Advanced Measurement Technology 142AH Посібник з експлуатації та обслуговування - Сторінка 6

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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 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.

NOTICE

2. Discharge the detector bias circuitry before

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.