checking mosfets on pcb 1011

CHECKING MOSFETS ON PCB 1011
Figura1 MEASUREMENTS ON GOOD MOSFET
Figura2 MEASUREMENT ON GOOD MOSFET
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MOSFET CHECK ON PCB 1011
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CHECKING MOSFETS ON PCB 1011
Figura 3 MESUREMENT ON GOOD MOSFET
Figura 4 MEASUREMENT ON A BRANCH WITH ONE OR MORE DEFECTIVE MOSFET
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MOSFET CHECK ON PCB 1011
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CHECKING MOSFETS ON PCB 1011
Figura5
Figura 6 MOSFETS READY TO BE CHECKED “IN HOT”. DISTANT 3MM FROM HEATSINK
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CHECKING MOSFETS ON PCB 1011
Figura 7
Figura 8
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CHECKING MOSFETS ON PCB 1011
Figura 9
Figura 10
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MOSFET CHECK ON PCB 1011
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CHECKING MOSFETS ON PCB 1011
Figura 11
Figura 12
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CHECKING MOSFETS ON PCB 1011
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.
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CHECKING MOSFETS ON PCB 1011
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).
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CHECKING MOSFETS ON PCB 1011
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).
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