System 80s open, with the 48k Black Label machine on the right about to receive a PSU from the 16k unit on the left.

Figure 2. The DS System 80 PSU part-way out of its compartment. The arrows show the PSU connector

Figure 3. Machine lower left is the 16k Black Label with PSU 1/2 pulled out. The 48k Black Label on the right will be fitted with this.

Result

After fitting the replacement PSU in the 48k Black Label, I tested the voltage on the expansion card edge again. This time it was 5V instead of 4V! Time to attach drives and see what happens?

Success! Complete stability. The machine was used for hours with no reboots and no sign of problems. Normally, the computer would have misbehaved in minutes! A faulty PSU was indeed the cause of this particular problem. My 48k Black Label (and very first computer) was now fine!

Figure 4. A fully-working Black Label Dick Smith System 80

Reflections

This diagnosis and repair took much less time than I'd expected. I hadn't considered a faulty PSU would be the problem. I'd expected it to be some component in the expansion card circuitry that I would have to track back to. Just goes to show you never can tell!

Of course I was lucky in that I had a spare PSU as a replacement. Of more value to the DS System 80/Video Genie/PMC 80 community would be information on how to fix such a faulty PSU. At some stage I'll attempt a repair. It may be just a capacitor or two that needs replacing. (Update: 30th June, 2010. Repair has been done!)

I'm please to get my original machine in fully working order again. There was a price though. The symptom first came to my attention a few years ago. At first I thought the issue was a faulty drive so I dumped what was probably a fully operational Tandon TM100!! Moral of the story. Never dump suspected faulty gear unless you KNOW it is faulty.

And even then, only after trying to fix it! (-:

Tez
29th June, 2010