Chauvin Arnoux Conta 20011

[HalFET]

I saved this one from the trash pile quite a while ago, and could find no real data on it other than a few pictures of long passed eBay sales and some references patent texts. It's a portable/bench multimeter, and a fairly crust one at that. It runs of six 1.5V C batteries, or you can choose to directly feed it 6V, though I couldn't find any plug that fit. But it seems to have a few curious design features, especially the connector at the top, which is for an earthing resistance measurement add-on apparently ( http://www.freepatentsonline.com/4590421.pdf ). The main body consists out of plastic, but the front and rear panel are made out of aluminium which was coasted with some sticky rubbery plastic which has started degrading. The earth terminal on the back hooks up to both of these panels, and nothing else. Based on the label, this one was probably used by a government agency or the military, and acquired/calibrated in May 1984, and has been gathering dust since the early 90s most likely. Or at least, that's what the state of the batteries in the thing led me to believe. Calibration was due November 1984, so it's about 33 years out of cal but still seems to be sort of ok versus a Keysight U1232A. Given that every part is labeled 1980-1982 (other than the fuse which is from 84) we can probably assume it was manufactured in 82 - 83.

Now at first glance this thing looked incredibly crusty, and they probably had significant issues with noise. The internal shield (which seems an afterthought) is a thin PCB mounted at an angle, a similar approach was used for shielding the precision resistor array (Caddock), they attached a piece of copper tape to it and then ran a very long wire to what seems like a grounding point. A similar shielding approach was applied to the bottom of the board, just a thin piece of circuit board with a wire soldered to it. The calibration adjustment resistors are on a vertical board, with what seems like a high turn count potentiometer. There are some pretty big bodges in there, such as wires directly soldered to pins of socketed components. But at the same time they went for an AD536A true RMS converter, which couldn't have been cheap back in 1982. It runs on a ICL7135 4.5 digit converter, with an ICM7211 display driver. Garden variety of opamps is present (CA3130, LM308, etc.) Switching seems to be done using HEF4051 chips. Interestingly enough, they do seem to use a MC14046B VCO/PLL to generate the sample clock, and since I can't seem to find any immediate reference around I presume they're using the zener diode in this chip as both frequency and voltage reference for the entire instrument. The negative voltages were generated by two ICL7660 switch mode converters. The case itself was also made in France, with threaded metal inserts mind you!

So overall it's an interesting little thing, they certainly didn't cheap out on it at first glance. I'm curious if anyone has more information on it. I've attached some pictures, I'll take some higher resolution ones once the camera is fixed again.

Front panel:


Back panel:


Top:


Botom:


Battery terminal:


DC voltage measurement:


Fuse:


Module connector:


Unit side view:


Behind shield:


In front of shield: