jmk wrote:

General statement: The Zeftronics unit is more accurate and more
reliable... and cheaper. In general, replacing the old unit (rather
than repair) makes a lot of sense.

Specific: However, some aircraft will experience electrical noise
problems in the avionics after replacement. The digital Zeftronics
puts out continuous broadband RF (as opposed to the old mechanical
units that put out various pops and clicks at random times). A lot of
older Piper's especially tend to have the wiring for the VR (up under
the dash) bundled with antenna wires and power lines to the avionics.
The result is a "frying bacon" sound on the ADF on the lower bands -
especially when tuned to a weak NDB. [The needle will also probably
track somewhere well OFF the station.]

I have had exactly the opposite experience. It is easier to suppress an
electronic regulator than it was to suppress the mechanical
regulator...

ADF (and Loran, remember those?) operate at low-rf frequencies between
100Khz and 500Khz. The "frying bacon" noise that effects these is
usually generated by arcing as the brushes jump across the segmented
commutator in a generator, or arcing as the brushes rub on the
slip-rings in an alternator. This happens regardless if the regulator
is mechanical, electronic or is not there at all...

Preventing the brush RFI from radiating to the ADF/Loran is
accomplished by putting a RFI suppression filter (usually just a
coaxial HyPass capacitor) on the output terminal of the gen/alt.
http://www.lonestaraviation.com/prod...tor%20Filt er
shows such a capacitor. BTW, contrary to what Lonestar claims, putting
such a filter on a gen/alt does nothing to cure alternator whine in
aircraft audio/intercom systems; only in "bypassing" RFI which would
otherwise radiate from the gen/alt and its wiring to the ADF or Loran
antenna. Also note that if the aircraft doesn't have an ADF or Loran,
then a filter is not even necessary, because the gen/alt brush hash
doesn't effect VHF/UHF/GPS receivers... Putting a filter between the
gen/alt and the aircraft electrical bus is not without consequence,
see:
http://www.ntsb.gov/ntsb/brief2.asp?...FA284& akey=1

Shielding the Field wire running between the regulator and the gen/alt
is usually sufficient to prevent radiation off the field wire. It
matters not if the reg is mechanical or electronic.