C3-Poor AM Radio Reception on 74' AM/FM Stereo

[Robert S.]

When I purchased my 74' the radio did not work. I have since pulled the radio and convector and had them repaired. I subsequently determined that both speakers were bad as was the antenna lead and I replaced these parts with new parts. I was quite careful in not bending the antenna lead during installation. The dealer from whom I purchased the car had replaced the antenna mast. The mast looks correct and appears to fit correctly.
At this point FM and FM stereo reception is very good. However AM is very weak with only the two strongest stations in our area able to be received and both stations fade and come back quickly when driving the car. I have tried the antenna trimmer on the radio with no effect. If I touch the antenna with my hand the AM reception immediately improves and the farther up the mast I touch the antenna the better the AM reception.
When I replaced the antenna lead I did nothing with the ground plane. The ground strap was bolted to the frame and the plate itself showed the effects of 36 years exposure to the elements but nothing that caused me to remove and replace it. I believe I have the new antenna lead inserted into the ground plane the same as the one removed.
Anyone have any ideas of what the problem might be? Should I pull the ground plane and clean it or replace it?

[Jack H.]

Re: C3-Poor AM Radio Reception on 74' AM/FM Stereo

Integrity of the ground plane can/does affect reception, but if you're in a major metro area with SOLID AM signal strength, my hunch is there's something else afoot...

Your comment that signal reception increases DRAMATICALLY when you add your body capacitance to the external antenna, tends to point to poor antenna-radio coupling.

You can play with the ground plane, but that may NOT be the major problem source. When/if you do, make sure the ground strap to ground plane connection is clean & oxidation free. Plus, double check the order of component installation at the other end of the ground plane's ground strap... Folks OFTEN get it backwards!

The stack-up is: (1) chassis, (2) star washer, (3) flat washer, (4) ground strap connector, and (5) fastener bolt head. The star washer is there to 'bite' through chassis paint and render a solid electrical connection to the frame.

You can also consider visually inspecting and making sure the antenna mast to body mount is clean & solid as well as the lead-in wire to antenna mount connection.

If all that checks out, consider the 'acid' test... Carefully unplug the radio lead-in cable from the rear of the radio chassis. Temporarily connect an 'el cheapo' replacement radio antenna with captive lead-in cable (Wal-Mart, made in China, approximately $8-10) and move it outside the car through the passenger side window.

Does your AM reception improve DRAMATICALLY? If so, that's pointing to the integrity of the lead-in cable back to/through the antenna at the rear of the car. Note, when you conduct the 'acid' test, there's no ground plane to assist the temporary antenna... You should expect that level of reception (or better) from your car's built-in antenna components.

[Edward J.]

Re: C3-Poor AM Radio Reception on 74' AM/FM Stereo

Bob, There is a Am antenna trim screw behind the turning and balance knob, its been awhile since I have played with one, but I believe what you do is tune in a am station that has a weak signal, and then turn the trim screw slightly left or right to try and strenghten the signal. ( The trim screw uses a small flat blade screw driver). Good luck Ed.

[Edward J.]

Re: C3-Poor AM Radio Reception on 74' AM/FM Stereo

Jack, does that sound right about triming screw?

[Jack H.]

Re: C3-Poor AM Radio Reception on 74' AM/FM Stereo

Yeah, the instructions are in the AIM as I recall. The only thing I remember from the description that's omitted is you're looking for a weak AM signal at the TOP of the dial for best results in 'tweaking' the capacitive trimmer in the radio chassis.

[Robert M.]

Re: C3-Poor AM Radio Reception on 74' AM/FM Stereo

From a 1970 owner's manual, probably applicable to a 1974.

"Peaking the Antenna
Remove the right radio knob and spacer.Tune the to a signal at or near 1500 KC on the AM scale. Set the volume to maximum volume. Insert a small screwdriver in the antenna trimmer screw and rotate left or right until maximum volume is received. Your radio antenna will be trimmed to maximum reception at any dial setting after adjusting."

[Robert S.]

Re: C3-Poor AM Radio Reception on 74' AM/FM Stereo

I have tried peaking the antenna per the AIM and owners manual with no effect.

[Jack H.]

Re: C3-Poor AM Radio Reception on 74' AM/FM Stereo

That's what I expected... The trimmer is essentially a 'fine tuning' device intended to 'tweak' that radio chassis dead-on to match the specific capacitance characteristics of it's mating lead-in wire/antenna components.

In my experience, the only time you can see this matching adjustment making much of a difference is under SEVERE fringe reception conditions (weak received signal strength). There WAS a design center (target) for the antenna components and the lion's share are pretty close to matching the design center of the radio receiver's 1st stage signal amplifier.

I'd suggest going back to the 'acid' test. Just because you bought and installed new antenna components (lead-in wire), doesn't mean they were spot on or defect free on delivery. OR, despite the care you took during installation, you didn't manage to somehow damage the antenna lead-in wire...

It's also possible for the mating bannana jack socket in the radio chassis to be loose and/or oxidized despite your having sent the radio out for bench work. Remember, we're dealing with EXTREMELY weak AM signals at the antenna to 1st stage amplifier interface. It doesn't take much to put things out of wack...

But, when everything IS right, the system should be robust and last for a LONG time!

[Robert S.]

Re: C3-Poor AM Radio Reception on 74' AM/FM Stereo

Thanks - I will buy a cheap antenna to plug into the radio directly and then go from there.

[Robert S.]

Re: C3-Poor AM Radio Reception on 74' AM/FM Stereo

Delay due to vacation but on my return I purchased an inexpensive antenna and cable and AM reception was much improved with the antenna laying on the sunvisor. Since the cheap antenna had no ground plane I am presuming that my issue is not with my ground plane but is with either the antenna mast (new part) or the antenna cable (new part) or the connection between the devices. I will borrow another corvette antenna to check that possibility but I am thinking it is probably the antenna cable - hate to have to change that out again. Thanks to all for their advice.

[Jack H.]

Re: C3-Poor AM Radio Reception on 74' AM/FM Stereo

That's why I always suggest the 'acid' test when debugging complaints of poor radio reception... Hint: when replacing a radio lead-in cable, do NOT presume that just because it's 'new' that it's 100% A-OK.

Connect it to the antenna, then jury rig it STRAIGHT to the radio for a quickie functional test BEFORE you go through the process of routing the cable through the rear fiberglass, under the carpet and across the dash to the radio.

That little gross functionality test can detect a 'shelf worn' lead-in cable BEFORE you execute the install labor + uncover a faulting antenna mast assy to boot!

[Robert S.]

Re: C3-Poor AM Radio Reception on 74' AM/FM Stereo

Yes I now realize I should have done that. I borrowed an original corvette antenna mast off of a 75' (large ball type) and it was about the same as my antenna so I guess it is the antenna lead that is defective.

[Robert S.]

Re: C3-Poor AM Radio Reception on 74' AM/FM Stereo

Since another person on this forum was having the identical issues after replacing their antenna cable, I decided to look closer at the ground plane (only radio antenna system part that I did not repair/replace) before concluding the cable was defective.
I removed the ground plane today. It was in reasonably good shape for 36 years with some minor corrosion on the aluminum plate, and moderate rust on the small riveted plate. I cleaned everything up and made sure I had good clean contact where the copper strap bolts to the frame and reinstalled. The AM reception significantly improved. I am not ready to declare radio issue resolved but I am close to doing so.
Lesson learned: Ground plane is there for a reason and is important to AM radio reception so do not ignore when chasing radio issues.