Well I finally got the courage to tear apart my duplexer. The spring and finger stock appear to be in excellent condition with good contact pressure. However...
If you look at this picture: http://www.n1bug.com/cavity.jpg You can see the end of the tuning plunger. The tuning rod is soldered to this end plate. Silver solder I'd guess. What you can't see in this picture is that at the top of the plunger there is a similar metal plate... the rod runs through a hole in that plate, but is NOT soldered to it, and there is considerable "play". This is where the audible vibration comes from when I lightly tap the tuning knob. My first thought was since this is up inside the center part of the cavity there wouldn't be any RF voltage/current at that spot. BUT... this cavity is extremely sensitive to vibration, producing severe desense with the slightest tap on or near it. I can find NO other sign of anything that might cause it. Could this "loose" metal to metal contact where the tuning rod enters the top of the plunger cause this problem? If so, why would it not have this problem when new, but develop it after many years? Seems there must have always been a bit of play there... no? I used this successfully for several years, then it became progressively more sensitive to vibration and started having intermittent periods of severe noise desense... but a tap or two here and there on the duplexer will either greatly aggravate it or temporarily cure it. If that could be the problem, the challenge is getting at it to fix it. The plastic knobs apparently have a metal insert and are securely SOLDERED onto the rod, so getting them off without burning them up is not trivial (at least not for me). If I unsolder the rod from the plate at the bottom of the plunger I would be able to pull the plunger part out the bottom, leaving the rod in place... but then there's no way to solder or otherwise secure the rod to the top end of the plunger to fix the problem. The only solution I can think of is to cut the rod just below the knob, and screw the whole thing out the bottom... solder the rod to the plunger top end plate, screw it back into the cavity and put a new knob on it. Any thoughts, please???? Paul N1BUG
