BIRD TERMALINE 8251 Series User manual

TERMALINE®LOAD RESISTOR
SERIES 8251
Operation Manual
©Copyright 2016 by Bird Technologies, Inc.
Instruction Book Part Number 920-8251S Rev. G
Termaline® is a Registered Trademark of
Bird Electronic Corporation

i
Safety Precautions
The following are general safety precautions that are not necessarily related to any specific part or procedure, and
do not necessarily appear elsewhere in this publication. These precautions must be thoroughly understood and
apply to all phases of operation and maintenance.
WARNING
Keep Away From Live Circuits
Operating Personnel must at all times observe general safety precautions. Do not replace
components or make adjustments to the inside of the test equipment with the high voltage
supply turned on. To avoid casualties, always remove power.
WARNING
Shock Hazard
Do not attempt to remove the RF transmission line while RF power is present.
WARNING
Do Not Service Or Adjust Alone
Under no circumstances should any person reach into an enclosure for the purpose of
service or adjustment of equipment except in the presence of someone who is capable of
rendering aid.
WARNING
Safety Earth Ground
An uninterruptible earth safety ground must be supplied from the main power source to
test instruments. Grounding one conductor of a two conductor power cable is not
sufficient protection. Serious injury or death can occur if this grounding is not properly
supplied.
WARNING
Resuscitation
Personnel working with or near high voltages should be familiar with modern methods of
resuscitation.
WARNING
Remove Power
Observe general safety precautions. Do not open the instrument with the power on.

Safety Precautions
ii
Safety Symbols
Note: Calls attention to supplemental information.
WARNING
Warning notes call attention to a procedure, which if not correctly performed, could result
in personal injury.
CAUTION
Caution notes call attention to a procedure, which if not correctly performed, could result
in damage to the instrument.
This symbol indicates that a shock hazard exists if the precautions in the
instruction manual are not followed.
The caution symbol appears on the equipment indicating there is
important information in the instruction manual regarding that particular
area.
This symbol indicates that the unit radiates heat and should not be
touched while hot.
This symbol indicates that the item must not be discarded but instead
managed according to the collection and recycle of waste electrical and
electronic equipment (WEEE) as outlined in Directive 2002/96/EC of the
European Parliament and of the Council of 27 January 2003 on waste
electrical and electronic equipment (WEEE).

1 kW Termaline Coaxial Load Resistor Series 8251
iii
Warning Statements
The following safety warnings appear in the text where there is danger to operating and maintenance personnel,
and are repeated here for emphasis.
See Pages 4 , 7 , 10 , and 11
See pages 5 , 8 , 9 , and 10
See pages 5 , 7 , 8 , and 9
Caution Statements
The following equipment cautions appear in the text and are repeated here for emphasis.
See page 4
See page 5
See page 7
See page 9
WARNING
The vent plug must be used at all times when the unit is operating or cooling. Failure to do
so could result in an explosion or severe burns.
WARNING
Disconnect the unit from all power sources before servicing. The unit may be energized
from multiple sources. The potential for electric shock exists.
WARNING
Never attempt to connect or disconnect RF equipment from the transmission line while RF
power is being applied. Leaking RF energy is a potential health hazard.
CAUTION
This load is designed for operation in a horizontal position only, with the vent plug up. Do
not use in any other manner.
CAUTION
The thermoswitch has a factory preset temperature adjustment screw. Do not adjust this
screw.
CAUTION
Make sure the interlock is properly connected when overloaded to prevent damage to the load.
CAUTION
Use only Bird coolant, P/N 5-1070, to prevent damage to the load.

