I am having a really bizarre problem with my recently purchased used Mark V
Field. When I bought the rig, the noise blanker button was intermittent -
that is, you had to hit it several times to engage or disengage the noise
blanker (no beep and no LED toggle). I just bought the roofing filter mod
and some CW filters from Inrad, and while I was installing the mod, I
decided to fold down the front panel and see if there was anything obviously
wrong with the PC board assembly that houses the NB button. I checked
continuity on the button, and electrically everything seems fine. I then
powered up the radio (with the front panel still loose), and as I am folding
in the front panel to screw it back into place, the NB button works perfect
until just before the panel reaches vertical. If I tilt the front panel
just a few degrees outward, all is well again. I have tried this same
experiment with no power applied and a multimeter checking the continuity of
the NB button while holding down the button, and the switch always seems to
make good closure, no matter what the position of the front panel assembly.
However, when I power it back up, same story as before - just before you get
the panel vertical, the NB button quits working. I unscrewed the two
"hinge" screws at the base of the front panel assembly, kept the front panel
vertical, and moved it slightly backward and forward (closer and farther to
the chassis). Again, the NB button quit working when I got within close
proximity of the chassis.
There is a pc board of some variety with a metal can around it that resides
right behind the NB and filter selection buttons, approximately 3-4 inches
tall and about an inch wide. I am guessing this is the shield for the high
voltage power supply that runs the front panel display. My current theory
is EMI getting into a control line and corrupting the pushbutton data, but
at this point, I am out of airspeed, altitude, and ideas.
Has anyone else seen this behavior? Can anyone offer some helpful tips?
Still scratching my head,
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