1968 Corvette AM/FM Sterero Mutiplexer Relay Noise

[Joseph M.]

Hello,
I fired up my factory AM/FM stereo in my 1968 Corvette and I noticed that a relay in the multiplexer unit starts buzzing when I'm on a channel in my area that lights the Stereo light. I plugged in a non Corvette antenna and I pick up a lot more channels and the buzzing appears to decrease. The tip of the antenna that slides into the radio is a little loose and the reception appears to change somewhat when I wiggle the antenna wire at the radio. In addition, the antenna on the car seems to be the original unit so I figured that it might be a good idea to start out fresh and replace the 40 year old antenna assembly, antenna cable, and ground straps (some are missing) Please let me know if I'm going down the right path.

[Jack H.]

Re: 1968 Corvette AM/FM Sterero Mutiplexer Relay Noise

Yes, you're heading in the right direction! The lead-in antenna cables are notorious for breaking especially when a car's interior carpet has been replaced and the cable saw rough handling.

The lead-in cable has a shielded outer sheath with a human hair, thin, main conductor wire inside. If that wire is fractured, you wind up with intermittent continuity to the antenna mast. The effects are MOST dramatic when trying to tune in AM stations vs. FM stations.

You've done the right diagnostic work (connect a substitute antenna with captive lead-in wire), to determine that AT LEAST one of the problems you're fighting is the existing antenna lead-in cable....

[Joseph M.]

Re: 1968 Corvette AM/FM Sterero Mutiplexer Relay Noise

Jack,

Thanks for the info! A non-Corvette antenna increased the reception dramatically and reduced the multiplexer relay noise so I'm going to go with replacing the antenna cable, antenna mask assembly, and ground straps. Also, will adjusting the set screw on the side of the radio improve reception? I noticed that the screw near the tuning know appeared to increase the AM volume/reception.

Thanks,
Joe

[Jack H.]

Re: 1968 Corvette AM/FM Sterero Mutiplexer Relay Noise

Expect a 'modest' effect...

The situation is this. The antenna is capacitively coupled to the radio chassis. It's done this way (vs. a direct DC path) to thwart receiver overload in the event you drive beneath/along side of AC power lines that are radiating 60 Hz like a banshee. That's why rote continuity tests of some antenna lead-in cables fail (there's an in-line capacitor in the cable that blocks DC current flow).

That said, the specific capacitance of a given antenna will vary from unit to unit dictated by the actual capacitance of the actual lead-in cable that's used. So, when you tweak the adjustment pot on the radio chasis, you're actally turning a variable capacitor to match your car's antenna spot on...

FM is rather forgiving of antenna mis-match while AM reception is pretty critical of antenna considerations. That's why the fine tuning adjustment proceedure has you tune to an AM station that's rather faint at/near the top end of the dial, and gently turn the adjustment pot to 'peak' signal reception.

Under major metropolitan radio reception conditions, received signal strength is typically STRONG, overwhelming minor antenna issues and FM reception is QUITE forgiving compared to AM operation. So, tweaking the radio chassis for optimum antenna coupling efficiency is an issue that really only matters when you're out in the 'boondocks' trying to haul in fringe signal strength stations especially AM....