Safety Precautions
iv
Safety Statements
USAGE
ANY USE OF THIS INSTRUMENT IN A MANNER NOT SPECIFIED BY THE MANUFACTURER MAY
IMPAIR THE INSTRUMENT’S SAFETY PROTECTION.
USO
EL USO DE ESTE INSTRUMENTO DE MANERA NO ESPECIFICADA POR EL FABRICANTE, PUEDE
ANULAR LA PROTECCIÓN DE SEGURIDAD DEL INSTRUMENTO.
BENUTZUNG
WIRD DAS GERÄT AUF ANDERE WEISE VERWENDET ALS VOM HERSTELLER BESCHRIEBEN,
KANN DIE GERÄTESICHERHEIT BEEINTRÄCHTIGT WERDEN.
UTILISATION
TOUTE UTILISATION DE CET INSTRUMENT QUI N’EST PAS EXPLICITEMENT PRÉVUE PAR LE
FABRICANT PEUT ENDOMMAGER LE DISPOSITIF DE PROTECTION DE L’INSTRUMENT.
IMPIEGO
QUALORA QUESTO STRUMENTO VENISSE UTILIZZATO IN MODO DIVERSO DA COME
SPECIFICATO DAL PRODUTTORE LA PROZIONE DI SICUREZZA POTREBBE VENIRNE
COMPROMESSA.

1 kW Termaline Coaxial Load Resistor Series 8251
v
SERVICE
SERVICING INSTRUCTIONS ARE FOR USE BY SERVICE - TRAINED PERSONNEL ONLY. TO AVOID
DANGEROUS ELECTRIC SHOCK, DO NOT PERFORM ANY SERVICING UNLESS QUALIFIED TO DO
SO.
SERVICIO
LAS INSTRUCCIONES DE SERVICIO SON PARA USO EXCLUSIVO DEL PERSONAL DE SERVICIO
CAPACITADO. PARA EVITAR EL PELIGRO DE DESCARGAS ELÉCTRICAS, NO REALICE NINGÚN
SERVICIO A MENOS QUE ESTÉ CAPACITADO PARA HACERIO.
WARTUNG
ANWEISUNGEN FÜR DIE WARTUNG DES GERÄTES GELTEN NUR FÜR GESCHULTES
FACHPERSONAL.
ZUR VERMEIDUNG GEFÄHRLICHE, ELEKTRISCHE SCHOCKS, SIND WARTUNGSARBEITEN
AUSSCHLIEßLICH VON QUALIFIZIERTEM SERVICEPERSONAL DURCHZUFÜHREN.
ENTRENTIEN
L’EMPLOI DES INSTRUCTIONS D’ENTRETIEN DOIT ÊTRE RÉSERVÉ AU PERSONNEL FORMÉ AUX
OPÉRATIONS D’ENTRETIEN. POUR PRÉVENIR UN CHOC ÉLECTRIQUE DANGEREUX, NE PAS
EFFECTUER D’ENTRETIEN SI L’ON N’A PAS ÉTÉ QUALIFIÉ POUR CE FAIRE.
ASSISTENZA TECNICA
LE ISTRUZIONI RELATIVE ALL’ASSISTENZA SONO PREVISTE ESCLUSIVAMENTE PER IL
PERSONALE OPPORTUNAMENTE ADDESTRATO. PER EVITARE PERICOLOSE SCOSSE
ELETTRICHE NON EFFETTUARRE ALCUNA RIPARAZIONE A MENO CHE QUALIFICATI A FARLA.
CONNECT INTERLOCK TO TRANSMITTER/GENERATOR/AMPLIFIER BEFORE OPERATING.
BRANCHER LE VERROUILLAGE À L’ÉMETTEUR/GÉNÉRATEUR/AMPLIFICATEUR AVANT EMPLOI.
CONECTE EL INTERBLOQUEO AL TRANSMISOR/GENERADOR/AMPLIFICADOR ANTES DE LA
OPERACION.
VOR INBETRIEBNAHME VERRIEGELUNG AM SENDER/GENERATOR/VERSTÄRKER
ANSCHLIESSEN.
PRIMA DI METTERE IN FUNZIONE L’APPARECCHIO, COLLEGARE IL DISPOSITIVO DI BLOCCO AL
TRASMETTITORE/GENERATORE/AMPLIFICATORE.

