Ludlum Measurements 78 User manual

LUDLUM MODEL 78 AND 78-1
DIGITAL ANALOG STRETCH SCOPE
March 2021
Serial Number 299335 and Succeeding
Serial Numbers

LUDLUM MODEL 78 AND 78-1
DIGITAL ANALOG STRETCH SCOPE
March 2021
Serial Number 299335 and Succeeding
Serial Numbers

Ludlum Measurements, Inc. March 2021
STATEMENT OF WARRANTY
Ludlum Measurements, Inc. warrants the products covered in this manual to be free of
defects due to workmanship, material, and design for a period of twelve months from
the date of delivery. The calibration of a product is warranted to be within its
specified accuracy limits at the time of shipment. In the event of instrument failure,
notify Ludlum Measurements to determine if repair, recalibration, or replacement is
required.
This warranty excludes the replacement of photomultiplier tubes, G-M and
proportional tubes, and scintillation crystals which are broken due to excessive
physical abuse or used for purposes other than intended.
There are no warranties, express or implied, including without limitation any implied
warranty of merchantability or fitness, which extend beyond the description of the
face there of. If the product does not perform as warranted herein, purchaser’s sole
remedy shall be repair or replacement, at the option of Ludlum Measurements. In no
event will Ludlum Measurements be liable for damages, lost revenue, lost wages, or
any other incidental or consequential damages, arising from the purchase, use, or
inability to use product.
RETURN OF GOODS TO MANUFACTURER
If equipment needs to be returned to Ludlum Measurements, Inc. for repair or calibration, please
send to the address below. All shipments should include documentation containing return shipping
address, customer name, telephone number, description of service requested, and all other necessary
information. Your cooperation will expedite the return of your equipment.
LUDLUM MEASUREMENTS, INC.
ATTN: REPAIR DEPARTMENT
501 OAK STREET
SWEETWATER, TX 79556
800-622-0828 325-235-5494
FAX [phone]

Ludlum Measurements, Inc. March 2021
Table of Contents
Introduction 1
Getting Started 2
Unpacking and Repacking 2-1
Battery Installation 2-1
Operational Check 2-2
Specifications 3
Identification of Controls and Functions 4
Display 4-1
Display Status Definitions 4-1
Front Panel Controls 4-2
Main Board Controls 4-2
Cal/Switch Board Controls 4-3
Safety Considerations 5
Environmental Conditions for Normal Use 5-1
Warning Markings and Symbols 5-1
Cleaning and Maintenance Precautions 5-2
Maintenance 6
Recalibration 6-1
Batteries 6-2
Technical Theory of Operation 7
Detector Input/Amplifier 7-1
Discriminator 7-1
Low Voltage Supply 7-1
High Voltage Supply 7-2
Detector Overload 7-2
Microprocessor 7-2
Audio 7-2
S1 (Function) 7-3
S2-S4 7-3

Model78 TechnicalManual
Ludlum Measurements, Inc. March 2021
LCD Drive 7-3
Backlight Drive 7-3
Instrument Setup 8
Entering or Changing Switch Board Parameters 8-1
The Function Switch 8-1
Function Switch Position Descriptions and Variables 8-2
Loading Default Parameters 8-4
Calibration 8-4
General Detector Setup Information 8-5
Exposure Rate Calibration 8-5
Detector Overload (OVL) Calibration 8-6
Recycling 9
Parts List 10
Model 78 Digital Analog Stretch Scope 10-1
Main Circuit Board, Drawing 272 x 591 10-1
Cal/Switch Board, Drawing 272 × 352 10-3
Display Board, Drawing 408 × 259 10-4
Chassis Wiring Diagram, Drawing 272 × 596 10-5
Drawings and Diagrams 11

