Inco Decade resistance box MDR-93-5a - Page 1
Forum section: electronics / testgear

author: pizzigri
author_rank: Regular Contributor
timestamp: September 30, 2022, 11:15:53 am
content: Hi everyone,i just found on Ebay an Inco Decade resistance box MDR-93-5a, this was made in Poland in 1975 and declares a basic accuracy of +/- 0.05%.It only goes up to 11K and change - well, theoretically11111R to be precise - so it's not the most useful thing in the world, but still...I got it awfully cheap, something along the lines of 30 Euro, and it was originally an educational tool at an Italian Technical/Industrial institute in Rome. I expected an unusable, burnt resistor box (since in the pics it also had one corner broken, instead it's not bad! Being a hobbyist and not having lots of money - I wanted to build a resistor box for ages but the cost of the resistors alone was out of this world, let's not talk about rotary switches... I imagined that for the price i could reuse the switches themselves at least.But here ìs the box:

author: pizzigri
author_rank: Regular Contributor
timestamp: September 30, 2022, 11:25:15 am
content: And here is a small teardown...removing the broken Bakelite sides. Probably going to remake them out of plywood, the finish is faux walnut anywayThe switches are sealed, and sincerely I did not want to risk not being able to put it back together at this time.All resistors are hand made, using constantan and manganin I suppose. Lots of man hours into this. All connections seem to be made of Silver.Detail of the 1K decade ressitors.I was curious if the individual resistors were inductive... so I connected a cheap but accurate LCR bridge to one of the resistors.And it does seem to be inductive... but now I'll see how it performs, with a recently calibrated Agilent 34401A.

author: Kean
author_rank: Unknown
timestamp: September 30, 2022, 01:10:24 pm
content: Quote from: pizzigri on September 30, 2022, 11:15:53 amNice score, and in pretty good condition for its age.You do have appear to one too many zeros/decimal places in that basic accuracy spec above though!

author: pizzigri
author_rank: Regular Contributor
timestamp: September 30, 2022, 01:42:47 pm
content: You are absolutely right! I was copying the faceplate specs, and an additional zero somehow found its way in... haha, I'll just edit it now. Thanks!But here's some measures. I am not using any metrology techniques here, I only nulled the leads as I do not (yet) have proper Kelvin clips (Frankie alas does not ship to Italy, and I had some other expenses in the meantime. I still lust after a good Siglent SDS 2000X Plus).In any case, I am amazed at the quality of the switches, they are real smooth and have perfect contact! after all these years and abuse at the hands of bored teenagers, the box still works great, and for a hobbyist as myself it is more than enough. Sure, it only has five decades, but hey...too bad the posts do not accept banana plugs, but they have channels etched that allow good contact with classic probes.Set at 1 KOhmall knobs set to 1Everything set to zero, leads nulled, resistance of the boxETA: why the forum software does not place the images where I want them to? I followed all the instructions but it just shows a line of text instead of the pics

author: Jacon
author_rank: Regular Contributor
timestamp: September 30, 2022, 01:46:34 pm
content: It seems they are still manufactured:http://kompart-pomiary.pl/kalibratory/Mdr93.htm(sorry - in Polish only...)

author: pizzigri
author_rank: Regular Contributor
timestamp: September 30, 2022, 02:01:29 pm
content: Incredible... wonder what they cost.ETA: apparently, the updated and current version of the one I have goes for about 1905,64 zł, which is just under 400 Euro. Ouch. Thanks Jacon for the link, there's a price list in the last page...I had a friend tell me that these boxes were found in most technical schools in Italy in the 80s and 90s.Accuracy to my eyes is quite good - especially for a 30 Euro purchase. It's possible that there are more floating around as schools dump old gear at scrap cost. I regret that it's impossible to get to the gear while they scrap it.