About This Manual
vi
About This Manual
This instruction book covers the following models:
Changes to this Manual
We have made every effort to ensure this manual is accurate. If you discover any errors, or if you have suggestions
for improving this manual, please send your comments to our Solon, Ohio factory. This manual may be periodically
updated. When inquiring about updates to this manual refer to the part number and revision on the title page.
Chapter Layout
Introduction — Describes the features of the 8251 series load and indicates the capabilities of each model in the
series.
Theory of Operation — Includes the details of the loads construction as well as a description of the coolant used
and the interlock provided.
Installation — This chapter details installation, from unpacking the unit to proper mounting, and RF connections.
Operating Instructions — Provides safety details necessary to follow during load operation.
Maintenance — This chapter provides troubleshooting and repair information, along with specifications, storage
and shipment instructions.
8251 8251N 8251D 8251D7-16
8251T-230 8251T-300

vii
Table of Contents
Safety Precautions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .i
Safety Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii
Warning Statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii
Caution Statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii
Safety Statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv
About This Manual . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .vi
Changes to this Manual . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi
Chapter Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi
Chapter 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
Items Supplied . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Chapter 2 Theory of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Load Resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Coolant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
Thermal Interlock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Chapter 3 Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Unpacking and Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
Mounting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Installing Thermoswitch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Connecting RF Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Using a “QC” Connector Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Using a 13-30 Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
Using a Swivel Flanged Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Using a Unflanged Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
Chapter 4 Operating Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Normal Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Operating Under Abnormal Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Shutting Down . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Emergency Shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
Chapter 5 Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Troubleshooting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
DC Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Coolant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Repair . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
RF Connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
Rear Seal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
Load Resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

Table of Contents
viii
Storage and Shipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Storing the Load Resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Shipping the Load Resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Customer Service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Replacement Parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Available “QC” Type Connectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Limited Warranty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

1
Chapter 1 Introduction
Bird 8251 Series Loads are portable, general purpose 50 ohm coaxial RF transmission line terminations. They are
accurate, dependable, and low reflection, dissipating up to 1000 watts of RF power. See Figure 1 on page 2 for
frequency ranges.
The load has a coolant chamber surrounded by radiator fins. A handle is recessed in the radiator fins. The front and
rear fins form mounting flanges which can be used as supports for freestanding use or as brackets for fixed
mounting. A vent plug at the top of the unit relieves internal pressure from coolant expansion. The load’s simple
and rugged design minimizes maintenance requirements.
Applications
Use an 8251 load:
As a substitute antenna for tuning transmitters under nonradiation conditions or for making routine tests
and adjustments.
As a substitute for any circuit loading element.
To measure, with a suitable meter, the power output of coaxially transmitted signals.
Items Supplied
Load Resistor: Pre-filled with coolant at the factory
Shipping Plug
Vent Plug
Instruction Manual

Introduction
2
Figure 1 Outline Drawing

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 



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Model Connector Frequency Range Thermoswitch
Temperature
8251 Female LC DC – 2400 MHz N/A1
1 Optional thermoswitch kit (P/N 2450-129) available.
8251N Female N DC – 1000 MHz N/A1
8251D2
2 Requires bullet (P/N 4712-020) – not included
1-5/8” EIA Swivel 470 – 860 MHz N/A1
8251D7-16 7/16 IEC 470 – 860 MHz N/A1
8251T-2303
3 Bullet included
1-5/8” EIA Fixed Flanged DC – 2000 MHz 230 °F (110 °C)
8251T-30031-5/8” EIA Fixed Flanged DC – 2000 MHz 300 °F (149 °C)

3
Chapter 2 Theory of Operation
Load Resistor
Bird 8251 series loads consist of a thin-film-on-ceramic resistor immersed in a dielectric coolant. The resistor,
individually selected for its accuracy, is enclosed in a special housing. When surrounded by the coolant, this
produces a uniform, practically reflectionless line termination over the specified frequencies.
Coolant
The load is cooled by natural fluid and air convection currents. The coolant, chosen for its dielectric and thermal
characteristics, carries heat from the resistor to the walls of the cooling tank, where radiator fins surrounding the
tank transfer the heat to the air.
When the coolant is heated, thermal expansion causes an increase in the internal pressure. An expansion tank on
the upper rear of the load allows for coolant expansion due to the rise in temperature, and is equipped with a vent
plug (see
Figure 1 on page 2 ) to relieve this pressure while protecting the opening from dirt or other contaminants.
Thermal Interlock
Loads can be supplied with an optional passive overtemperature thermoswitch. Normally closed, it opens at either
230 or 300 °C (110 or 149 °F), depending on the model, turning off transmitter power. The interlock system will not
permit use of the transmitter until the load has reached a safe temperature.

