Calibration of a THS720A DMM  - Page 1
Forum section: electronics / repair

author: mwbarth36
author_rank: Regular Contributor
timestamp: April 07, 2020, 09:44:46 pm
content: I have acquired a Tektronix THS720A that has the error message "DMM Uncalibrated" and would like to attempt calibration. Problem is one of the calibration steps requires supplying 600 Volts RMS 500 Hz Sine wave. That is a 1696.8 Volt p-p 500 Hz Sine wave. Does anyone know of a generator capable of providing this signal?

author: Chris56000
author_rank: Frequent Contributor
timestamp: April 08, 2020, 07:09:16 am
content: Hi!That sort of voltage is normally supplied by a "Multi Product Calibrator" of the Fluke or Datron type costing five figures – they have a very accurate Wien Bridge sinewave oscillator with a V.C.F. and PLL added, and then a very high voltage feedback stabilised power amplifier, plus a very accurate a.c. & d.c multimeter circuit stable and reliable enough (almost 3458A quality!!) to control both the input and h.t. supply voltages fed to the power amplifiers via the control interface!(A look at a Datron 4700/4800 S.M. will give you an insight into the operation and complexity of these devices!)However it is possible to do this at home, but you need another meter of similar accuracy to the THS720, a sine or function generator with a 50 ohm output, and four identical 6V output good quality 230V mains transformers about 6VA each, plus a 9:1 divider probe, made from a 900k and a 100k VR37 resistor, plus a good quality audio power amplifier capable of about 50W output.1) Make up a 9:1 divider probe from 10 X VR37 100k metal glaze high voltage resistors, then connect the black lead of your "standard" meter to one end of the chain, and the RED lead of your standard meter to the junction of the first and second resistor from the black meter connection – I'll call this the "COMMON" end.2) Connect the primaries of 4 x 6VA 6V mains transformers in series, 230V on the first to 0V on the second and so on, to give a total of 920V.3) Connect the FULL chain of 10 x 100k resistors to the 960V output of the series connected transformer primaries. Connect the THS720 to the full voltage across the 920V chain of four primary windings.4) Connect the 6V secondaries of the transformers in parallel, 0V to 0V and 6V to 6V etc.5) Connect the 6V windings to the output of your power amplifier (they should be in parallel) and turn the volume about ⅔ of the way towards max., and the function generator's 50 ohm main output to the "line" input.6) Switch your standard meter to 1000VAC., turn the output of your function generator to minimum and set it to give 500Hz sine output.7) Very gradually increase the output of your function generator until the standard meter reads 60V a.c.(The reason I suggested the potential divider initially, is to prevent damage to your standard meter through accidentally applying over–voltage to it! IF your standard meter will withstand 1000V input you can connect it to the full 960V directly!)Check your standard meter is showing the 600V called for in the THS720 calibration instructions, with the SAME fed to the THS720, then follow the steps given in your THS720 manual to enter this.Most good quality chassis–mount transformers (RS, etc., should be capable of operating at about 500Hz for a short period – enough to calibrate your meter!It takes far longer to write this than it does to do it!If you're calibrating a number of meters, you could buy function generator and power amplifier modules from eBay, etc., & make up a "Calibration Box" with a built in mains supply, two terminals for your "Meter Being Calibrated" and two terminals in parallel for the "Standard" meter, and a switched attenuator between the function generator and power amp modules made of 20 turn presets, etc., plus the transformers of course!There's so many different modules available on eBay, etc., it's not worth trying to draw it out as it depends entirely on what you buy!Chris WilliamsPS!You'd be able to make a Calibration Box along the lines of what I suggested for less than a Calibration Lab would charge, although I believe RS did once offer a Meter Calibration Service for £70 in the dim, distant past!

author: mwbarth36
author_rank: Regular Contributor
timestamp: April 08, 2020, 02:45:38 pm
content: Thanks Chris56000! I knew about the Fluke equipment and the cost. Never thought about using the audio amplifier for a substitute. One even has a fairly precise control of the amplitude that way. Sounds like the way to go. I have all the equipment necessary except the transformers. Thanks again!

author: Chris56000
author_rank: Frequent Contributor
timestamp: April 10, 2020, 05:00:30 pm
content: Hi!I can draw you out an example with eBay & RS item numbers if you'd like one!Chris Williams

author: mwbarth36
author_rank: Regular Contributor
timestamp: April 12, 2020, 04:21:30 pm
content: Thanks, but I get the idea, it just did not occur to me. I have the amp to do it, and am getting the other components. Thanks again.