Hi Adrian, welcome.
Congratulations on your acquisition, if it's the one I think it is, the listing says "All switches and resistors visually checked. Cal checked" and mentions the 6-7 switch issue on the 0.01 decade to be fixed. I'm not sure if you have any comeback there.
It sounds as if it's generally in good condition anyway, and I can't see how any problem on that specific setting could be caused by any sort of overload. You might find the occasional unit that has been used as a rheostat, but for voltage divider use, they are pretty bulletproof. I would expect it to be well within spec anyway.
The contacts are solid silver stud with silver graphite wipers. The listing mentions DeOxit has been used. If the contacts are showing 'crud' then, yes, a good clean with IPA would be certainly be a good idea. The switches are supposed to be self-lubricating, but if it's been sitting for a long time, there's still the possibility of oxides or sulphides forming. Exercising the switches should be enough to clear that but I've found that the smallest trace of Electrolube EML, or even Vaseline keeps the surfaces well protected.
It was quickly evident that the end-end resistance of one side of the decade in question was exactly and precisely one resistor's worth too high at 1K0 rather than the 900R I was expecting, sooo:
1/ Hard to be sure by looking but based on the number of solder joints I think I see per stage, am I correct in my assumption that each of the resistors is actually wound as two in parallel to cancel winding inductance?
From memory, and looking at my photo above and the linked post, yes, I think most of the decades use parallel counter-windings to minimise inductance. I'm not sure about the highest one, I suspect that that one has enough resistance to swamp out the stray inductance.
2/ If (1) is correct then my hope was that it could just be a 'dry joint' (a cold joint on the other side of the Atlantic!) but the 2.5 joints I can actually see/get to look fine so I have not tried a reflow (yet!).
I suppose it's possible that one of the joints is dry, but construction quality is very high. Make sure you aren't seeing the effect of the next decade down, which is being switched in series with the resistor chain that is fooling you - it's best to measure the individual resistors and make sure you understand where the wiper position is when measuring them. I've attached the data page, which includes basic schematic below.
3/ If (2) is actually a possibility, how easy is it to take things apart enough to get to any solder joints on the reverse side of the card and, indeed, are there any?
Yes, that's entirely possible, the resistor cards would have been adjusted and matched before they were soldered into the switch banks. Maybe you can peer through between the solder joints from the other side of the switch with the aid of a strong light. Unfortunately, as you can see in the above photo, some of the 'pins' of the resistor card extend into the little holes in the switch body (bottom in the photo) for mechanical support, so you would need to unsolder all of its switch stud connections to get a card out. Entirely possible with care, but you need to be sure you need to first.
As a check I soldered a 200R type RC55Y precision resistor I happened to have in stock across the appropriate switch wires and the thing now works as it should.
Hmm, it does sound as if there's a problem then, but if the switch is twitchy then I'd address that first.
Home
Help
Search
Login
Register