4
Chapter 3 Installation
This chapter provides information for on-site requirements, unpacking, inspection, and preparing the load for use.
Unpacking and Inspection
1. Carefully inspect shipping container for signs of damage.
2. Do one of the following:
If the shipping container is damaged, do not unpack the unit. Immediately notify the shipping carrier and
Bird Electronic Corporation.
If the shipping container is not damaged, unpack the unit. Save shipping materials for repackaging.
3. Inspect unit for visual signs of damage.
Note: If there is damage, immediately notify the shipping carrier and Bird Electronic Corporation.
Setup
Note: Before first using the load, get a resistance baseline for future maintenance. Refer to "DC
Resistance" on page 9 for details.
1. Remove the shipping plug from the load. Refer to Figure 1 on page 2 for location of the plugs.
2. Verify the o-ring seal is in place on the vent plug, then install in the load.
Mounting
Place the load in a dry, dust and vibration free environment. Do not use outdoors or in areas of condensing
humidity. For operation above 300 W, allow at least 6" (15 cm) of clearance on all sides of the load.
The load is equipped for either portable use or fixed installation. The mounting brackets on the front and rear faces
have four mounting slots arranged in a 518” x 15” rectangle (130 x 381 mm). Use a screw with a 14” (6.4 mm)
diameter max.
WARNING
The vent plug must be used at all times when the unit is operating or cooling. Failure to do
so could result in an explosion or severe burns.
CAUTION
This load is designed for operation in a horizontal position only, with the vent plug up. Do
not use in any other manner.

1 kW Termaline Coaxial Load Resistor Series 8251
5
Installing Thermoswitch
Bird 8251T loads are identical to the 8251 except for a factory-installed interlock thermoswitch. It is normally
closed, opening at either 230 or 300 °F (110 or 149 °C), depending on the model. The terminals are rated for 10A @
120 Vac and 5A @ 240 Vac.
1. Unscrew the cover screws and remove the thermoswitch cover.
2. Thread the interlock wires through the grommet on the side of the cover.
3. Wrap the interlock wires around the terminals and clamp them in place with the terminal nuts.
Note: Do not apply more than 10 in.lb. (1.1 N-m) of torque to the terminal nuts.
4. Replace the cover.
5. Check the requirements of the transmitter interlock and make connections as required.
Figure 2 Thermoswitch
Connecting RF Power
Note: After installing the load, the RF transmission line can be attached using standard coaxial line
coupling kits.
WARNING
Disconnect the unit from all power sources before servicing. The unit may be energized
from multiple sources. The potential for electric shock exists.
CAUTION
The thermoswitch has a factory preset temperature adjustment screw. Do not adjust this
screw.
WARNING
Never attempt to connect or disconnect RF equipment from the transmission line while RF
power is being applied. Leaking RF energy is a potential health hazard.

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

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%%
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&!
'

Installation
6
Using a “QC” Connector Coupling
Use a 50 ohm coaxial cable such as RG-218/U or -220/U (-17A or -19A), appropriate for the frequency and power
level of operation.
Note: Use a cable connector which will mate with the one on the load.
Using a 13-30 Coupling
Use 50 ohm coaxial cable such as RG-8A/U, RG-9U, RG-213/U, or equivalent with a male 13-30 plug.
Using a Swivel Flanged Coupling
1. Insert the center bullet.
2. Push it in until it is fully seated.
3. Connect the coaxial input in a straight line
4. Push, carefully, on the center conductor to close.
Note: The swivel flange on the load makes connection independent of the orientation of the fixed
flange on the coaxial input outer conductor.
5. Insert the bolt sets and tighten evenly all around to transmission line manufacturer’s recommended torque.
Note: Use all of the bolts.
Figure 3 Swivel Flanged Coupling
Using a Unflanged Coupling
1. Insert the center bullet
2. Push until it sets the midpoint nibs.
3. Position the outer sleeve, with clamping bands, over the input connector.
4. Set the transmission line snugly against the coupling stops.
5. Position the clamping bands evenly about 3/4” from the ends of the sleeve.
6. Tighten the clamping bands.
Figure 4 Unflanged Coupling
After the transmitter has been connected to the load, proceed according to the transmitter manufacturer’s
instructions. When reconnecting the antenna, it may be necessary to slightly readjust the transmitter due to
differences in VSWR between the load and the antenna system.
   



