I picked this Hunting Hivolt Check IT Insulation tester cheap a few years back from ebay. I haven't managed to find a schematic or manual on the web, the nearest I have is spec. data from a 1984 Electroplan catalogue. The specification is pretty useful for its compact size: Up to 10kV DC output set by multi-turn pot and three leakage current ranges... 1uA, 10uA and 100uA max. Trip current is approx 130% of range. Reasonably safe too (unless you start charging capacitors, that is) with peak output current limited to 250uA. Price in 1984... £420 + VAT.
It was in a pretty sorry state - Dropped at some point, Voltage setting not working correctly, half the front panel buttons missing and worst of all, a broken edgewise moving coil meter. It has been sitting unloved in a corner for a couple of years so definitely time to get it working. It's a shame I didn't think to take a 'before' photo, but internal constuction wasn't helpful - every internal wire is orange (even the original transformer live connection) with terminations made fragile by solder wicking. There was no insulation on the very exposed mains fuse holder and transformer connections either. The voltage multiplier end of the PCB was dipped in wax during manufacture to improve leakage so looks a but messy. The 14.4V NiCd battery pack was (of course!) dead and corroded.
The first problem to solve was the meter, really hard to find these days and expensive. It was easy to glue the casing back together, but the drop damage included loss of the needle counterweight, making it more useful as a spirit level than a meter. I managed to fix this with some stripped kynar with the end coiled to the right diameter to slide onto the stub of the original and fixed with superglue. I was then able to weight the looped other end of the kynar with solder blobs until it balanced. I managed to dislodge the moving coil termination in the process which needed re-soldering, definitely a microscope job!
Due to the fragile wiring I restricted reverse engineering to the voltage feedback circuit, the fault turned out to be the LM747H dual opamp. I successfully replaced this with a bodged on LM358. I managed to find some C&K button caps that fitted well enough when glued. I also replaced the two very dim red LEDs with some nice bright noticable ones. I haven't bothered replacing the battery pack.
Now it's fixed, it performs really well. The voltage can be set very accurately, even right down at the bottom end using the multiturn pot. The meter defaults to current, with a momentary action voltage check button. The current readings on all three ranges are accurate, and the trip works predictably. Not bad considering it only has one internal trimmer and one selected resistor and zener regulated supply.
Luckily I had some silicone EHT cable kicking around. I had to bodge up a suitable BNC plug using just the PTFE inner parts and heatshrink, a standard BNC plug flashes over internally (the BNC socket on the unit, on its own, flashes over at about 7.8kV). I stripped the plug outer collar, fitting an O-ring into the rotating clamp to act as a strain relief. The Alden EHT plug on the other end makes a pretty good test probe. I notice from internet pictures that the Check IT Mk2 uses a better output connector, I'm not sure what other internal differences there are. The ouput polarity is negative with respect to Earth.
I finally have a way of non-destuctively testing second hand transformers, chinese SMPSs etc. which, given current prices for anything above 1kV, I'd never have otherwise.
P.S. If anyone has a manual for the Check IT (either original or Mk2) I'd be really grateful for a scanned copy.
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