In article ,
Winfield Hill wrote:
[... mosfet as rectifier ..
look up
Please do. The setup and source of diode charge and discharge currents
isn't clear from your description.

I have someone else checking their records. I didn't turn up a copy of
the schematic. I think I, remember where the charge current came
from.


Remote preamp Long cable Main unit
.................. +15 .....................
+15V .--------------------------.
! . GND .
/ .--------------------------.
\ . .
/ . .
\ . Signal in question .
!----------------------------------------- Reciever
!- . .
!- Q1 . .
!- . -15 .
! .--------------------------.
-15V . .
.................. .....................


With Q1 on, the signal is at -15V.
With Q1 off the signal is at +15V
This works fine until we add some more info.

There is some big currents switching in the pre-amp section. Q1 turns on
just as the other thing switches off and remains on for a few mS. The
other circuit switching causes the preamp's GND to jump vs the ground in
the main unit. The combination of the cable capacitance and the receiver
circuit caused Q1's current to flow backwards. Either the bias on Q1
wasn't enough to keep the voltage below 0.7V or the current was backwards
just when Q1 was supposed to go off.

The cable could be as much as a mile long. The large signalling voltage
swings were needed to deal with the ground differences.


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