Inside my ancient Tandy resistor substitution box

I think I bought this thing about 40 years ago, and it still sees very regular use. I wish they still made them like this, but even with 3D printing and a suitable PCB, it's complicated by the need for sliding contacts.

There are 36 resistor values spaced evenly around the PCB, with a matching alignment hole for the spring loaded ball bearing to sit into. Note the two rings of solder-resist to prevent the solder from flowing into the slip ring and contact areas.

The resistors start with unusual values at the lower end, with 5, 10, 22, 33, 47, 56 and 100 ohm, but then settle into standard decades based on 10, 15, 22, 33, (47), 56 and 82 all the way up to 1 Megohm, but omitting 470 ohm. presumably to allow a good range of values within the 36 available positions.

I think if I was designing something like this now, I might just have an array of resistors and a little flying jumper to select the desired value.



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