Re: Digital O-scopes for TC use

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>" <evp-at-pacbell-dot-net>

Tesla list wrote:
> 
> Original poster: "Daniel McCauley by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <dhmccauley-at-spacecatlighting-dot-com>
> 
> Actually, I know it is done, we do it all the time when working on radar
> transmitters.  We'll float the scopes at up to 100kV.  HOWEVER!  We only use
> some old junky cheap battery powered scopes.
> 
> We never use our "good" scopes and I would never recommend people using
> their personal scopes to do this.
> 
> Dan

	I was involved in a similar project back in 1971, while working at
Hughes Aircraft.  We were building airborne high-power (90 kW peak, 3 kW
average) Ku Band TWT transmitter, and the cathode voltage was right
around 80 kV.  Whole transmitter insides were sitting in a great big
tank of oil to facilitate testing.  We RENTED a battery powered scope to
look at the grid drive waveforms.  It was hung on cords from the
ceiling, and attached to the cathode circuit.  One day someone managed
to arc the case of the scope to ground several times, and for some
mysterious reason the thing survived!  Returned to the renter who never
knew what had happened.....   Somewhat later on a similar project the
guys managed to drop the scope into the tank, and that time they had to
buy it.

	The HV power supply used transistor choppers running at about 30 kHz to
drive the primary of the HV transformer.  DC came from a 3 phase full
wave bridge feeding a big filter capacitor and then into three H bridge
switches which fed the three legs of the transformer.  This same setup
would have worked fine for an SSTC running at that frequency, although
the old Delco DT-200 transistors we used couldn't run much fastor.  As I
recall about 4 were used in parallel at each side of each driver, and
accidents happened regularly which would blow all of the transistors in
one leg, with spectacular results.  Transistors normally die without a
whimper, but when that big filter capacitor discharged in to the shorted
transistors there was a bang like a 22 rifle, and sometimes holes  were
actually blown in the cases.  Lab was littered with the carcasses as
they were expensive, slow delivery, and "too good to throw away".  I
still have some leftover transistors and am using one right now to drive
a small flyback transformer.

	Interestingly enough, once the transmitter got into the final package
and into the B-47 test aircraft we had no failures in almost 150 flight
hours.

	Sort of off the topic, I'm afraid, but illustrates some of the
potential difficulties with floating an instrument at HV.  I suspect a
cheap used laptop with suitable A/D converters would be a cheaper way to
go than to try to float a good scope and battery.

Ed