A few years back DEC (Digital Equipment Corp) who produced the PDP and VAX ranges had a problem with their monitors.

You could turn off and unplug a monitor then four hours later touch the live and neutral pins on the mains plug and get a shock of over 400v from stored charge.


An old trick in the electronics industry was to take a small high voltage capacitor and tape the legs down on each side. Charge it up and then drop it into someone's jacket pocket ... some time later there would be the "What the ... Which one of you people with an unknown father, did that ?" scream!

Agreed, I should have used the long-hand, 16 to 32 micro-Farads.

Thanks.

or even uF ...

That was around the late 1980s, as "Dumb Terminals" spread rapidly in large quantities around particularly electronics factories, for remote access to central computers such as the VAX - Virtual Architecture Extended (and its basic Timing Unit of 1 Clunk).

It applied to more than the PDP/VAX range; being mainly the VT100 and VT102 CRT Monitors - BUT it also applied to various others such as the WYSE.

The charge was from the Filter Capacitor across and built into the Mains Input Socket, which became isolated from the discharge effect of the internal circuitry when the Monitor was switched OFF using its own Mains Switch, before switching off and/or unplugging from the wall socket.

We received a Safety Warning from our Central Safety Laboratory, which in fact gave the WRONG procedure for Safe Disconnection.

Myself and a former colleague (of about 20 years previously) at another location both spotted the error, resulting in a revised version version of the Safety Warning being issued very quickly.

Later, a small suitably shaped and punched PCB with a Discharge Resistor became available to push into the fixed male receptacle on the Monitors.

The PCB had rectangular, through-plated holes which made contact with the Live and Neutral Pins in the male receptacle.

The hole for the Earth/Ground pin was not plated etc.

t was "hidden" when the cable connector was plugged in.

In fact, to our consternation, we realised that it was a problem with virtually all the similar connectors used on the equipment we designed and manufactured in substantial quantities.

Thanks for reviving the memory!

We picked it up in late 1982 ...

And it was generally when the monitors were being "picked up" to be moved, that the effect might become apparent, although I do not recollect anyone reporting such "pin-[censored]" shocks to us.

The capacitors were very small in the pica-farad range, for dealing with RF Interference.

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Regarding the various smells associated with faulty, overheating equipment, how many remember the "Burnt Cabbage" smell emanating from Selenium Rectifiers?

Rotten Eggs from Ebonite insulation material, both when overheated and when being machined.

It was surprising when teaching in the RAF, how many trainees asked-
"What does burnt cabbage smell like?"

Reconstituted Dried Cabbage and Dried Potato Powder, both from Government Emergency stocks, featured regularly in the main meals of the day.

The viewing of those pics maxed out my webspace allowance (first time ever).

Have deleted them, and they can now be downloaded in a .zip file via:

http://www.freefilehosting.net/psupics

Edited by camieabz (Thu 03-Jan-13 23:31:25)