Lincoln Electric LF-72/74 service manual Theory of Operation, Return to Section

Page 82

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E-2

THEORY OF OPERATION

E-2

FIGURE E.2

GENERAL DESCRIPTION & INPUT RECEPTACLE, CONTROL BOARD AND OPERATOR CONTROLS

OPTIONAL BURNBACK TIMER

OPTIONAL REMOTE VOLTAGE CONTROL

 

GAS

3.75 AMP

SOLENOID

OVERLOAD

WIRE

PROTECTION

 

DRIVE

WIRE DRIVE

MOTOR

TACK

RECEPTACLE

To Gun Trigger

 

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14PIN

INPUT CABLE RECEPTACLE

24/42 VAC

CONTROL BOARD

 

 

 

 

 

 

WIRE FEED

 

TRIGGER

 

 

 

INTERLOCK

 

 

SPEED POT.

 

2-STEP

 

 

 

 

 

 

COLD FEED GAS PURGE

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GENERAL DESCRIPTION

The LF72 and LF74 are single head, 24vac or 42vac input, 2 roll (LF72) or 4 roll (LF74) wire feeders. They have a single calibrated control for wire feed speed WFS and an optional K2339-1 remote voltage control kit to be used in conjunction with a Lincoln constant voltage (CV) direct current DC power source.

The LF72 and LF74 offer the user 3 individual features Cold feed, trigger interlock 2 step trigger control and gas purge. Presettable burnback and postflow timers are also available with an optional K2330-1 timer kit.

The motor is equipped with a 3.75amp current over- draw protection circuit, which is indicated by a THER- MAL LED located on the case front.

INPUT RECEPTACLE, CONTROL BOARD AND OPERATOR CON- TROLS

The 24 or 42-volt input is applied to the LF72 or LF74 through the 14 pin amphenol type connector. The remote trigger leads are accessed through 14 pin receptacle. The input voltage is rectified by a full wave bridge and is sent to a switching power supply in the control board where it is filtered. This filtered DC volt- age is applied to a switching power supply. The switching power supply manufactures +15vdc and +5vdc regulated supplies for the control board circuit- ry.

The control board receives commands from the user operated controls for the wire feed speed WFS pot, the optional timer kit and the optional voltage remote control kit. It compares this feedback information with the user command signals and generates appropriate signals to control wire feed speed, arc voltage and gas flow.

The control board is also responsible for the operation of the trigger interlock, 2-step, cold feed and the gas purge features of the LF72 and the LF74.

LF-72/74 SINGLE/DUAL

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Contents LF-72/74 Safety Electric Shock can kill Welding Sparks can cause fire or explosion IiiPrécautions DE Sûreté Sûreté Pour Soudage a L’ArcMaster Table of Contents for ALL Sections Installation Section Table of ContentsSection A-1 Installation Amp Rating Duty CycleLocation Safety PrecautionMounting Boom Mount Bench MountSwivel Mount SuspendedWeld Cable Sizes Coaxial Weld Cables Weld Cable ConnectionAnalog Control Cable INSTALLATIONA-7Control Cable Connections Analog Miller Control Cable Adapter K2335-1 Miller Power Source Lincoln Wire FeederPin Function Remote Sense Lead Specifications Wire Drive SystemsWelding GUN/WIRE Feeder Trigger Connector High Frequency ProtectionWelding GUNS, Torches and ACCES- Sories Magnum GUN and Cable AssembliesWire Drive Configuration Procedure for Changing Drive and Idle Roll SetsRolls.FIGURE A.4 Gun Receiver For use With Bushing Pressure ARM AdjustmentSet the pressure arm as follows See Figure A.6 Wire Reel Loading Weld Wire Routing Figure A.9Shielding GAS Connection Conduit Kits Installing ElectrodeTo install Lincoln conduit See Figure A.10b For Codes 11209, 11210, 11211 and aboveAluminum Wire Preparations Figure A.12bINSTALLATIONA-18 Base Model Bench Model Standard Duty Bench Model Heavy DutyTypical System Configurations CableLF-72/74 Operation Section Section B-1Operation GAS Input Work ConnectionProduct Description Common Welding AbbreviationsRecommended Processes Front Panel Controls and Connections Figure B.1 Case Front Controls DescriptionStep Trigger Interlock Switch Burnback and Postflow Timer KITCold FEED/GAS Purge Switch Remote Voltage Control KITGUN Receiver Bushing K1500-2 Wire Feed Speed KnobTrigger Connector 5-PIN Amphenol Trigger InterlockAccessories Section Section C-1Accessories Optional KitsLF-72s Accessories Key LF-72/74 Maintenance Section Section D-1Calibration Specification MaintenanceMaintenance Routine MaintenanceFigure D.1 Figure D.3 Major Component Location Section AA-1 Section AAAA-2 WFS Range Wire SizesModel AA-3 Figure AA.1AA-4 AA-5 AA-6 Coaxial Weld CableAA-7 INSTALLATIONAA-7K1797-xx AA-8 Drive Roll Kits Steel or Cored Wires AA-9AA-10 Figure AA.3Lation or changing drive rolls and/or AA-11Changing the Gun Receiver Bushing GuidesAssembly of Drive Rolls and Wire Guides AA-12AA-13 Set the pressure arm as follows See Figure AA.8AA-14 AA-15 Loading 16 to 44 lb .3 20kg Spools See figure AA.8AA-16 Figure AA.11AA-17 Figure AA.12aAA-18 Figure AA.12bAA-19 AA-20INSTALLATIONAA-20 Common Equipment SET-UPSFigure AA.15 High Volume Production AA-21Figure AA.16 Portable Welding AA-22Section BB-1 Section BBBB-2 Fcaw Innershield or OutershieldRequired Equipment BB-3BB-4 Figure BB.1 Case Front Controls DescriptionK2426-1 and K2426-2 models BB-5K2426-3 model Switch BB-6Section CC-1 CC-2 CC-3 CC-4 CC-5 CC-6 Section DD-1 Section DDWhen inching with gun trigger, electrode To verify the wire feed speedNal wiring WFS knob setting Could remain energized Several seconds MAINTENANCEDD-3 DD-3Figure DD.3 Major Component Location DD-4Theory of Operation Section Section E-1Theory of Operation Return to SectionControl Board LF-72/74 Troubleshooting & Repair Section Section F-1Troubleshooting & Repair HOW to USE Troubleshooting GuidePC Board Troubleshooting Procedures Function Problems Function Problems Troubleshooting & Repair Drive Motor and Tach Feedback Test Test DescriptionTest Procedure Troubleshooting and Repair GAS Solenoid TestTest Procedure Figure F.3 Leads 552, 552BDiode Bridge Test 552B + 552A Wire Speed Potentiometer Test Wire Speed Potentiometer Test 14F-14PC Board Removal and Replacement Control PC Board Removal and Replacement Procedure Removal Procedure Replacement ProcedureTroubleshooting and Repair Wire Drive Motor and Gear BOX Removal and Replacement ProcedureProcedure Replacement Procedures SET UP Operation VerificationLF-72/74 Electrical Diagrams Section Section G-1Electrical Diagrams LF-72/74Timer KIT Board Normal Speed Micro Internal panel Motor Connecter Summary High Torque Setting engages Schematic Common Analog Control PC Board G4559 Sheet Electrical Diagrams Electrical Diagrams PC Board Assembly Common Analog Control G4560-2E0
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