vi
NOTICE
This preamplifier has been shipped to you with its
2. Discharge the detector bias circuitry before
protection circuit connected into the input circuit.
The protection circuit prevents destruction of the
input FET due to large transients under abnormal
operating conditions and serves as an impedance
matching termination for the input cable. The
presence of the portection circuit imposes only a
slight resolution degradation. With the protection
circuit installed, the preamplifier is immune to
almost anything the operator is likely to do that
causes transients either at the detector input or at
the bias input connector. It is very important that
the protection circuit be installed when using bias
voltages greater than 3 kV so that the input FET
is protected from high-energy transients during
possible momentary breakdown of the detector or
the bias supply filtering circuitry.
The protection circuit does not protect the
detector, but even if the detector breaks down as
a result of over-voltage, the preamplifier will
survive the resulting large transients if the
protection circuit is in. This, of course, is not true
if the protection circuit is out, in which case the
input FET is very susceptible to destruction by
transients at the detector input connector.
If the If the input protection circuit must be taken
out for any reason, this involves disconnecting
one transistor lead and installing a jumper across
two series resistors. The Warranty on the 142AH
is void if the protection circuit is taken out unless
all of the following precautions are taken:
1. COMPLETELY DISCHARGE the detector bias
circuit before connecting a low impedance, a
cable, or any other capacitive device to the
Detector Input connector on the preamplifier.
making ANY connections to the Detector Input
connector and before disconnecting the
preamplifier from the detector.
3. To discharge the detector bias circuitry,
connect a low impedance (short cap preferably)
across the Detector Bias connector on the
preamplifier.
The input circuit will be destroyed if the Detector
Input connector is shorted while the detector bias
components are charged, and the quality of these
capacitors is such that they will retain a charge
through a long period of time. Such a short could
result from connecting a detector, cable, or other
capacitive device such as a voltmeter probe. A
short circuit, either short term or continuous, will
cause the applied bias voltage (stored on C34) to
be coupled through C2 directly to the input
transistor, causing a catastrophic breakdown.
If a variable supply is used, merely turning down
the voltage control to zero and leaving it for at
least one minute will suffice since the bias
circuitry can discharge itself through the output of
the bias supply.
Sometimes it is necessary to simply disconnect
the bias supply, such as is the case when using
batteries for bias. This situation leaves no
discharge path, so a path must be provided by
placing a short circuit or low impedance across
the Detector Bias connector on the rear panel of
the unit. DO NOT SHORT the Detector Input
connector on the front panel of the instrument.
