1
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 detector. 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 detector,
but even if the detector breaks down as a result of
overvoltage, 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 a 51
S
series resistor. The Warranty of the 142IH is void if
the protection circuit is taken out unless all of the
following precautions are taken:
1. COMPLETELY DISCHARGE the bias circuitry
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 detector.
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.
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.
Summary of Contents for 142IH
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