electro-Physics: 17.3409 volts

From: Gordon JC Pearce <gordon_at_gjcp.net>
Date: Tue Dec 14 16:30:11 2004

Tom Jennings wrote:
> One of the worst heisenbugs I can recall was a tape drive
> (unknown manu) on the Nova 1200 development system at Ocean
> Research Equip., late 1970's. Just an ordinary drive, somewhat
> low in the rack. It would "randomly" fail, tape went slack,
> but only in the afternoon.
>
> The heisenbug aspect was: it would work fine when you were
> standing in front of it. If you walked away, slack tape.
>
> The problem was found when we determined it worked also, in
> the afternoon, unattended, with it's smoked plexi door closed,
> which was always left open because it was annoying to open and
> close all the time. Turns out it was a ray of afternoon sun
> beaming in the window directly into the tape-in-place sensor
> near the head/capstan area.
>

Many years ago (oh, ok, about 20) when I was not even yet a spotty
teenage geek, I was playing about trying to coax some life from an old
valve radio. Poking about with high-impedance headphones suggested that
the problem was in the final stage of the audio amp. It would play
quietly, getting fainter, until eventually there was nothing...

So, I figured I should be seeing about -6 volts on the control grid wit
respect to the cathode. Whip out my Dad's AVO 8, apply red croc clip to
the cathode pin, apply black croc clip to the grid and *DAMN THAT WAS
LOUD*!!!

Reduce volume to a sensible level, shrug, and listen to the radio - took
a lot longer to get faint, as I bumped the volume up and up, but
eventually it faded completely. Volume to minimum, probe grid pin,
slowly increase volume, worked for a few minutes more.

Hmm.

Consult rough sketch of the circuit around the output amp that I'd
drawn. What was this funny 1M resistor from the grid to ground? And
why was it reading damn near open? Of course, the grid leak resistor,
without which the control grid would build up enough negative charge to
cut off the valve completely...

Gordon.
Received on Tue Dec 14 2004 - 16:30:11 GMT