View Single Post
  #3   Report Post  
Posted to rec.audio.car
[email protected] jnbearden@aol.com is offline
external usenet poster
 
Posts: 2
Default Problem Solved (Was: Audiovox Wired FM Modulator Whines)

The high-freq. noise was there whether anything was plugged into the
RCA connectors or not. It was slightly louder with a DVD player
connected and with the engine running, but it was evident even when
just plugged into the radio antenna, even running on its own 12v power
supply (i.e., not the car battery).

The modulator comes with an isolation transformer which Audiovox tech
support advised me should be installed. It was, but had zero effect.
The spurious noise was not coming in through the audio cables or RCA
connectors.

For anyone else who encounters this problem, the source was apparently
an RF choke molded into the power lead coming from the fuse and 12v+
wire and just before the on-off switch. Feeding 12v directly into the
switch connector (i.e, bypassing the fuse and choke) eliminated the
noise. Though Audiovox wasn't able to solve my problem, I have to give
them credit for responding quickly to my email and then following up
with an "OK" when I proposed removing the choke.

Can't guarantee the same fix will work with anyone else's system (FWIW,
mine is installed in a 1999 Chevy Express van). Or explain why it
works: the choke was still there in my testing, dangling from the
on-off switch with the fuse and 12v connector wire trailing off it.
It--or the system comprising the choke and a few inches of wire on
either side--apparently generates a signal or acts as an active antenna
when there's 12v across it, but is harmless when connected to the power
lead so long as there's no voltage drop across it.

BTW, the wired modulator is a nice idea: a reasonably easy way to add
an aux. input to a car radio for a DVD, VCR, and/or satellite radio
without the problems of the wireless FM modulators. Sound quality is
good and it completely disconnects the car antenna from the radio when
it's switched on, with no discernable effect on radio reception when
it's switched off. We have a moderately strong station on 89.1 Mhz (one
of the two freqs. it uses to squirt the audio signal into the radio)
but the station is totally inaudible, with no interference, when the
modulator is switched on (unlike the wireless FM modulators that can't
share a freq. with an active station).