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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). |
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