MD's 1281 and some measurments have been made

[mendip_discovery]

So a few years back [1], an XDevs member better known as Branadic mentioned to me that a 1281 was up for sale on eBay. I looked, and it was only in Bristol, a short 20ish-mile trip. I made a few bids and ended up winning it, for £1562. Turns out it's had a hard life, should have guessed as the seller was a director of a calibration lab, and it was broken. Various traces on the board were vaporised and even after fixing them and replacing some relays, I still had issues with no signals getting through. So I left it in the corner as I didn't have the brain space to get my head around the repair. Well, recently I got around to getting it into the hands of a grown-up....after much nagging from the xDevs team. The grown-up is the splendid YT personality known as @IanJ to us, and via his channel https://www.youtube.com/@IanScottJohnston, he keelhauled me over, replacing relays unnecessarily, using heavy gauge wire and leaving flux on the board. But over 3 videos, he managed to SPOLIERS repair it.







But I am not here for repair stuff, this is metrology where we measure stuff.

The unit arrived at my work in the afternoon, so like a kid I unwrapped my present and plugged it in. The work 3458A was already running. After I finished for the day I was able to do a few measurements. I did do some voltages but I won't be showing those as the 3458A was out of ACAL and no nulling of leads had taken place but I did do some 4 Wire Resistance stuff.

The lab is 22.6 °C & 52.4 %RH, using a Transmille 3050A calibrated on 28 October 2025.
We have,

• What Transmille measured back in October, plus their uncertainty
• My 1281, 1yr spec, calibrated in 2020, set in TruOhms 8digits and "Filter"
• The work 3458A, calibrated, in June by Keysight, 1yr spec, 8 digits, NLPC 200, Delay of 2 seconds, offset on

• 10 Ω
  
• 100 Ω
  
• 1 kΩ
  
• 10 kΩ
  
• 100 kΩ
  

This bodes well, considering the 1281 has been without calibration for several years and has been sitting in the corner of my room, switched off and unloved. More measurements to follow. I need to find the key for the selfcal switch, as I put mine somewhere safe...so safe I can't find it.

What measurements should I do next? I need to find a suitable cal lab to send this to so it can get calibrated against a voltage/resistance reference more suitable to its quality.



Mendip_D
[1] May 2022

[IanJ]

First time I have used a 1281 in person let alone repaired one!.......but it was great fun, and in my usual style I tried to learn about and describe what I was looking at as I went along.

PS. The gold pins I did retro-fit to the replaced connector before shipping back the unit to Mendip_D.

Ian.

[Kleinstein]

After the repair it may be a good idea to check the meters input bias current in the DC voltage mode.
Ideally this would be at a few voltages in the 20 V range.