I Built the MMR-40 QRP SSB/CW Rig: PTO Tip

I’m claiming to be an intermediate builder now. I built the MMR-40 QRP rig designed by Steve Weber, KD1JV and sold by Doug Hendricks QRP Kits. This project was the my most ambitious kit building experience to date.

Right away, I knew this project would not be like the ones I built before. The first task described in the instructions is to solder three brass nuts to the board for the PTO. Before I built this kit, I didn’t know that PTO stands for permeability tuned oscillator, which is some sort of movable metal inside a coil to tune the VFO instead of a variable capacitor.
PTO Detail
Here’s my first MMR-40 building tip: there needs to be some play between the nylon spacer and the middle brass nut. Thread the entire coil assembly together, including snugging in the #6 nylon screw, and place the assembly on the board to see how much play there needs to be before you solder the nuts down. Once my PTO was assembled, I had trouble getting the screw to run through and had to move one of the nuts. Even though my kit came with enough #32 wire to wrap the PTO coil twice, I only had to wrap it once and solder it to the board twice. A note about the PTO coil turn count: the turn count on mine came to about 34. However, if you get within 5 or 6 turns of the 39 that are called for, the PTO will work. If you look at the PTO detail picture, you will see a few gaps as if the coil was not wound tight. The coil was tight until I adjusted the spacing a little to align the tuner. So far, I’m happy with the range I get from 7.001 to 7.075.

The MMR-40 is a fantastic kit. Each time I touch the rig I learn something more about how radios work.

Nearly finished

3 responses to “I Built the MMR-40 QRP SSB/CW Rig: PTO Tip”

  1. Dennis MArkell

    Where did you get that cool-looking pc board holder?

  2. Don (AC7FA)

    The board holder is available at Hendricks QRP Kits web site http://qrpkits.com.

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