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MFJ-4416B Troubleshooting Guide


MFJ-4416 Troubleshooting Guide



1.         For any failure Check Pin 2 of U1.  This is the 5.2 Volt reference voltage used by most of the circuitry.  It should read 5.2 volts.  If it is low then remove U2 and check again.  If it is 5.2 volts then suspect U2 as being bad.  If it is still low then suspect U1 as being bad.  If the voltage is high again remove U2 and recheck the voltage.  If it is normal 5.2 volts then suspect U2.  If it is still high then Suspect U1 as being bad.


2.         Also for any failure check the voltage at Pin 13 of U1 and Pin 3 of U2.  This voltage should be the same as the Input Voltage.


3.         Output won’t adjust.  R4, R7 and R3 set up a voltage divider circuit to reduce the output voltage to a value that is compared against the 5.2 volt reference voltage.  With the unit set at 13.8 volts the voltage at Pin 1 of U1 should be about 5.2 volts.  If the unit will not adjust then check the voltage at Pin 1 of U1.  With the unit in the not boosting mode the voltage should swing from about 2.5 volts to 7.0 volts as you move the trim pot from one extreme to the other.  If it does not check the 3 resistors listed above.  If it still does not adjust then check to make sure R6 and C6 are soldered properly.


4.         Always boosting.  There are several things that control this.  The first check to make is to check Pin 10 of U1.  With the unit set to not boost this pin should be at about 3.8 volts and when the boost button is pushed it should go 0 volts enabling the boost circuit.  If this pin is always 0 then remove the banded end of Diode D5.  If the unit still does not stop boosting when you turn the Boost Switch off then suspect Q6 as being bad.  If the unit does function then the problem is Q5 or Q7.  Look at the voltage on the banded end of D5 that you removed.  This voltage will initially be high when power is first applied but should drop to Zero.  If the voltage is high and remains high then look at the voltage on the ends of R16 it should  be about the same on both ends.  If it is low on the end that is toward U1 then suspect Q7 as being bad.  If  the voltage is  close  then suspect Q5 as being bad.


5.         Shorts out supply.  If this condition exists and neither the input terminals nor the output terminals show a dead short then it could be caused by the same condition as in #3 above.  Ensure that the screw is removed from the SCR and it is lifted off the board a little.  If the problem goes away suspect D6 or D7 as being bad.  If it still shorts out the supply check the output voltage and see if it is high if so then check Pin 1 of U1 as listed in #3 above.



6.  Won’t turn on when the transmitter is keyed.  If this failure occurs then take a 10K resistor and attach one end to ground.  Then take the other end of the resistor and touch the End of R16 that is toward U1.  If the Boost LED comes on then suspect Q7.  If the boost LED still does not come on then suspect Q5. 


Article details
Article ID: 79
Company: MFJ
Date added: 2017-01-12 14:41:10
Views: 19

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