13-04-2020, 03:19 PM
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
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
