Model 78 and 78-1
Technical Manual
Section 4
Ludlum Measurements, Inc.
Page 4-3
March 2021
HV LIMIT (R027)
: a multi-turn potentiometer (approximately 20 revolutions) sets the
maximum HV limit with the front-panel
HV
control adjusted to the maximum clockwise
position. It is adjustable from 1250 to 2400 Vdc.
VOLUME (R002)
: a multi-turn potentiometer (approximately 20 revolutions)
varies the audible click-per-event and alarm audio. Adjust the control to the
maximum clockwise position for maximum volume. If the
VOLUME
control
is adjusted to the maximum counterclockwise position the clicks-per-event or
the audible alarm(s) will not be audible when active.
Cal/Switch Board Controls
Note:
In order to access the following controls, open the latches at both ends of the
instrument and remove the electronics.
DISC (Discriminator)
: a multi-turn potentiometer (approximately 20
revolutions) used to vary the detector pulse-counting threshold from 2 to 100
mV. A Ludlum Model 500 Pulser or equivalent should be used in checking or
adjusting the pulse discrimination parameter
Note:
When making adjustments to the
HV
potentiometer, make note of the following
precautions: Use a Ludlum Model 500 Pulser or high-impedance voltmeter with a
high-voltage probe to measure the high voltage at the detector connector. If a
Ludlum Model 500 Pulser is not available, ensure that the impedance of voltmeter
used is 1000 megohms or greater.
HV1 and HV2
: multi-turn potentiometers (approximately 20 revolutions) that independently
vary the detector voltages from 200 to 2400 volts. The maximum high-voltage output is
adjusted by the
HV LIMIT
potentiometer located on the internal main board.
