EHT!

[ppppenguin]

Paul Marshall has recently restored the EHT generator of a 1950s vintage picture and waveform monitor: https://becg.org.uk/2020/04/13/mkiii-pwm-eht-module/

Does anyone have EHT stories?

My worst encounter was with about 1.2kV, mains derived, on a Telequipment D61 scope. The probe insulation wasn't quite good enough. It hurt. My arm ached for a couple of days due to the volent contraction.

ZAP!

[radioman]

Hmm, lets see....
How about a large static discharge ?
When I was working on a BBC micro with the lid off, I leaned forward over it to check one of the leads at the back.
My hair (I had more back then) got attracted to the front of the Microvitec Cub colour monitor, which charged me up nicely...
I then decided to power-reset the computer and moved my left hand towards the rear of the PSU and earthed myself very well on the top of the power supply.
The crack I heard and pain I felt seemed to reach up to my elbow !
Luckily it wasn't continuous...

Andy

[BusyBee]

Static discharge lol! It reminds me of a trick I used to do with old TV's. When with someone in a room with the TV on I used to wipe my hand across the screen and touch them on the nose to give a little spark and tingle. That worked well until one day I did it and the resultant discharge felt like my finger had blown off! It put me off to say the least 

Tracy

[peter scott]

I think my worst EHT adventure wasn't from the current passing through me but rather the laceration of my hand as I withdrew it from the TV cabinet. It was only a B&W 17" flyback EHT but the aquadag coating could hold a reasonable charge.

Peter

[ppppenguin]

Peter, that's true of quite a few shocks. When you jump to fall off the ladder/scrape your hand/bang your elbow etc.

I have a sort-of feather duster  made from nylon fibres or similar. It holds the dust well, partly due to static. Once I decided to clean it with a vacuum cleaner. The rush of air built up a huge charge which inevitably discharged through me. It really stung and made me jump.

The picture is from 2009, taken at the Amberley Museum. Amazing what a Van-de-Graaf machine does with long hair. She says she barely felt anything.

[beamcurrent]

Mine's a 'scope story too. I was mending a rather nice Rohde & Schwarz video waveform monitor. It was a powerful beast with lots of EHT to give a bright trace of just a small section of a video waveform.

The EHT bit was working just fine, but alas  as I reached across to take a low voltage measurement, the DVM probe strayed to near the EHT cap and a hefty spark leapt out to the tip of the probe.  The probe did not mind much but the expensive DVM (in 1986) did!

The LCD went a deep black and could not be revived. With much sadness I put it away. I looked at it again in a couple of months and it had come back to life as the charge had leaked away. It soldiered on to give another 30 years service.

I have mended a few of those EHT generators it is part of the Marconi MkIII camera http://www.tvcameramuseum.org/marconi/mk3/bd687p1.html It's a driven oscillator and step-up transformer type, not a flyback one.  Makes about 9Kv.

The Visconol's are easy to replace, more concern about the sectional wave wound transformer! The picture shows the EHT generator in the side of the picture & waveform monitor, dated to 1953.  Note the anti corona ring round the octal rectifier base.

[peter scott]

I have a sort-of feather duster  made from nylon fibres or similar. It holds the dust well, partly due to static.

Jeffrey,

Your feather duster looks very human.

Peter 

[Panrock]

A terrible tale.

When I was 19, I built a VCR97-based television display unit.  I was ignorant, inexperienced and reckless. The EHT was derived from the HT secondaries of two large 425-0-425 mains transformers in series, then rectified and smoothed with some large 32uF caps in series (with balancing resistors). The result was 2500v from a low impedance.

Worse still, it hadn't occurred to me to use negative EHT, feeding just the relatively few components at the CRT's cathode end. No, instead the entire timebase chassis, comprising six valve stages or maybe more, was at this lethal +2500v (since the tube used electrostatic deflection).  This live chassis was mounted vertically at the rear of the horizontal earthed chassis bearing the signal chain components, separated by just  a three-inch wide piece of hardboard. 

One fine day, I had a lapse of concentration and momentarily touched both chassis at once, arm to arm, across the heart! I couldn't let go.  There was a gigantic muscular shuddering and the chassis crashed onto the floor.

Afterwards, I found the mains toggle switch had been flicked off. I had no recollection of doing this myself but this maybe is what saved my life. It remains a mystery to this day.

The moral: don't monkey with EHT, especially mains EHT! Fortunately over the subsequent fifty years, I have never forgotten this lesson.

Steve

[peter scott]

Steve,

That truly is an horrific tale!

Peter

[ppppenguin]

We're going to have to try very hard to beat Steve's story. Actually it would be better if we didn't try too hard Anyone who's got a better story probably isn't around to tell it.

I rather hope Graeme will contribute to this thread. He's worked in nuclear research with some seriously lethal voltages and currents.