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Way back in the mid 1980s I worked for a small software house that used NCR hardware. Their work ranged from mini systems to micro based stuff and before the World went completely PC focused, NCR made the Decision Mate V, my employer bought them and I rather liked them.

The machine in the photo above is a recent addition to the collection and scratches an itch I’ve had for some time. As you can see it has a half hight 5.25″ floppy and a Winchester (size currently unknown).

This is a Z80 machine running CP/M-80 as with lots of machines of the day. Is slightly unusual in that the expansion slots are accessible from the back of the machine without removing any covers.

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When I bought this machine I was warned that some capacitors had been removed from the power supply. These are 10,000uf, 35V electrolytics and easy to find. I’ve fitted three new ones but sadly this machine is not running yet. The 12V line is dead at the moment and the diagnostic LEDs at the back are saying RAM fault. so some investigation is needed.

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Part 2. Looking for clues.

Looking at the schematic of the power supply it looked logical that Q1 had failed. It’s a big transistor that looks to be doing the heavy lifting. Whenever I work on old machines from the late ’70’s onwards I am always impressed by the fact that components’ fittings and shapes haven’t changed much. A quick trip to eBay and a new BDV64A Darlington transistor was in my hands. Sadly it didn’t help.

Sadly it didn’t help

That should have been it really but no. Disconnecting the PSU from the motherboard however did help. The PSU came up, the fan started spinning and the 12v line was good and healthy.

Capacitors! I remember hearing once that electrolytic capacitors tend to fail open circuit and tantalum capacitors tend to fail closed circuit. The theory now was that a tantalum cap had gone short and was crowbaring the PSU but the PSU was a high quality item and hadn’t blown up or blown the cap up.

That’s an easy one to check. The mother board only had three tantalum caps on it and sure enough the one across the 12v line had gone short. A quick replacement later and we have the fan running and the motherboard still complaining about a RAM fault.

The RAM is made up of eight 4164 DRAM chips. Sadly they aren’t socketed and so I decided to try and remove them. This is tricky and my attempts to do it elegantly failed. Without know which chip(s) it was (were) I took a punt and removed four by cutting the legs and desoldering the pins. I cleaned the holes and fitted four turned pin sockets and four new 4164s.

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Bingo.

RAM fault gone. It seems I got lucky.

Now it’s complaining that it has a keyboard error. Understandable as I haven’t got a keyboard yet!

Watch this space.