CHECKING MOSFETS ON PCB 1011

Transcription

1 Figura1 MEASUREMENTS ON GOOD MOSFET Figura2 MEASUREMENT ON GOOD MOSFET MOSFET CHECK ON PCB 1011 Pag 1 de 9 Septiembre 2004 ruc

2 Figura 3 MESUREMENT ON GOOD MOSFET Figura 4 MEASUREMENT ON A BRANCH WITH ONE OR MORE DEFECTIVE MOSFET MOSFET CHECK ON PCB 1011 Pag 2 de 9 Septiembre 2004 ruc

3 Figura5 Figura 6 MOSFETS READY TO BE CHECKED IN HOT. DISTANT 3MM FROM HEATSINK MOSFET CHECK ON PCB 1011 Pag 3 de 9 Septiembre 2004 ruc

4 Figura 7 Figura 8 MOSFET CHECK ON PCB 1011 Pag 4 de 9 Septiembre 2004 ruc

5 Figura 9 Figura 10 MOSFET CHECK ON PCB 1011 Pag 5 de 9 Septiembre 2004 ruc

6 Figura 11 Figura 12 MOSFET CHECK ON PCB 1011 Pag 6 de 9 Septiembre 2004 ruc

7 Figura 13 INTRODUCTION Each RF Power amplifier have two H bridges with four branches of three transistors in paralell.( See SCH1011), so, each time you measure one MOSFET of each branch you are checking the whole branch. This means that if one MOSFET is defective, you will see as if all of they are defective. PROCEDURE With a digital multimeter as in photos, check all branches including PWM amplifier. In fig 1,2 and 4 you can see how to check the MOSFETS and the meter indication as for good MOSFET In fig 4 you can see the meter indication if one or more MOSFET are defective in that branch. When you detect a defective branch you need to know which transistor is defective. To know which transistor is defective in that branch, unscrew the screw that fix transistors to the heat sink and move the transistors ( do not unsolder) in order they are not in contact with heatsink ( see fig 6). With the transistors as in fig 6 plug the RF amplifier into the transmitter.do not connect the DB25 connector with the flat cable. MOSFET CHECK ON PCB 1011 Pag 7 de 9 Septiembre 2004 ruc

8 Turn on the transmitter for about one or two minutes and then remove the RF amplifier board an touch carefully the loosen transistors with your finger.caution IT CAN BE VERY HOT. Defective transistor becomes hot. Good transistors stay cool. Change the defective transistor and check again all branches as indicated in fig 1 to 3. It is recommended to check in reverse polarity as shown in fig 5. ( good transistor are shown) ATTENTION: DO NOT FORGET TO MOUNT TRANSISTOR WITH THE FERRITE BEAD ON DRAIN PIN. When there is a big damage it is recommended to check also PWM transistor as shown in fig 7 to 13. Note: If PWM transistors has been damaged,it is very probable that TC 4420 IC and his output resistor are also damaged. Alter repair is completed and RF modulated amplifier plugged into the transmitter, test point indications are as following: TP1.- If his voltaje is zero, it indicates low RF drive signal or that the external inhibit pin is activated (JP2 installed) or there is excesive RF output current. When all is OK on pin 12 of U2 must be +5V Inputs are: Pin 1 U2 indicates low RF drive. If OK it must have +5V (with transmitter in OFF but main switch ON). Pin 2 U2 indicates external inhibit. If OK it must have +5V(with transmitter in OFF). Pin 13 U2 indicates RF output overcurrent.if OK it must have+ 5V, (with transmitter in OFF). TP2.- If voltaje in this pin is +5V module is OK. If voltaje is Zero, there exist the following posible failures: unbalance because of defective transistor on PWM circuit, or any of the failures describe don TP1. Inputs to U2B are :son las siguientes: Pin 5 U2 indicates unbalance. If OK there must be + 5V, (with transmitter in OFF). Pin 4 U2 indicates TP1 status. If all OK There must have +5V, (with transmitter in OFF). Pin 3 U2 indicates if PWM transistors are working OK. If OK there must have +5V (with transmitter in ON). MOSFET CHECK ON PCB 1011 Pag 8 de 9 Septiembre 2004 ruc

9 TP3.- If there is no failure it must measure +5V. For this all inputs must measure +5V. Inputs are: Pin 11 U2 indicates +270V power is OK if +5v exist. (UIT transmitter in ON). Pin 10 U2 indicates status of TP2.Must have 5V if all OK, (with transmitter in ON). Pin 9 U2 indicates PWM drive signal is OK if +5V on it. (with transmitter in OFF). MOSFET CHECK ON PCB 1011 Pag 9 de 9 Septiembre 2004 ruc