Mike Spera wrote:
It appeared to work well for over 2 months, however, it started
fritzing out again the other day.

What else changed?

Some suggested that the Narco nav/coms were not properly shielded.

This might be true for the case where you just tune something in (the
ILS or a particular comm frequency) but don't transmit. As others have
mentioned, there is a low-power local oscillator running in the receiver
all the time.

While others speculated that the interference is coming straight from
the antennas for the nav/com.

This is more likely to be true in the case where the GPS doesn't drop
out until you transmit on a particular frequency for a while.

How do I shield the box in the stack or the GPS antenna to see if it
makes a difference?

Just moving the antennae further away from one another is one way. It's
probably easier to move the GPS antenna further away, since it's a new
addition and its cable isn't buried in a harness somewhere. As far as I
know, most GPS antennae have built-in amplifiers, so moving it another
few feet from the GPS receiver isn't that big of a deal. If you're
buying a new GPS antenna, look for one with a thicker coaxial cable -
this is _usually_ an indication of a better-shielded cable.

For shielding the box in the stack, sometimes it helps if you can narrow
down where the interfering signal is coming from. It could be radiating
right out of the box itself, or it could be sneaking out along the
antenna, power, or mic/headphone leads and radiating from them. If
tuning in the ILS will kill the GPS on the ramp, troubleshooting will be
a lot easier. Assuming that you can reproduce it on the ramp, you might
try it with all the headsets and PTT switches unplugged. If the GPS
comes back, start looking at the audio wiring. If all the audio wiring
is tip-top, you _might_ be able to install a filter on the audio inputs
and outputs on the radio.

If the GPS stays dead with the audio stuff unplugged, if you can reach
the connector, and if this is something a mortal in your situation is
allowed to do, you might reach up behind the nav/com radio and unscrew
the antenna connector. (Don't try transmitting with the antenna
disconnected, but receiving is OK.) If that brings the GPS back, start
looking at the nav/com antenna cable and especially the connector. Some
of these connectors are very massive and the shield conductor should be
soldered with a 1.21 gigawatt soldering iron, but it instead gets
soldered with a generic 30 watt pencil, leading to bad connections. Or,
the shield on the coaxial cable can simply get damaged over the years.

If that doesn't help, it might be coming from the power leads, but that
is somewhat harder to rule out. You can make sure the power connections
are clean and tight, but beyond that, it's "try it and see" - like
rigging a small battery right next to the radio for testing, or adding
a filter across the power and ground lines.

For the "box itself" case, (again, if mortals are allowed to do this),
you can take the box out of the panel and look it over. Make sure all
the cover screws are present and tight and make sure all of the covers
are making good contact with one another. The physicists will tell you
that it is possible to add fingerstock and jumpers to make sure the case
parts are really solidly connected to one another, but the lawyers may
object. There is probably a ground connection in the multi-pin
connector, the antenna shield is probably also grounded, and the radio
may get a bit of a ground through its mounting. You can at least make
sure all the fasteners that hold the radio into the panel are present
and tight, and it may be possible and helpful to add a braided ground
strap from the radio case to the airframe.

Disclaimer: This is based on experience with two-way radios, GPS
receivers, and audio equipment in ground vehicles and in fixed
installations. I don't have an A&P; I don't even have a TG&Y. Some of
this may not be allowable owner maintenance. Your mileage may vary.

Matt Roberds