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Restoring my ancient resistor decade box

  • This resistor decade box (AKA resistor substitution box) came with the same lot of equipment I bought half a century ago. I have no idea who made it or when but I have always thought it was probably homemade (but if it was it was well done).
    BTW: I know where the knob that matches went but I won't be able to get to it for about a week or so.
    [​IMG]

    When I was using it a while ago I noticed that its resistance was approx. 10% high on just about every setting so I opened it up thinking some of the switch contacts were probably dirty (in retrospect, I should have realized it was unlikely that the X10 rotary switch's contacts would have 1-10Ω resistance and the x100 switch's contacts would be 100-1000Ω). When I saw the resistors inside it I started measuring them and nearly every one is close to 10% higher than it should be. I know that carbon composition resistors can drift out of spec over time (one of the reasons you don't often see them these days) but some of these resistors were leaking wax (I've never seen that before except on very old capacitors) and most of them don't have standard colour coded stripes. I wouldn't be surprised if these were WWII surplus mil spec parts (there were still a lot of those in surplus stores in the 1970s). Definitely overdue for replacement with modern resistors.
    I looked up power ratings vs size for carbon comp resistors and concluded that they were probably 1 or 2W so I ordered 20 each 2W 1% resistors in 10Ω, 100Ω, 1kΩ, 10kΩ and 100kΩ and put it aside.
    [​IMG]

    The resistors came today and I didn't have another project on the bench so a few minutes after I took the pics above it looked like this.
    [​IMG]

    Once it was apart I cleaned up the case and took it to the garage shop for a fresh coat of paint, then put an 800f tip in the soldering station and unsoldered all of the old resistors & wires from the switches.

    April 1 update:
    After the paint dried overnight I used a heat gun to get it to too hot to comfortably touch, let it cool while I soldered the resistors onto a switch and repeated until the new resistors were all soldered and there was very little solvent smell left from the paint (ideally I would let it cure until there was absolutely no smell left but it should be good enough).
    [​IMG]

    And no, I didn't paint it to match the new resistors. I'm trying to use up old paint for things like this so this time I used some Pontiac Blue Engine Enamel left over from painting a motorcycle about 25 years ago.
    [​IMG]

    I also got the 5th matching knob and cleaned all of the hardware in the sonic cleaner. It is only a an old motor & eccentric denture buzzer but it works surprisingly well for stuff like this and there was a lot of dirt in the bottom of the tank. I didn't want to take a chance on removing the paint from the vintage Mallory dial plates so they'll have to do with a good rub with a rag and soapy water.
    [​IMG]

    BTW: The mismatched nuts, washers and lock washers seem to confirm my idea that this was home made.

    April 4 final update:
    It is often the fiddly stuff that takes most of the time and lining up the dial plates and the switches was no exception. The process went something like
    1) set the switch to the middle position (5), then thread one nut onto the switch all the way & add the lockwasher, put the switch through the panel, put the dial plate over it and add the plain washer and the 2nd nut (finger tight)
    2) line up the dial plate and the flat on the switch shaft by eye and tighten the nut a bit more
    3a) for the first switch (x10Ω) put the knob on and adjust everything again
    3b) for all other switches set the knob's set screw in just far enough that it can go on and turn the shaft, turn the knob a step or 2 to get at the set screw and tighten it, turn the knob back to 5 and adjust everything again
    4) remove the knob (for all except x10 turn the knob go get at the set screw first), tighten the nut, put the knob back on, check that 5 on the dial is still centred on the panel and that the knob points to it in the correct position
    5) remove the knob again, loosen the nut and repeat from step 2 until the dial plate and switch actually are lined up correctly
    6) repeat until all switches are mounted

    Once that was done soldering on the new wires was next
    [​IMG]

    I started to put it together with the original screws but changed my mind and used some #4x1/4" Robertson hear sheet metal screws. It was originally labelled with dry transfer lettering and whoever did it didn't clear coat over the labels to seal & protect them (you can see where I touched up missing digits with a marker in the before picture). I could have done the same but I didn't want to wait for the clear to sure before assembly so I decided to use my new Dymo labeller instead. I always found it mildly confusing to have the labels between the dial plates (only mildly because it wasn't hard to figure out if you looked at all of the labels) so I put the new labels where there is no doubt which one is for which switch.
    [​IMG]

    And that's the project finished. The new resistors average about 0.3 - 0.4% low (much better than 10% high) and it should be good for at least another half century (well beyond the time I expect to last).
Martaine2005 likes this.
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