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

 

7
Chapter 4 Operating Instructions
Normal Operation
Bird 8251 series loads have no indicators or operating controls. They require no special operating procedures or
surveillance when their performance limits are not exceeded. Follow the instructions for the specific transmitter
equipment.
Operating Under Abnormal Conditions
The load can be moderately overloaded for short periods.
Shutting Down
Note: These loads are passive devices, so have no way of being turned off. Turn off RF power at the
source.
Emergency Shutdown
Turn off RF power at the source.
Note: If the interlock thermoswitch is properly connected, RF power will be automatically turned off
when the coolant temperature reaches an unsafe level
WARNING
The vent plug must be used at all times when the unit is operating or cooling. Failure to do
so could result in an explosion or severe burns.
CAUTION
Make sure the interlock is properly connected when overloaded to prevent damage to the load.
WARNING
Never attempt to connect or disconnect RF equipment from the transmission line while RF
power is being applied. Leaking RF energy is a potential health hazard.

8
Chapter 5 Maintenance
This chapter covers cleaning, inspection, troubleshooting, and specifications for the Bird 8251 series loads.
Troubleshooting
The table below contains troubleshooting information for problems which can occur during normal operation. This
manual cannot list all malfunctions that may occur, or their corrective actions. If a problem is not listed or is not
corrected by the listed actions, notify a qualified service center.
Maintenance
Cleaning
The outside surface of the instrument should be wiped free of dust and dirt when necessary. Excessive dust on the
cooling fins will interfere with heat dissipation. Clean the RF connector, both metallic and insulating surfaces, with a
dry, non-residue forming solvent.
Problem Possible Cause Correction
Leaking coolant
Loose clamping band Tighten the clamping band
Defective or improperly installed front
O-ring
Replace the O-ring
(
See “Load Resistor” on page 11. )
Defective or improperly installed rear
O-ring
Replace the O-ring
(
See “Rear Seal” on page 10. )
High or low DC resistance
Loose RF input connector Tighten connector
Faulty RF input connector
Models with “QC” type connectors: Replace connector
(
See “RF Connector” on page 10. )
All other models: Return the unit for service
Faulty resistor Replace the resistor
(
See “Load Resistor” on page 11. )
Overheating radiator
RF power too high Lower RF power (
See “Specifications” on page 13.
for maximum RF power)
Coolant level too low Check the coolant level. Add coolant if necessary (
See
“Coolant” on page 9. )
Coolant degraded Replace the coolant
(
See “Coolant” on page 9. )
Faulty resistor Replace the resistor
(
See “Load Resistor” on page 11. )
WARNING
Never attempt to connect or disconnect RF equipment from the transmission line while RF
power is being applied. Leaking RF energy is a potential health hazard.
WARNING
Disconnect the unit from all power sources before servicing. The unit may be energized
from multiple sources. The potential for electric shock exists.

