v
NOTICE
This preamplifier has been shipped to you with its
protection circuit connected into the input circuit.
the protection circuit prevents destruction of the
input FET due to large transients that may occur
during abnormal operating conditions and serves as
an impedance matching termination for the input
cable from the proportional counter. The presence
of the protection 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 at either the detector input or the bias
input connector.
The protection circuit does not protect the
proportional counter, but even if the proportional
counter 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, would not be true if the protection circuit
were taken out, in which case the input FET is very
susceptible to destruction by transients at the input
connector on the preamplifier.
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 142PC is void
if the protection circuit is taken out unless all of the
following precautions are taken:
1. COMPLETELY DISCHARGE the bias circuit
before connecting a low impedance, a cable, or any
other capacitive device to the Input connector on
the preamplifier.
2. Discharge the bias circuitry before making any
connections to the Input connector and before
disconnecting the preamplifier from the proportional
counter.
3. To discharge the bias circuitry, connect a low
impedance (shorting cap is preferred) for at least
one minute across the Bias connector on the
preamplifier.
4. Do not short the Input connector to ground, as
the voltage-carrying center pin can become pitted
or burned and form an area where corona discharge
will then develop under normal use.
The Input circuit will be destroyed if the Input
connector is shorted while the 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
(stored on C2) to be coupled directly to the input
transistor, causing a catastrophic breakdown.
If a variable bias supply is used, merely turn down
the voltage control to zero and leave it for at least
one minute. This 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 Bias connector
on the rear panel of the preamplifier. DO NOT
SHORT THE INPUT CONNECTOR on the front
panel of the instrument unless the input circuitry
has been completely discharged.
Содержание 142PC
Страница 4: ...iv...
Страница 8: ...viii...
