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Systron-Donner 9000 Digital VOM


This digital voltmeter is a precision laboratory instrument, made in US in around 1968. It has a 3.5 digit display, consisting of 3 nixie tubes and an oblong neon lamp serving as leading "1". The nixies used are National Electronics' type NL-905. These have a Noval base with 3 extra pins in the middle. The digits are upside down, so the main board that contains them, is mounted upside down. In addition, there are 3 overrange, "+" and "-" indicators.

SD9000 inside

A look a the inside, view from the front.
The main board has been tilted backwards.

The logic circuitry is TTL (transistor-transistor logic). Its ranges are:
DC Voltage: 1 V, 10 V, 100 V and 1000 V,
Resistance: 1 kΩ, 10 kΩ, 100 kΩ 1 MΩ and 10 MΩ,
Display range is 0-1500, but the maximum measured value is 1000, an overrange indication is given for values above 1000.

The meter comes with a tilt stand to be placed on the workbench, as you can see on the photograph above. It contains two circuit boards, the main board and the power supply. The A/D converter is a dual slope converter, rather advanced for its time. The display counter consists of three 7490 decimal counters and a 7473 dual JK flipflop, followed by 7475 latches and 7441 decoder/drivers.

Display tubes digits upside down

A close look at the display tubes.
These are mounted upside down.

I bought this nixiefied meter on the quarterly NVHR radio flea market. It appeared to be in good condition, but did not work properly. The display froze at times, or did not reset after power on. There were a number of dry solder joints on both boards. Resoldering these cured the meter.

The readings were off. Fortunately, the manual came with it, so I had the callibration procedures. After callibration, it's a good usable instrument now.

Copyright © 2002 by Onno's E-page         published 2002-12-01