I have owned 2 of the FPM-300 MK II's now and always wished the dial skirt was adjustable as in the Drake B line to allow for 1KHz hash marks. Like the Drake, the FPM-300 has a 25KHz/revolution tuning rate and the dial skirt has 25 hash marks.
I had a spare knob and decided to see if I could emulate the drake scheme. I used a narrow cutoff tool on my lathe to separate the dial skirt from the knob. This leaves an opening of just under 1.1" in the center of the skirt. I made a disk of the appropriate O.D.with a center hole of 0.250" for a press fit into the skirt and epoxied it in place. My skirt white hash marks were badly worn- so I first polished the skirt with Novus #2 plastic polish and then restriped the hash marks. I did a bright red stripe for each 5KHz mark.
Before mounting the skirt I removed all but 1 felt washer from the tuning shaft and placed the skirt over the 1/4" shaft followed by a neoprene rubber washer. This allows the main tuning knob to grab the skirt. I suspect an O ring, or even a proper grommet would work as well. When remounting the knob- apply a little inward pressure to the knob while tightening the shaft screws so it engages the rubber washer.
Just as in the Drake line, turn on the calibrator, turn the main tuning dial until the 100KHz (or 50KHz) main dial mark is perfectly lined up with the window red line. Then simply slip the skirt until a red hash mark is straight up. That's all there is to it. I find good correlation up to at least 50KHz on my current MK II.
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This page last updated 30 Nov 2024
