and now to something completely different:
Restoring a Kontron / Tabor Electronics DMM 4021 that was saved from the scrap heap:
Shown here measuring outdoor temperature with a PT1000 sensor. Replaced some caps with modern 105 deg low ESR types, adjusted it with the aid of my K2000 and built a new NiCD rechargeable battery pack.
More TEA p*rn:
Lots of Dale precision resistors, main voltage divider Caddock 1776-101, input components isolated from PCB by Teflon isolators, protection diodes low leakage in metal cans (PAD50 by Siliconix), ADC is a LD120/121 combo from Siliconix. The datecodes point to a manufacturing date of mid 1982. Price in Germany back then 1000DM.
The most interesting feature is "self calibration": by pressing otherwise unused button combinations you can check the accuracy of the DMM. There is a calibration assy on the main board:
You can see a collection of Dale 0.1% resistors for checking resistance and current, and a REF-02EJ precision bandgap reference in a metal can. It should have stabilized a bit during the last 38 years
Another button combination allows to check the internal battery pack. During operation, the battery pack is trickle charged, as soon as the meter is switched off, charging with 150mA starts and runs as long mains is connected. This charging method works well with NiCd cells but wreaks havoc with NiMh. Luckily I had some cells from a power tool lying around that went into the new battery pack.
Main problem now is finding a schematic and calibration information. The schematic over at radiomuseum.org seems to be a bad scan of a seventh generation photocopy and is missing part of the power supply. Another problem are the plastic front and rear frames: they are extremely brittle and glueing them together didnt last for very long.
Apart from being a fun project, this DMM was my first one at work, 33 years ago...
Rainer