Running with only a minor fault

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After setting up the ROM Emulator for a 700 series machine there was no harm in bypassing basic tests and attempting to boot up a complete 700A microcode image.

… and it worked.

The machine came up with just one blank Nixie in the Y register. All other functions seemed to work properly, at least to an initial checkout. The blank Nixie looks physically OK, so hopefully it’s a driver transistor or similar, rather than a dead tube.

The ROM Emulator installs nicely in a space behind the PCBs, two of the Volt Clamps are visible beside the Emulator board. The ROM connectors pass easily through existing gaps in the chassis and the entire installation can be done without any modifications to the metalwork. The ROM Image selector switch can be accessed between the blades of the cooling fan, so no case modifications required.

Nixie Driver Repair

Nixies in vintage calculators are sometimes slow to light, perhaps a combination of cathode degradation and/or driver transistor deterioration. It’s often worth waiting to see if the cathodes will recover after being under power for some time.

In this case the Y register tube did not show any signs of activity and testing showed that the 2N3490 anode switch transistor was bad.

Substitution with a BC639 restored the display. It was not easy to find 2N3490’s so the appearance of a modern plastic package must be tolerated.