Model78and78-1 TechnicalManual Section1
Ludlum Measurements, Inc. Page 1-1 March 2021
Introduction
he Ludlum Model 78 and 78-1 Digital/Analog Stretch Scope features two radiation
radiation detectors mounted on a 3.7 m (12 ft) extendable pole. The two energy-
compensated, Geiger-Mueller (GM) detectors measure gamma radiation from 0.1 mR/hr
mR/hr to 1000 R/hr (1µSv/h –10,000mSv/h). The Model 78 measures in R (Roentgen)
(Roentgen) units while the 78-1 measures in Sieverts.
Both an analog meter and a backlit 4-digit LCD (liquid crystal display) are utilized for display. The
4-decade logarithmic meter face spans from 0.1 to 1000. The digital readout displays the level of
radiation in either mR/hr or R/hr (µSv/h), depending upon the detector selected. The LCD also
has indicators for low battery, alert, alarm, overflow, and detector overload.
Other instrument features include Dead Time Correction (DTC) to compensate for detector dead
time; audible click-per-event with programmable 1, 10, 100, and 1000 divide-by; LCD backlight
with programmable "on" time; programmable fixed or variable response time; and a count
overflow visual alarm.
Warning
There is a reduced response in pulsed fields due to the detector dead time.
The Model 78 incorporates independent adjustable alarms for both detectors. There are two alarm
indications. The first-level alarm is indicated by display of ALERT on the LCD. The second-level
alarm is indicated by display of ALARM and a continuous audible tone. The audible alarm can be
silenced (acknowledged) by depressing the RESET switch.
The electronics front panel is equipped with switches for control of detector selection, audio
on/off, fast or slow response times, LCD backlight, display reset, and power on/off. A headphone
jack is also available. Two “D” cells located in the handle provide power to the electronics. Battery
life is approximately 250 hours.
The stretch scope is made of stainless steel tubing that retracts to an overall length of 105.9 cm
(41.7 in.). The handle can be removed for storage, reducing the length to 92 cm (36.2 in.). The
electronics are housed in an aluminum body with a beige powder-coat finish and subsurface-
printed Lexan front panel. Front-panel switches are sealed with rubber “boots” for resistance to
moisture.
Section
1
T

Model78and78-1 TechnicalManual Section2
Ludlum Measurements, Inc. Page 2-1 March 2021
Getting Started
Unpacking and Repacking
Remove the calibration certificates and place them in a secure location. Remove the
instrument, detectors, and accessories (batteries, cable, etc.), and ensure that all of the items
listed on the packing list are in the carton. Check individual item serial numbers and ensure
calibration certificates match. The Model 78 serial number is located on the front panel
above the analog meter face. Most Ludlum Measurements, Inc. detectors have a label on the
base or body of the detector for model and serial number identification.
Important!
If multiple shipments are received, ensure that the detectors and instruments are
not interchanged. Each instrument is calibrated to a specific detector(s), and is
therefore not interchangeable.
To return an instrument for repair or calibration, provide sufficient packing material to
prevent damage during shipment. Also provide appropriate warning labels to ensure careful
handling. Include detector(s) and related cable(s) for calibration.
Every returned instrument must be accompanied by an Instrument Return Form, which
can be downloaded from the Ludlum website at www.ludlums.com. Find the form by
clicking the “Support” tab and selecting “Repair and Calibration” from the drop-down
menu. Then choose the appropriate Repair and Calibration division where you will find a
link to the form.
Battery Installation
Ensure the Model 78 ON/OFF switch is in the OFF position. Remove the handle by rotating
the handle counterclockwise. Install two "D" size batteries in the compartment with the
posts pointing toward the instrument. Re-install the handle, rotating clockwise.
Section
2

Model78and78-1 TechnicalManual Section2
Ludlum Measurements, Inc. Page 2-2 March 2021
Note:
The center post of the battery is positive. Batteries are placed in the battery
compartment with the posts pointing toward the instrument.
Operational Check
Turn the ON/OFF switch to the ON position.
The display goes through an initialization sequence showing all 8s with decimal points. Check
on all segments to ensure they are on as illustrated:
The LCD then shows the firmware number in the format “P XX YY.” The “XX” is the
firmware number, and the “YY” is the firmware version. (The following figure is for example
only, to illustrate location of display.)
Note:
The Model 78 is not sensitive to neutron radiation.

Model78and78-1 TechnicalManual Section2
Ludlum Measurements, Inc. Page 2-3 March 2021
Expose the detector to a check source if the background count is too low to generate a display
reading. Place the AUD ON/OFF switch in the ON position and confirm the external unimorph
produces an audible click for each event detected (audio divide by "1" parameter). The AUD
ON/OFF switch will silence the clicks if in the OFF position.
Note:
It is recommended to use the SLOW response setting when measuring low
radiation fields.
Increase the source activity or lower the alert and alarm points to initiate an ALERT and ALARM
condition. (Refer to Section 8 for parameter change procedures.) Depress the RESET to
acknowledge the audible alarm
Decrease the radiation activity below the ALERT and ALARM threshold and depress RESET to
clear the alarm conditions. If an alarm condition is not present, depressing RESET the first time
will reset the alert condition and zero the ratemeter.
Position a check source to produce a reading of 0.5 to 2.0 mR/hr (0.005 to 0.02 mSv/h) while
observing the ratemeter fluctuations, select between the fast and slow response time
(FAST/SLOW) positions to observe different variations in the display. The SLOW position should
respond approximately five times slower (for fixed response mode) and three times slower (for
the variable response mode) than the FAST position.
Note:
The slow response position is normally used when the Model 78 is displaying low
numbers that require a more stable display. The fast response is used at high count
levels.
Depress and release the BKLITE switch. The backlight located behind the LCD should
illuminate for pre-programmed "ON" time.
Select the desired FAST/SLOW and AUD ON/OFF parameters and proceed to use the instrument.
Note:
The warm-up time on this instrument is less than 10 seconds on every range.

