Radio static question

[JC68vert300]

My friend has an all original 1972 Dodge Dart Custom, very pristine, not a bubble in the vinyl top,no body rust. He owns it for 46 years. it has the 225 slant six, no electronic ignition. His radio plays fine on accessory, but once the engine is running the radio is full of static. what should I tell him to inspect? Being that it is clear with the key on accessory, I didn’t think it would be a bad ground or antenna. I was thinking to start with checking the age of the spark plugs and wires ( he said he doesn’t remember when they were changed), so maybe a better insulated wire. Then check the alternator? Thanks for any input.

 

[Mopars & Missiles]

There were two capacitors that were used to reduce AM radio static from the ignition system when the car was running on many of these cars. One was connected to the coil itself, the other one was on the back of the instrument panel connected once again to the +12 volt supply to the IP. I've never had a '72, only '70's and back, but his '72 may very well be the same as older cars.

You didn't say, but I'm guessing you are talking about AM radio. If its FM, then the problem most likely is NOT these capacitors.

All this is covered in the FSM, so consult with the appropriate '72 manual to check the placement of the capacitors for his car.

 

[Big_John]

It could be a lot of things... First, I assume this is an AM radio.

Does the static go up and down with engine RPM? That would mean ignition related. Bad plug wires or solid core plug wires can be the culprit. There should be a condenser/capacitor on the coil for static and that is often missing. Another on the instrument panel, but I believe that's more for alternator caused static.

You can also do a "trim" adjustment on the radio. This might not help, but it's just a few minutes work.

Extend antenna to 40 inches.

Tune radio to a weak signal between1400 and 1600.

On one of the lower rear corners of the radio, I think the right side, there should be a hole that you insert a small flat blade screw driver and tune until thee best reception is found.

 

[CBODY67]

The one time I tried to trim the radio on the '66 Newport (which is done from the front side of things), I got no better reception from the orig factory setting. So, I concur with Big_John in that it probably will not help.

Is the engine-running static from the ignition system (changes with engine rpm) AND with the hood open or closed. Usually, ignition noise is worse with the hood open, as then the static goes into the radio antenna mechanism and then to the radio. Otherwise, with the hood closed, very minor amounts of ignition noise happens as it all has to come in via the antenna.

How old are the spark plug wires? OEM was the carbon-core resistance wires used with non-resistor spark plugs. Some people liked to use resistor spark plugs with solid wire conductor plug wires.

As a diagnostic for the lead-in cable, get some aluminum foil and wrap the parts of the lead-in which are under the hood as an extra layer of shielding. IF that helps/fixes the issue, then a new radio antanna lead-in cable might be needed.

Just some thoughts,
CBODY67

 

[Mopars & Missiles]

For the record, ignition system static or noise (for the most part) does NOT enter the radio through the antenna.

The ignition static is transmitted through the cars +12 volt power system. It is everywhere there is +12 volts within the car. AM radio is AMPLITUDE modulation, thus the voltage spikes created from the ignition and superimposed on the +12 volt power cause the radio to "crackle" with every spike from the ignition.

FM radio (FREQUENCY modulation) was developed (in part) to eliminate this static noise problem that is inherent in AM radio.

 

[CBODY67]

For the record, ignition system static or noise (for the most part) does NOT enter the radio through the antenna.

The ignition static is transmitted through the cars +12 volt power system. It is everywhere there is +12 volts within the car. AM radio is AMPLITUDE modulation, thus the voltage spikes created from the ignition and superimposed on the +12 volt power cause the radio to "crackle" with every spike from the ignition.

FM radio (FREQUENCY modulation) was developed (in part) to eliminate this static noise problem that is inherent in AM radio.

Then why is it worse with the hood open than closed? I can see your point with pops and such when a brake light switch is activated, or some noise when the wiper motor operates, for example. We know that radio antenna lead-in cable has a wire mesh shield as one of its layers, as any coax cable will. I recall that such 75 ohm coax cable was touted in the residential market over the two-wire 300 ohm flat lead-in as being resistant to outer electrical interference. Would not a similar coax cable do the same in an automobile environment? Obviously it must help as that has been typical for automotive radio antenna lead-in well before FM radio was used in automobiles, although FM bands were used in law enforcement vehicles well before FM consumer radio was availeble.

After I installed low-resistance plug wires on our '66 Newport, with normal Champion non-resistor spark plugs, I noticed more ignition noise with the hood up than before. But it was greatly diminished when I closed the hood (which afforded greater shielding from ignition noise getting to the antenna itself). I also noticed a similar situation on our '69 Chevy pickup with resistor spark plugs in it, too. Which is why I made the suggestion to put aluminum foil around the antenna lead-in cable. An easy thing to try. If it doesn't work, then try something else.

Years ago, in trying to decrease the ignition static on weak stations and in damp weather, I sanded/cleaned the capacitor ground surfaces, with no difference made in the radio static issue. NOT to say that the capacitors might fail internally and cause issues.

Just working from my experiences. No more, no less. Your experiences might be different.
CBODY67

 

[Gerald Morris]

Then why is it worse with the hood open than closed? I can see your point with pops and such when a brake light switch is activated, or some noise when the wiper motor operates, for example. We know that radio antenna lead-in cable has a wire mesh shield as one of its layers, as any coax cable will. I recall that such 75 ohm coax cable was touted in the residential market over the two-wire 300 ohm flat lead-in as being resistant to outer electrical interference. Would not a similar coax cable do the same in an automobile environment? Obviously it must help as that has been typical for automotive radio antenna lead-in well before FM radio was used in automobiles, although FM bands were used in law enforcement vehicles well before FM consumer radio was availeble.

After I installed low-resistance plug wires on our '66 Newport, with normal Champion non-resistor spark plugs, I noticed more ignition noise with the hood up than before. But it was greatly diminished when I closed the hood (which afforded greater shielding from ignition noise getting to the antenna itself). I also noticed a similar situation on our '69 Chevy pickup with resistor spark plugs in it, too. Which is why I made the suggestion to put aluminum foil around the antenna lead-in cable. An easy thing to try. If it doesn't work, then try something else.

Years ago, in trying to decrease the ignition static on weak stations and in damp weather, I sanded/cleaned the capacitor ground surfaces, with no difference made in the radio static issue. NOT to say that the capacitors might fail internally and cause issues.

Just working from my experiences. No more, no less. Your experiences might be different.
CBODY67

AM radios pick up ALL KINDS OF **** EM from every source around. Short of getting a spectrum analyzer to try reading all possible sources, If you want to shield an antenna lead, it will help to ground it to the body, usually. I concur its an easy experiment to attempt.

I LOVE old school AM radios BTW, including crude cat-whisker diodes w big coils, and mega-ohm earphones. Trimming capacitors sometimes helps, though it likely will help more to replace the old dielectric in the trim caps, meaning, replace the caps. Sundry after market choke coils to put in the antenna lead used to abound on store shelves, as I think you can recall. Tell the OP to go hunt for some on ePay, under the Vintage **** headings. Some of these did a bit of good.... SOME of them. I tried a couple in the late 70s, admittedly, on Fords. Who knows?