Service Manual or Schematic for Simpson 444? - Page 1
Forum section: electronics / testgear

author: don.r
author_rank: Frequent Contributor
timestamp: January 10, 2018, 10:49:03 pm
content: I recently acquired a Simpson 444 Micro Ohm Meter and was wondering if anyone had a service manual or schematic for it. Its in working order but may need calibration and does need a new NiCad pack and a bit of TLC. Also, does anyone know if the leads are simply current and sense connected to the BNCs?

author: Vgkid
author_rank: Super Contributor
timestamp: January 11, 2018, 06:14:46 am
content: I look forward on seeing what you can find. I was eying a parts one , then I actually ran out of money

author: mtdoc
author_rank: Super Contributor
timestamp: January 11, 2018, 06:42:25 am
content: Here's the operator's manualin case you don't have it. No schematics though.Per the manual, connections are via Kelvin probes to each BNC

author: don.r
author_rank: Frequent Contributor
timestamp: January 11, 2018, 06:10:51 pm
content: Thanks. I figured as much about the probes. I'll have to build my own as the unit came without probes.

author: don.r
author_rank: Frequent Contributor
timestamp: February 23, 2018, 10:47:46 pm
content: Trying to figure out the secret sauce for the Simpson. It appears that the only magical component aside from a few precision resistors and some LT1001 op amps is a small Dale transformer which I gather would be unobtainium if it failed. I still haven't tracked down the signals but it looks like that, after passing through the DUT, the 40hz sq wave goes into the transformer and the secondary is then rectified and, I assume, measured by the Maxim 4.5 digit ADC/LCD driver. I guess the trick is getting usable results from a 100uV applied signal.Still no luck getting a service manual or schematic.

author: Vgkid
author_rank: Super Contributor
timestamp: February 24, 2018, 11:38:53 pm
content: Regarding to the workings , I wonder if it is similiar to the keithley 580. But thinking about it , I wonder if it runs a precision dc rectifier /rms/absolute converter for the ac portion , and a manually offset mode for the dc only mode. Though for dc a 765X would be a better fit than the lt1001.

author: don.r
author_rank: Frequent Contributor
timestamp: February 25, 2018, 12:03:53 am
content: Possibly but its 5ma vs 100ma for the Keithley and 100uv vs 20mv. According to the theory of operation, the pulse mode is basically the DC mode running at 40Hz and doing the error calculations automatically. Maybe similar techniques but orders of magnitude in voltage.I doubt I'll get any calibration/service manual/schematic as these units are still being sold new and Simpson and friends probably make big money on cal and service. Only difference I can see so far for the 444-2 vs 444 is the 444-2 has a 100-240V PSU whereas the 444 has an internal fixed jumper to set operating voltage.

author: Vgkid
author_rank: Super Contributor
timestamp: February 25, 2018, 04:26:11 am
content: With that low of an excitation voltage , it makes it rather impressive that it is running that much gain(10,000) without using a chopper amp, especially for the DC test. This is assuming that the test voltage is 0-100uVPeak. If it was +/-50uvPP a chopper amp wouldn't be needed , though it would get interesting to simply measure that accurately. I'm thinking a speedy precision sample/ hold + peak detector.

author: don.r
author_rank: Frequent Contributor
timestamp: February 25, 2018, 04:31:17 am
content: Further investigating will have to wait unfortunately as I have an HP 54645D and a 3456A coming in that will need a little repairing... hopefully not a lot.