Ohmite 3416 Decade-Ranger, Decade Box

[Jeff_Birt]

I bought this from EBay and had to take it apart before heading off to work this morning. I noticed two screws missing after taking it out of the box which is never a good sign, but I did find one of them in the box. In general it is in good shape except for 4 burnt 1K resistors 

Looking at Ohmite's website it does not look like they still make the same series of resistor that is in the box. The 1K resistors are marked:

GP 1/2, 1K, 0.1%, TC25

Which I take as 1/2W, 1K 0.1% (duh), and a TC of 25PPM. The closest I saw on Ohmites website was the Arcol MRA series. Arcol is hardish to get in the USA though, not a large selection have I found. I did find some suitable Vishay resistors in the CMF55 series (about $1 ea.). I sent Ohmite a note to see if they have any suggestions from their product line, after all they have to use some product of theirs to make them as they are still a current offering.

[Vgkid]

Thanks for the teardown. I  almost bought one a few years ago.

[Jeff_Birt]

I heard back from Ohmite who confirmed the decoding of the burnt resistor part number and that the MRA Arcol series was the closest match. Alas, these are hard to get in the USA so I chose the Vishay resistors which were ordered and came in yesterday.

I was able to take the damaged switch loose without having to disassemble the whole unit which was a big time saver. The burnt resistors were extracted and replaced with the Vishay parts. Both resistors are 1/2W metal file, 25PPM TC but the Vishay resistors are noticeably smaller in diameter.

After reinstalling the switch it was tested with my freshly calibrated HP 3478A. I calibrated the meter yesterday in 2 wire mode so understandably the single ohms range reading was off more than 0.1%. But the higher ranges were all well within spec. The zero ohm reading (all switches to zero) was 70 milli-ohms, but given the meter can only read down to 30 milli-ohms and I was using it in 2 wire mode I would not bank on that value, but non-the-less the switch contact resistance is low.

[Jeff_Birt]

And finally the fixed switch. Since the switch body is ceramic the residue from the burnt resistors was easily cleaned up with a Q-tip and alcohol. Even with having to do the repair I am very happy with this purchase.