Got out to the hangar with my watt meter to see if I had a high VSWR that
might be putting RF where it wasn't wanted. When I turned the slug to the
reflected position, the meter reading was so low I could barely read it.
That eliminated that possibility.

I next found out that I could not duplicate the problem. The only difference
this time was that the top cover over the instrument panel had been removed
for access. I called EI in Bend Oregon for some tech assistance.

Their take on it, was that the garbage was getting in via the ground lead!!
They said to shorten it and the trouble would be gone. Since they make the
instrument and have seen this before, I followed their instructions, pulled
the ground lead off the buss and made a separate ground as close to the
instrument as possible. I did fudge a bit and wound the ground lead through
a ferrite bead right where it exits the instrument case. They kind of
ignored my comments that the problem only occurred with the radio was keyed
AND the strobe was on.

If this does it great. If not I think my next attack might be to get a
ferrite bead on the power to the strobe up on top of the vertical stabilizer
and filter the input to the fuel flow instrument. I have a skylight in my
PA22 and looking up through it while fiddling with the wiring I see the
antenna is perhaps 24 inches from the instrument. I guess not too many
fabric airplanes have fuel flow instruments installed. :-(


Thanks Jim for the tip on the hash being additive and hitting a threshold in
the instrument.

Cheers:

Paul
N1431A