Compass calibration is somewhat of a black magic art, but my suspicion is
that some steel parts of your aircraft have become magnetized and are a very
good source of compass errors.

You said that you replaced a radio. Remove the radio from the rack. Take
an unmagnetized steel screwdriver and see if any of the steel screws holding
the rack to the airframe are little magnets. Do the same with any steel
that you can find in the general vicinity of the compass. Try the engine
mounts as well.

Go to a TV repair shop that has been around for a while. In general, the
dingier, dirtier the store the better chance you have for finding what you
need. Ask the nice TV fixit guy if he has an old degaussing coil and ask
him if he would rent it to you.

Remove all instruments with moving coil/magnet meters (VOR/ILS indicators,
analog ammeters/voltmeters, etc.) from the panel.

Turn the airplane to the compass heading with the WORST error. Now remove
the compass. Run that degaussing coil over the whole instrument panel
slowly. When you think you are done, do NOT switch the degaussing coil off
until you are at least a foot or two away from the aircraft. If you screw
this up and switch the coil off close to the panel you will have more
problems than when you started.

Now put the compass back in its mount. Did it clear up the error? If not,
remove the compass again and run that degaussing coil S L O W L Y over the
engine mount, switching it off again when you get it a foot or two away from
the mount. Did that clear up the error?

If not, Sacramento Sky Ranch has a degausser on steroids that they rent out
for problems just like this.

Mumetal shields are a patch for the real problem ... which is that you are
carrying a magnet around with you that is sucking the compass towards it on
all headings. Solve the problem; don't patch it.

Jim

--
"If you think you can, or think you can't, you're right."
--Henry Ford

"Road Dog" wrote in message
m...

I took my Piper Warrior in for its annual and the mechanic
said that the compass card was missing and that he'd have
to swing the plane to generate a new one.

After swinging it, he claimed that he couldn't get the
E-W deviation within the minimum. So he suggested we
try "mu" shield (or something) to block the source of
magnetism which he claims is coming from one of the
instruments. (He says the compass works fine outside
the plane.)