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4.1. GENERAL

Figure 4.1 is a simplified block diagram of the

circuits in the 142AH Preamplifier. When the

protection circuit is in, the diode between the two

series input resistors to the amplifier stage is

connected. When the protection circuit is out, the

diode is disconnected and a jumper is used to

short across the two series input resistors.

4.2. DETECTOR BIAS

The amount of bias required by the detector is

specified in the data furnished with the detector.

The bias accepted into the preamplifier through

the SHV Bias connector is furnished through the

load resistance (approximately 100 megohms) to

the Input SHV connector of the preamplifier. If

the detector leakage current is appreciable, a

notable voltage drop will occur across the series

load resistor in the preampliflier, and this must be

added to the detector requirement when the bias

supply level is adjusted.

When the detector bias must be raised to a level

greater than 3000V, there are inherent problems

associated with obtaining low noise and high

performance. Careful design and good

4. OPERATION

manufacturing control, as well as extensive

testing under full bias, assure that each

preamplifier is free from spiking problems and

that no degradation of noise performance will

result when the required bias voltage is applied.

But dust, dirt, and other surface contamination

can collect within the Input connector and cause

spiking problems due to corona discharge in the

Input connector and associated cabling. This is

especially true when the contamination occurs on

the voltage-carrying center pin of the Input

connector while the preamplifier is operating with

bias voltages greater than 3000 V. These

problems can usually be cured by directing a

stream of clean compressed air or nitrogen into

the end of the connector to blow off the surface

contamination. This is a very subtle problem

source because enough contamination can

reaccumulate on the Input connector within

several seconds after cleaning and cause

recurrence of the corona. A dust cap is provided

to keep the Input connector covered when it is not

in use, and this will protect the interior surfaces

when a cable is not connected. When the cable is

connected, it can also transfer contaminants to

the connector that can cause some spiking

problems, even after the cable is removed.