Unwelcomed attraction.

[DrStrangelove]

I recall the HV probe in Practical Television.

I think it used a string of 10M resistors in a paxolin tube filled with paraffin wax.

If I could be bothered I'd go & have a look in the Practical TV book.

[Mike Watterson]

Though paraffin was a bad choice. Water gradually dissolves in it.

I bought HV rated 33M resistors to make a probe, but I've never assembled it yet.

Polystyrene or PVC cement (available then) are better. Or epoxy now.

[ppppenguin]

My HV probe isn't potted. The string of 33M is in a PVC tube which is within a paxolin outer tube. The HV end is sealed with Araldite. There's a SOT resistor in series to set calibration and a 4M7 from the meter end to the earth clip. It originally worked with an ordinary 50uA analogue multimeter to give 25kV FSD. So you have to mentally allow a factor of 2. I now use it with a DMM so I still have to do mental arithmetic.

[boater sam]

SOT resistor? Not transistor?

[ppppenguin]

SOT = select on test. Nominally 5M to make up the total to 500M. I have a dim memory of how I calibrated it when I first built it. I had access to a DVM that went up to 1kV and a stabilised PSU that went to more than 1kV. I set exactly 1kV on the PSU and used a small resistor, possibly 4K7, at the "cold" end of the resistor chain as a potential divider. Then used the DVM to measure the voltage across that. The rest was simple arithmetic.

That sort of testgear was fairly exotic in 1974. But I was working in a professional lab with lots of nice kit. For example HP180 series scopes, which seemed to be the preferred type, rather than the solid state Teks of that era (465?). When I went to Imperial College in 1975 I was rather disappointed to find very mediocre scopes etc in the labs. Elderly Telequipment etc.

[boater sam]

Thanks.

Ferranti's in the late 1960's it was all valve Techtronics, we used them to heat the R&D lab in winter!

Sam.

[DrStrangelove]

When I went to Imperial College in 1975 I was rather disappointed to find very mediocre scopes etc in the labs. Elderly Telequipment etc.

IIRC the scopes in the electrical dept of Swansea Tech were ancient ?Airmec? devices: 3" round screen things. No idea what the bandwidth was but it wasn't much.

When I returned as a technician 40 years later I was much happier to see 100MHz Tektronix LCD scopes.

We ended up with 3 different versions of them as I bought more over the years.

It made introducing the First Years to the wonders of the oscilloscope rather interesting.