1 kW Termaline Coaxial Load Resistor Series 8251
9
Inspection
Inspect the unit every six months. Check for coolant leakage around the clamping band and the thermoswitch. Also
check for corrosion.
DC Resistance
Measuring the DC resistance between the inner and outer conductors of the RF connector shows changes in the
load over time, a good check of the load resistor’s condition. Under normal operating conditions, the resistor should
provide at least 5,000 hours of operation before requiring any additional service. DC resistance tracking must start
before the load is put into service, and should be measured annually.
Perform the following steps and record the value for future comparison. Make sure that you have an ohmmeter
with an accuracy of ±1% at 50 ohms and that the load temperature is between 20 and 25 °C (68 to 77 °F) before
starting.
1. Turn off the RF power and interlock circuitry.
2. Disconnect the RF line.
3. Connect the multimeter test leads to the center and outer conductor of the load resistor.
4. Compare the measured value with the previous measurement and with the baseline resistance, measured
when the load was put into service.
Note: If the new value differs from either of these by more than 1 ohm this could indicate a failing
resistor.
Figure 5 Measuring Resistance
Coolant
Coolant lifetime will vary greatly depending on operating conditions. For heavy use (full RF power for long times,
high ambient temperature), check the coolant every 500 hours. If the load has only had light duty (fraction of full
power, low ambient temperature), then coolant inspection may only be necessary every 2,000 hours.
Note: Correct any coolant leakage before inspection.
1. Remove the load resistor (Refer to “Load Resistor” on page 18).
WARNING
Never attempt to connect or disconnect RF equipment from the transmission line while RF
power is being applied. Leaking RF energy is a potential health hazard.
WARNING
Disconnect the unit from all power sources before servicing. The unit may be energized
from multiple sources. The potential for electric shock exists.
CAUTION
Use only Bird coolant, P/N 5-1070, to prevent damage to the load.

Maintenance
10
2. Inspect the coolant.
The coolant should be clear, with a faint yellow tinge, and have a slightly sweet smell. If it is black with a
burnt or acrid smell, drain it and add about 2.9 gal (11 L) of coolant.
With the load still on end, the coolant level should be 434to 5 inches (125 mm) below the top surface of
the resistor assembly mounting ring, at ambient temperature.
3. Add coolant if necessary
Repair
RF Connector
The Bird 8251 and 8251N have a special Bird “QC” connector which allows easy changing of the RF connector. This
does not disturb the coolant seal or affect the electrical continuity of the load. To change the connector, proceed as
follows:
1. Remove the four screws at the corners of the RF connector.
2. Pull the connector straight out.
3. Push the new connector in.
4. Ensure that the center pin on the connector is properly seated in the mating socket on the load.
5. Replace the screws.
Note: If not using the connector normally supplied, frequency and power must be limited to the capa-
bilities of the connector.
Rear Seal
If the coolant has not been drained from the load, stand the unit on end with the RF connector down before
disassembly.
1. Unscrew the four screws at the corners of the guard cover.
2. Unscrew the tube nut from the tank nozzle, using a small wrench if necessary, and pull the nozzle free.
3. Unscrew and remove the clamping band.
4. Remove the rear cover, including the attached escape tube and the diaphragm seal.
5. Inspect the diaphragm seal and replace it if it is not pliable or has surface cracks.
CAUTION
Use only Bird coolant, P/N 5-1070, to prevent damage to the load.
WARNING
Disconnect the unit from all power sources before servicing. The unit may be energized
from multiple sources. The potential for electric shock exists.
WARNING
The vent plug must be used at all times when the unit is operating or cooling. Failure to do
so could result in an explosion or severe burns.
WARNING
Disconnect the unit from all power sources before servicing. The unit may be energized
from multiple sources. The potential for electric shock exists.

1 kW Termaline Coaxial Load Resistor Series 8251
11
6. To reassemble, reverse these steps.
Figure 6 Rear Seal
Load Resistor
1. Remove the vent plug.
2. Install the shipping plug.
3. Stand the unit on its back, with the RF connector up.
4. Brace the unit to prevent it from tipping over.
5. Unscrew and remove the clamping band.
6. Lift the load resistor assembly out of the tank and allow any coolant to drip back into the tank.
Note: The O-Ring should be free of twists and positioned evenly around the flange of the resistor
housing. If the O-ring shows signs of deterioration (e.g. is no longer pliable or has surface cracks)
replace it.
7. Replace the entire load resistor assembly.
Note: It cannot be further disassembled.
8. Put the clamping band in place and tighten it.
9. Remove the shipping plug. Install the vent plug.
WARNING
The vent plug must be used at all times when the unit is operating or cooling. Failure to do
so could result in an explosion or severe burns.