Model78and78-1 TechnicalManual Section3
Ludlum Measurements, Inc. Page 3-1 March 2021
Specifications
DISPLAY: 4-digit LCD with 1.3 cm (0.5 in.) character height
Model 78 RANGE: 0.1 to 999.9 mR/hr
Model 78-1 RANGE: 1 µSv/h to 9999 mSv/h
DISPLAY LINEARITY: within 10% of true value with connected detector
INPUT SENSITIVITY: adjustable from 2-100 mV; negative pulse response; normally set to
50 mV through coiled cable
ACCURACY: digital readout and analog logarithmic meter are accurate within 10% of true
value
HIGH VOLTAGE: adjustable supply from 200 to 2500 V; regulated to within 0.2% at 1000
Vdc; maximum load of 50 µA at 1000 Vdc; two set-points normally set to 550 Vdc (HV1)
and 550 Vdc (HV2)
RESPONSE TIME: variable or fixed ratemeter response (All stated times correspond to a
range of 10% to 90% of the final reading). Factory default is ˝variable˝ so that the instrument
will automatically adjust the response time to the best setting for the current count rate
Variable Response: dependent on the number of counts present; typically 4 to
25 seconds for FAST and 4 to 60 seconds for SLOW
Fixed Response: The FAST response position is programmable from 1-199
seconds. The SLOW response position is approximately five times slower than
the FAST. For MDA-type measurements, the fixed response mode is
recommended
ANGULAR RESPONSE: within 30% as detector is rotated from 0 to 90 degrees (calibration
reference is with radiation field parallel to the long axis of the detector)
GEOTROPISM: referenced to the unit in horizontal position with display up. The indication
will not vary more than 2% of the full-scale reading with the unit in any other orientation.
Section
3

Model78and78-1 TechnicalManual Section3
Ludlum Measurements, Inc. Page 3-2 March 2021
WARM-UP TIME: unit may be used immediately after the LCD initialization sequence is
completed –approximately five seconds after turning on
ALERT/ALARM: visual and audible adjustable alarm points
CALIBRATION CONSTANT: 0.001 to 280 × 109counts/unit
DETECTOR DEAD TIME COMPENSATION (DTC): adjustable from 0 to 9999 microseconds
Backlight "ON" Time: 5, 15, 30, 60, 90, 120, 180, or 240 ±1 second
HEADPHONE JACK: size 0.32 cm (1/8 inch)
POWER: two standard "D" size batteries housed in a sealed handle; current drain
approximately 35 mA (backlight off); minimum battery voltage 2.2 ±0.1 Vdc
BATTERY DEPENDENCE: Meter readings vary by less than 3% from fully charged batteries
until the battery symbol appears, indicating the need for recharge or replacement.
BATTERY LIFE: approximately 250 hours for alkaline batteries; battery failure indicated on
display
SIZE: 12.2 x 10.9 x 105.9 cm retracted; 377.2 cm fully extended (4.8 x 4.3 x 41.7 in.
retracted; 148.5 in. fully extended) (H x W x L, fully extended L)
WEIGHT: 2.9 kg (6.4 lb), including batteries
CONSTRUCTION: Main instrument housing is fabricated of aluminum with beige powder-
coat finish and printed, membrane front panel. Detector housing consists of a polished
stainless steel telescope assembly with brass fittings.

Model78and78-1 TechnicalManual Section4
Ludlum Measurements, Inc. Page 4-1 March 2021
Identification of Controls and
Functions
Display
The Model 78 utilizes a four-digit LCD readout with a fixed decimal point. The
two smaller digits located in the lower right corner of the display indicate
exponential power when in the parameter setup mode. The
upper right of the LCD exhibits units and multiplier(s) R/hr,
mR/hr, or µR/hr; Sv/h, mSv/h, or µSv/h; C/m, kC/m, C/s, or
kC/s. However, normally only units of R/hr, mR/hr or µR/hr
are used. The bottom part of the readout displays ALARM, ALERT,
OFLOW, and OVERLOAD enunciators as well as the low-battery
icon.
Display Status Definitions
ALARM: ratemeter count has increased above the preset alarm threshold. An audible
continuous tone will accompany the "latching" ALARM condition. Depressing RESET will
acknowledge the audible ratemeter alarm. Depressing RESET a second time will reset the
ratemeter reading and ratemeter alarm.
ALERT: ratemeter count has increased above the preset alert threshold. To reset an ALERT
condition, depress RESET once if in the non-alarm condition and twice if in an ALARM
condition. (The first depression in the alarm condition acknowledges the audible alarm.) The
ratemeter will reset to the minimum displayable reading each time the alert is reset.
OFLOW (Overflow): indicates that the incoming count exceeds the capability to display stable
or reliable readings corresponding to the radiation level being measured. The overflow
symbol (OFLOW) will appear when the ratemeter exceeds 100 kcps or if the dead time
correction is greater than 75%.
OVERLOAD: indicates the detector is being exposed to radiation intensities greater than the
detector maximum operating limit. The overload alarm point is set by adjusting the
OVERLOAD control located on the CAL/SWITCH BOARD.
Section
4

