MFJ 90XX spotting switch


From: Monte Stark (
Date: Tue Jan 17 1995 - 10:57:25 EST

      SPOTTING SWITCH for the MFJ 9040
Monte "Ron" Stark, KU7Y
285 W. 4th Ave.
Sun Valley, NV 89433

        While listing to the NA contest this week end I
noticed something that seems to be getting worse with time.
When one station calls another station, it should be done on
the same frequency. Often the responding station is so far
off frequency that he is never heard. Many stations use
narrow filtering during a contest and if you find that
people seem to ignore you, this may be the problem.
        Some radios have features that let you know where
you are. Many of them are slow to use and just not handy. I
have not heard of any QRP kits that have a spotting feature.
With this in mind I looked over the schematics for the MFJ
9040 and the MFJ 9020. You need to provide 5 volts to pin 8
of U5. After a few minutes of studying and a few minutes of
soldering, I had a nice spotting switch. I have only done
this on my radio that is a 9040. I think this will work on
any of the 90XX series rigs.
                              You will need a momentary
                              switch and a diode. I used a
small push button switch and a 1N914 diode. One cut trace, a
few inches of insulated wire and you are done.
        To install a spotting switch, start by removing the
top cover. Unsolder the RF output wire that goes to the SO
239. Remove the bolt and nut from the final transistor.
Remove the six screws in the bottom of the radio. Locate the
two five volt regulators, U7 and U8. These are both near the
front panel. On the bottom side of the board, locate the
trace that brings the B+ from Q5 to U8. Cut this trace and
bridge the cut with the 1N914 diode. Orient the band toward
U8. This will allow the B+ to flow into U8 during transmit
but will stop any voltage from going to Q5 when the spotting
switch is used. Placing the diode in the supply side of U8
prevents the 0.6 volt drop from causing a problem.
        Now connect one wire to the output side of U8. This
goes to pin 8 of U5. Connect another wire to the regulated 5
volt output of U7. Connect these two wires to the momentary
    I mounted the switch in the unmarked hole on the back
panel. I did that because the hole was the right size for
the switch! Twist the two wires together and bring them up
through the hole at the back of the board where the tie wrap
is located.
        When reassembling the rig, be sure that the
washer is in place between the final transistor and the
case. Do not forget to connect the RF output wire to the SO
239. Also, clean the board where you worked on it with a
tooth brush and alcohol.
 When you press the spotting switch you hear a tone that is
                              equal to the
transmitter offset. In a rig that is properly aligned this
will be 700 cycles.
    To use the spotting switch, be sure to center the RIT
control. Then hold the
spotting switch "on" and tune the station using the main
tuning dial. When the tone from the station is the same as
the side tone you are on the same frequency. I checked
with the TS930S and could not tell any difference in the
tone between the station I was zero beating and my

        72's and enjoy, Ron, KU7Y

ps. In looking at the NorCal 40, I think the same idea
would work there also. I just don't have one to try it

.........KU7Y....................Monte "Ron" Stark..... Valley, Nevada....
.........ARRL.......NorCal #330.......NRA LIFE.........

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