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OPERATING INSTRUCTIONS
FOR:
VICTOREEN MODEL 496
Portable GM Count Rate Survey Meter
Part No. 496-1
Form: 1392-5-74
INSPECTION AND MATERIAL RETURN INSTRUCTIONS
Instruments should be examined and tested as soon as received by the purchaser. Claims for
damage, if any, should be filed at once with the carrier. Instruments returned to the plant for warranty
service, repair, calibration, replacement or credit must be accompanied by VICTOREEN's Return
Material Authorization, Form Number 9083D-12-74. This form is in the manual sent with the
instrument. No instrument will be accepted without authorization.
Material valued at $200.00 or more and/or weighing more than twenty pounds should be shipped
prepaid and fully insured by REA Express or other similar means.
We suggest that any instrument weighing over twenty pounds be wrapped in heavy kraft paper and
packed in a double corrugated carton or wooden box. Protect the instrument on all sides with at least
three inches of excelsior or similar padding. Mark the case plainly with suitable caution warnings to
insure careful handling.
WARRANTY
VICTOREEN INSTRUMENT DIVISION OF SHELLER-GLOBE CORPORATION warrants that
in the event any defects in material or workmanship should develop within one (1) year from date of
shipment, the company assumes full responsibility for servicing equipment of its manufacture without
charge.
Careful design and workmanship along with quality assurance control steps have gone into the
manufacture of this instrument.
The calibration of this instrument was correct within specified limits when the instrument left our
factory. VICTOREEN cannot be responsible for injury or damage resulting from improper use as well as
faults or calibration errors which develop subsequent to our shipment of the instrument.
If VICTOREEN determines that a fault has been caused by misuse, abnormal operating conditions
or repairs by unauthorized personnel during the warranty period, repairs will be billed at normal rates.
VICTOREEN reserves the right to perform warranty service in its own factory or authorized repair
station, or at the customer's installation.
VICTOREEN shall not be held liable for damages or delays .caused by defects beyond making
repairs or furnishing replacement parts, nor shall VICTOREEN be liable for any defective material
replaced without its consent during the period of this warranty.
This warranty specifically excludes the following items which are covered by the original
manufacturer's warranty; photomultiplier and other electron tubes, Geiger and proportional counters,
crystal and other solid state detectors, batteries, and major ancillary items of instrument systems.
TABLE OF CONTENTS
1.0 GENERAL DESCRIPTION
1.1 Purpose of Equipment ............... 1
1.2 Physical Description ............... 1
1.3 Specifications ................... 2
2.0 OPERATION
2.1 Battery Installation ................. 4
2.2 Battery Test ..................... 4
2.3 Radiation Measurements .............. 4
3.0 CIRCUIT DESCRIPTION
3.1 General ....................... 5
3.2 Power Supply .................... 5
3.3 Metering Circuit .................. 5
3.4 Aud-io Circuit .................... 6
4.0 MAINTENANCE
4.1 General ....................... 8
4.2 Power Supply Adjustment .............. 8
4.3 Calibration ..................... 8
4.4 Circuit Testing ................... 9
5.0 REPLACEABLE PARTS LIST
5.1 Electrical Components ............... 13
5.2 Mechanical Components ............... 13
LIST OF ILLUSTRATIONS
Figure 1. Model 496 Portable GM Count Rate Survey
Meter ................... iii
Figure 2. Interior View ................. 7
Figure 3. Calibration Setup ............... 8
Figure 4. Printed Circuit Board ........... 10
Figure 5. Schematic Circuit Diagram ........ 11-12
1. 0 GENERAL DESCRIPTION
1. 1 Purpose of Equipment
The Victoreen Model 496 provides a convenient, accurate means
for detecting low levels of gamma. X-ray, and beta radiation in
conjunction with an accessory Geiger-Mueller probe assembly such
as the 489-4, 491-30, and 491-40. Alpha particles may also be
detected with the 489-35 probe assembly.
An indicating meter and built-in speaker enable the operator to
measure GM tube count rates from very low background levels to
80, 000 counts per minute.
1. 2 Physical Description
The model 496 is a portable, self-contained instrument housed
in a two-piece, all metal case. The two operating controls, conveniently located on the case top, provide power on-off switching,
range selection, battery testing, and speaker on-off switching.
The batteries are retained in a high-impact plastic battery
compartment which cannot be corroded by battery leakage fluids.
The battery contacts are readily replaceable.
An accessory GM probe attaches to the input connector and
may be carried or stored in a clamp on the handle.
Maintenance is facilitated by a hinge assembly on the battery
compartment and printed circuit board.
Reliability and accuracy under rough field usage are assured
by the solid-state design and rugged construction.