Model78and78-1 TechnicalManual Section4
Ludlum Measurements, Inc. Page 4-2 March 2021
Low Battery Icon:indicates that the batteries have decreased to the minimum operating
voltage of 2.2 ±0.1 Vdc
COUNTING:appears upon power-up, but not implemented
Front Panel Controls
ON/OFF Switch: a two-position toggle switch that applies power to the instrument.
AUD ON/OFF Switch: The clicks-per-event audio may be silenced or enabled via this front-
panel toggle switch. The audible alarm is independent of the AUD ON/OFF switch and will
override the audible clicks-per-event. An audible alarm can only be silenced by toggling the
RESET switch.
mR/hr - R/hr Select Switch: a protected toggle switch that toggles between mR/hr and R/hr
ranges
F/S (Fast/Slow) Response Switch:a two-position toggle switch that selects fast or slow
counting response time
Variable Response: dependant on the number of counts present; typically
4 to 25 seconds for FAST, and 4 to 60 seconds for SLOW
Fixed Response: The FAST response position is programmable from 1-199
seconds. The SLOW response position is approximately five times slower
than the FAST. For MDA-type measurements, the fixed response mode is
recommended
BKLITE (LCD Backlight) /RESET: A momentary switch, which when pressed to the BKLITE,
position illuminates the LCD for a pre-programmed time. The backlight "ON" time can be
selected from between 5 and 240 seconds during parameter setup. In the non-alarm
condition, pressing the momentary switch to RESET resets the ratemeter display to the
minimum display readout. In an alarm condition, pressing RESET will silence the audible
alarm. Pressing RESET a second time will reset the ratemeter ALARM and/or ALERT
condition.
Main Board Controls
Note:
To access the internal circuit boards, unlatch the latches at each end of the
Model 78. Carefully separate the top chassis from the bottom cover (referred
to as a "can"). The can has the audio speaker (unimorph) with a two-
conductor cable attached to the main board. The audio plug may be
disconnected during the internal control adjustments.

Model78and78-1 TechnicalManual Section4
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.

Model78and78-1 TechnicalManual Section4
Ludlum Measurements, Inc. Page 4-4 March 2021
OVL (Detector Overload): a multi-turn potentiometer (approximately 20 revolutions) that
adjusts the detector current level, which must be exceeded to initiate an OVERLOAD alarm.
This control adjusts the current level discrimination point from 0.5 and 40 microamperes,
corresponding to the specific detector saturation point. The Cal/Switch board also utilizes a
16-position rotary switch (FUNCTION) to select the 16 setup parameters. (Refer to schematics
and component layout drawing near the end of the manual.) All of the setup parameters are
stored in the non-volatile EEPROM, which will retain data even after the Model 78 batteries
are removed. Changing parameters and information on switchboard controls, other than
those detailed above, are covered in Section 8 of this manual.

Model78and78-1 TechnicalManual Section5
Ludlum Measurements, Inc. Page 5-1 March 2021
Safety Considerations
Environmental Conditions for Normal Use
Indoor or outdoor use
No maximum altitude
Temperature range of -20 to 50 °C (-4 to 122 °F); may be certified for
operation from -40 to 65 °C (-40 to 150 °F)
Maximum relative humidity of less than 95% (non-condensing)
Pollution Degree 3 (as defined by IEC 664). (Occurs when conductive
pollution or dry nonconductive pollution becomes conductive due to
condensation. This is typical of industrial or construction sites.)
Environmental Rating: IP52
Warning Markings and Symbols
Caution!
The operator or responsible body is cautioned that the protection provided
by the equipment may be impaired if the equipment is used in a manner not
specified by Ludlum Measurements, Inc.
Caution!
Verify instrument voltage rating before connecting to a power converter. If
the wrong power converter is used, the instrument and/or power converter
could be damaged.
Section
5