H. W. Sullivan Dual Dial Non-Reactive Resistance Box Restoration

[krivx]

I found what looked like a liquidation of a commercial electronics lab on eBay a little while ago. I picked up two resistance decade boxes and a Systron Donner counter with Nixie display all for about €50. I thought some people might like to see some pictures as the resistance boxes are a little different from what I've seen in the past.


I was surprised when these were delivered, they are much larger and heavier than I was expecting. I can't find a production date but the stamped steel and Bakelite(?) controls suggest 1960s era. Both unit were covered in grime.


"Patented Dual Dial Non-Reactive Resistance 0.1% Grade". The 0.1% grade is what caught my eye, but the "dual dial" system is interesting. Essentially, when a decade is switched to the end of it's travel the switch can be rotated further to access a separate decade. The hundreds of ohms range can be switched to a hundredths range for example.




Opening up the case I realized why it was so large. All the switches are huge over-sized assemblies that look like they are machined from brass.




The switches are soldered together with bent strips of copper, there are no wires.


The resistors are custom wirewound around a shared form for each decade.


I did notice this capacitor connected across one resistor , I'm not sure why. It's marked 23pF.


After taking a look at the inside's I removed all the knobs and dials and cleaned up the front panel and case with some window cleaner. There is still a little bit of sticker residue left but I'll take that off with IPA later.


It seems to work well. I can dial in resistances form 0-10k in steps of 1 ohm. There seems to be slight settling time on the measurements but I'm using a slow meter (Extech 330). I might test this again with a Fluke 87 to confirm the values change as soon as the switch contacts move. It's hard to say how well the small ranges of 1mOhm to 0.1Ohm work, as this is right down below what the meter I had at hand can measure.

The other unit I have is similar - three dials covering a 1k-100k range with lower ranges of 0.1 to 100 ohms. I'm pretty happy with these, a little awkward to use but a bargain.

[amspire]

I think the capacitor in series with one of the resistors is probably damp out a high frequency resonance of the box.

If those switches have a contact resistance of 1 milliOhm or less, then they are incredible. You just cannot buy switches like that today.

You have done very well.

[JuiceKing]

Nice. The resistors and switch mechanics look very similar to those found in the Genrad resistor boxes of what looks like a similar vintage.

[krivx]

I think the capacitor in series with one of the resistors is probably damp out a high frequency resonance of the box.

If those switches have a contact resistance of 1 milliOhm or less, then they are incredible. You just cannot buy switches like that today.

The other box I have has a 0.05% tolerance and actually lists a nominally contact resistance of 5 milliOhm. That's including the two screw terminals, 3 decade swithes and the bent metal interconnects, so something in the region of 1 milliOhm per switch might be a reasonable estimate. I'm not sure if I can verify this without a 4-wire resistance measurement.

[Neilm]

I believe that H.W. Sullivan were absorbed by Megger

Yours

Neil

[Conrad Hoffman]

Nice. Those style switches were used by General Radio (GenRad in later years), Shallcross, L&N and some others. They will have milliohm resistance because of the multiple leafs and high pressure. Clean 'em and use a film of Vasoline for lubricant. Hopefully none of the resistances have been overloaded and burned. I've had to rewind a few with new resistance wire.

[krivx]

I am looking to move these on - please get in touch if you are interested.