Sullivan & Griffiths / H.W. Sullivan Non-reactive Resistor Decade Box
Problem
We bought a resistor decade box on ebay for a very reasonable price - it was evident in the listing that there would be some issues with it. The date on the unit is 1956 and the construction of the unit is very high quality.
The switches are formed of stacked copper shims and wipe against brass posts, like a very high quality mechanical distributor. Testing the measured value and cycling all the controls it became obvious that the 100s dial had issues.
The measured value was very close to the set value, all the way up to around 500 Ohms, then the reading was higher than expected from there on.
Another unexpected step occurred a few steps later on the same switch. Oddly, each time there was an unexpected change in the resistance, it was roughly 200 Ohms, instead of 100 Ohms.
Diagnosis
It turned out that each resistance element was formed from two paralleled windings of resistance wire on a ceramic former. It was wound so the inductive properties of the two opposing coils cancel out.
In each case where we found an issue, the value was 200 Ohms instead of 100 Ohms and the wire was burned out in one half of each resistance.
Since appropriate lacquered resistance wire was impossible to obtain and normal magnet wire would be impractical to achieve 200 Ohms, we opted to replace the damaged sections with modern precision resistors.
To maintain the non-inductive property we placed two 200 Ohm resistors in parallel and in opposing directions to mitigate any inductive effects as much as possible.
Solution
This was a good find, fun to repair and now this is a part of my test equipment.
It has several labels/legends with dates, hand written on the inside of the front panel is 15-10-56 1956 (originally manufactured), 27-05-(19)88 it was tested last and now it carries my own “Test passed” label, printed in 2020.
It also has a Plessey seal label which is broken but still a complete sticker, giving some interesting glimpse into the history of this unit.
If you are experiencing similar issues and would like to discuss or request a quote, email us at
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