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Another MMR40 fix


Another MMR40 fix

When I initially constructed the MMR40, I was going to use the original case.  So I put the band range switch on the bottom of the board to accommodate that.  Well with switching to the new case, I decided that I needed to move the switch to the front panel.  This had an unfortunate side effect.  It seems that the stray capacitance from the few inches of wire used to connect the new switch is enough to shift the VFO frequency down about17 kc.


Original Band Switch

I had spent a good amount of time adjusting the coils of the VFO to make it cover from 7025 to about 7150 kc on CW, which includes the most popular QRP frequencies of 7040 and 7030 kc.  With the 17 khz offset, the new range was shifted to 7042 to 7170 kc.

To rectify this problem, I pulled 1 turn off of the L2 toroid of the VFO circuit.

L2 Toroid

This shifted the VFO frequency up, and caused the rig’s final frequency to shift down.  So I now cover 7010 to 7147 kc in CW range, and 7126 to 7258 kc in SSB range.  I gave up some spectrum at the top, and gained some at the bottom.  I guess that’s OK, as 5 watts is a lot better using CW than it is using voice.  The Blue/White cable is the connection to the new switch.

MMR40 Mounted in the chassis

Large picture of band switch installation. I had a question about this, and found this higher res photo of how I wired the band switch. It looks like I wired the switch across the two terminals directly above the “S2” indicator on the silkscreen. I don’t remember why the wire colors are different here, but this looks like the connection…

 Migrated Comments:

3 Responses to “Another MMR40 fix”

  1. AC0AX Says:

    How did you wire up the band switch. Im currently trying to wire mine up but the pto never starts oscillating. It was fine before I tried to wire in a new switch.

  2. wb0smx Says:

    AC0AX, I added a hi-res photo to the post of how I wired the band switch. You can click on the photo and get a large one that will let you zoom in on the details. Good luck…. Randy WB0SMX

  3. ac0ax Says:

    Thanks Randy,
    I got it working. I must of had a cold joint. I touched up all the joints in the PTO and then it came back to life.


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