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Old 22nd Sep 2018, 2:24 pm   #1
trobbins
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Default British Physical Labs RM215-L Breakdown & Insulation Tester

This tester from circa 1970 is an old school beast, although more 'modern' than BPL's original RM215 that came out in 1955, but lacking a little refinement that AVO and Biddle added when they appear to have licenced the design from the mid-1970's. It's not lacking in test voltage, with up to 6kVAC, and a doubler to generate up to 12kVDC, with front panel adjustment from 0% to 100% and metering.

Luckily the power part of the tester has no need for repair (so far), although I did megger the 7.5kV caps and transformers to give some confidence, but haven't yet turned up the output too much to calibrate the meters(and confirm cap insulation and diode breakdown).

I'm not a fan of mains ac wiring running with secondary side wiring in cableforms, so I separated them, and added an interposing relay to remove the need for mains wiring being part of the 'LOW' probe.

The section that has needed some effort is the low level circuitry for sensing breakdown and providing speaker monitoring of ionisation. The Newmarket Transistors PC5 amplifier module was fine except for a few caps and resistors. But there were multiple hum/noise sources that over-loaded the speaker output, including the old rectifier diodes and 12V zener for the amp assembly, the poor pcb trace and wiring layout for grounding and location of filter caps, along with a crackly pot and BC109. And even with all noise sources managed, there is still a fundamental ground loop involving the mains neutral and protective earth and use of chassis ground for the DC ionisation detection signal being amplified, which I'll have to think about some more.

A restoration doc with schematic and AVO manual is https://www.dalmura.com.au/static/BP...V%20Tester.pdf.

If anyone has any data on the SEI high voltage diodes, or on the Newmarket Transistor amp modules (I identified a few media releases, and traced the circuit to see it was a Lin clone), that would be interesting. All the part date codes aligned pretty well, which was comforting.

Thanks to Ashley (cathy_vintage) for scanning the AVO manual and schematic, which kicked me off to start the restoration. I now just have to not kill myself from further testing

Ciao, Tim
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Old 24th Sep 2018, 1:29 pm   #2
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Default Re: British Physical Labs RM215-L Breakdown & Insulation Tester

I just used a mains isolation transformer to relocate the neutral-earth link from the house main switchboard, to the output of the isolation transformer. That did the trick, and gave me a clean neutral in to the tester, so that the tester speaker just has a discernible hum with the ionisation volume at max.

Sort of highlights the typically higher level of neutral noise experienced nowadays, compared to back when most loads were a lot simpler.

I'll just bypass the front panel fuse (its a death trap for exposed thread when checking the fuse) and that should be the last of the changes needed.
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