MODS to repair an MFJ 931 Artificial Ground

I recently bought this unit from a shortwave listener. Two faults became apparent.

The inductance selector switch wafer was cracked or broken in two places and did not operate properly. The switch might have been broken when the switch and coil were forced into the chassis as a unit. The mass of stiff-arm wires between the coil and switch can exert considerable force on the switch wafer.

The switch was not secured to the front panel, allowing it to turn when the knob is turned. This too may have put excessive force on the switch wafer. The damage can be seen in the photos above.

I removed the coil and switch. They have to be removed together because some of the solder connections are inaccessible if the switch is installed. Then I unsoldered the switch. I repaired the switch wafer with crazy glue. I was careful not to put excessive strain on the switch when I installed the switch and coil. To keep from straining the switch wafer, I only soldered two coil leads to the switch before a trial install. I adjusted the top lead to get the coil where I wanted it. Then I removed the coil and switch and soldered the rest of the leads for the final install.

I added a star washer between the switch and chassis to keep the switch from moving when the knob is turned. I did not use the fiber insulating washer that was originally installed; it was probably shorted out by the front washer anyway.

The supplied coil and tuning cap would not tune below 2.11 MHz. This prevented the 931 from tuning up on the 160 meter band. MFJ needs to use at least 50 per cent more coil to cover the advertised frequency range. To make good on the shortchanged inductance, my junkbox contributed the coil seen at the bottom of the picture. Before reassembling the switch and original coil, I removed the first (highest frequency) tap from the MFJ coil and moved the taps over one. The end of the MFJ coil that originally went to ground now went to the added coil and the last tap on the switch. The modified circuit now covers from below 160 to above 10 meters. The new coil is not well supported because I did not wish to drill mounting holes for it while the meter was mounted on the chassis.

The glue job on the switch failed so I ordered a replacement. The replacement switch is only 40 cents, so I ordered four. I was disappointed to find the replacement switch much smaller than the original. The replacement switch used a different nut and washer, which were not provided.