2.0 OPERATION
2. 1 Battery Installation
Snap open the pull catches at each end of the case and remove
the case bottom. This will expose the circuit board assembly and
the battery compartment. Two wires connect the speaker on the
case bottom to the case top. Do not strain these wires.
Squeeze the battery retainer clamp to remove it from the
compartment. Install standard "D" size flashlight cells in the
openings provided, observing the proper polarity. If operation
below 32°F (QOC) is contemplated, use alkaline batteries. Replace
the retainer clamp and the case bottom.
Remove all batteries if the instrument is to be stored for
any extended period of time.
2. 2 Battery Test
Turn the selector switch to "bat. " If the meter does not read
within the check band, the batteries must be replaced. The battery
test may be performed at any time, whether the instrument is in
a radiation field or not. Notice that the speaker will operate in
the "bat. " position if the "vol. " switch is on.
2. 3 Radiation Measurements
The Model 496 measures count rate from any of the standard
Victor een GM detector probes. Three operating ranges (XI, XI 0,
XI 00) are provided, which correspond to 800, 8000, and 80,000
counts per minute full scale. For best reading accuracy, switch
to the range which provides a reading in the upper 90% of the
scale.
An audible indication of count rate is provided by the built-in
speaker and amplifier circuit. The speaker operates independently
of the meter circuit, and may be switched on and off by means
of the "vol. " switch.
3.
0 CIRCUIT DESCRIPTION
3. 1 General
The overall operation of the Model 496 can best be understood
by referring to the schematic circuit diagram. Figure 5. The
external GM tube detector is supplied with a regulated high voltage
from the power supply via R4. When the detector is energized by
a photon of radiation, the current through R4 increases briefly,
causing a negative pulse to appear at C4. This pulse is coupled
via Q2 to the separate meter and speaker amplifiers.
3. 2 Power Supply
The power supply provides two regulated outputs: 900 volts
for the geiger tube, and 10 volts for the metering circuit. The
circuit operates as a blocking oscillator in the flyback mode.
The oscillator portion of the circuit consists of transistor
Ql, windings 3-4 and 5-6 of transformer Tl, resistors Rl and
R2, and the batteries.
The high voltage supply portion of the circuit consists of
winding 1-2 of Tl, rectifier CR2, resistor R3, capacitors
C2 and C3, and the Victoreen high voltage regulator diode VI.
The low voltage section is comprised of winding 5-6 of Tl,
diode CR1, and capacitor Cl. The regulating action of VI,
reflected back through the transformer, serves to regulate
the low voltage as well as the high voltage output.
3. 3 Metering Circuit
The metering circuit is a monostable multivibrator whose
output pulse width is determined by the 'setting of the range
selector switch, followed by a temperature and voltage stable
constant-current stage which drives the meter.
GM pulses from amplifier Q2 are fed through R7 to Ul, pin 12,
the base of a transistor (part of a 5-transistor integrated array).
This transistor, plus the one at pins 6-7-8, and their associated
components, form the monostable, or one-shot multivibrator.
The transistor at pins 9-10-11, in conjunction with CR3, Rl2,
Rl5, and R16, is the constant current generator. Shaped pulses
from the multivibrator turn the current generator on and off. Each
current pulse thus delivered to the meter is precisely controlled
in amplitude and duration.
The indicating meter is connected in the collector of the
current generator across resistor R14. This resistor provides
damping for the meter. Capacitor C8, in conjunction with Rl3,
R14, and the meter's internal resistance, controls the meter's
response speed.
Range switching is accomplished by switching timing
capacitors, which changes the charge per pulse available to the
meter. On the X10 range, the pulse width is 1/10 that on the XI
range. On the XI 00 range, the pulse width is 20% more than
1/10 that on the X10 range. The extra 20% compensates for GM
tube dead time, which would otherwise cause the instrument
to read about 20% low on the XlOO range. Thus the instrument
measures count rate accurately from a GM tube on all three
ranges, but responds about 20% high on the XlOO range with a
linear pulse generator.
3. 4 Audio Circuit
The audio circuit is a monostable multivibrator followed by
two stages of current amplification which drive a 3. 2 ohm
loudspeaker. Power for the circuit is taken directly from the
battery supply, since the current demands are high, and
regulation is not required.
GM pulses from amplifier Q2 are fed through R18 to the
transistor at Ul, pins 3-4-5. The multivibrator, consisting of
this transistor, plus Ul pins 1-2-3, and associated components,
has a pulse duration of about 100 microseconds. The output pulse
drives the speaker through transistors Q3 and Q4. The audio
circuit is independent of the metering circuit, and may be
switched on and off with switch S2.
Figure 2. Internal View