Baffling radio interference

Wes Tausend wrote:
...

Ahhh.

Each radio has a circuit called AGC for Automatic Gain Control. AGC allows close and/or distant stations to wend their way through the RF and audio amplification stages without creating an overload (too loud, distorted etc.).

It is done by a sort of variable "throttle" feedback process (think bootstrap or cruise control) and some circuit designs become unstable at the max gain levels needed for weak stations; i.e. overly distant stations. The highly leveraged circuit then starts to "motorboat", a slow audible "putt-putt" oscillation. That may be it. Just slightly higher truck voltage during increased rpm can vary this discharge frequency in a reverse way.

1.Hasta luego (see you later)
2.Adiós (good bye)
3.Hasta mañana (see you tomorrow)
4.Hasta pronto (see you soon)
5.Hasta la próxima (until next time), :B

Wes
...

Thanks, Wes! I was willing to live with the noise, once I figured out my radio wasn't "on the fritz." But I can't stand a baffling problem with no logical explanation. Yours makes perfect sense! Thanks.
Tom

...

Ahhh.

Each radio has a circuit called AGC for Automatic Gain Control. AGC allows close and/or distant stations to wend their way through the RF and audio amplification stages without creating an overload (too loud, distorted etc.).

It is done by a sort of variable "throttle" feedback process (think bootstrap or cruise control) and some circuit designs become unstable at the max gain levels needed for weak stations; i.e. overly distant stations. The highly leveraged circuit then starts to "motorboat", a slow audible "putt-putt" oscillation. That may be it. Just slightly higher truck voltage during increased rpm can vary this discharge frequency in a reverse way.

1.Hasta luego (see you later)
2.Adiós (good bye)
3.Hasta mañana (see you tomorrow)
4.Hasta pronto (see you soon)
5.Hasta la próxima (until next time), :B

Wes
...

Thanks for all the thoughtful replies. I really appreciate how everyone is trying to solve this problem for me. Yesterday, I had a thought. We have an RV garage that is long enough to keep our camper and the pickup, provided I park the pickup with the front end under the cabover of the camper. In order to make that work, I changed out the antenna from the OEM to a shorter mast, shortly after we bought the truck. It occurred to me that the antenna might be the problem. With engine running, I swapped the antenna masts and found that the noise on my favorite stations was the same with either mast. There was also no discernible difference in the quality of the reception. Just for fun, I hit the seek button on the radio. It quickly found a "local" station (remember, I said we live in a rural area), and the reception was perfect, with no noise! So, perhaps, I just need to find stations that are closer to home. Only problem is, I don't WANT to learn Spanish at this age!! :o

punomatic wrote:
Thanks again for all the replies. I am still curious, though, why the frequency of the popping noise decreases as engine RPM increases. Any thoughts on that specific question?

Is the little "static" ball in place on top of the antenna? How about wiggling the antenna plug at the radio to make sure the lead-in wire is grounded. I think I've actually run across this reverse peculiarity before, but I don't remember just what it turned out to be. It might be increased speed through the air slows the discharge rate.

Or maybe this. A long time ago I was doing some plaster work in a house, so I had half a clean bucket of water that I planned to use for washing up. It was winter, so I got the clear, pristine water from an outdoor spigot a day early so it would warm up. The best laid plans of mice and men...

The next morning I was shocked and appalled to discover a dead mouse in my clean wash water. I suspected a co-worker who was a practical-joker, so I carefully speared the tiny corpse on his chrome radio antenna. I, of course, used a convenient orifice on the soggy mouse.

When the co-worker discovered the frozen mouse, he immediately felt it was inappropriate and recklessly yanked the frozen little critter off his antenna. Unfortunately the little ball on top of the antenna came off with the stiff mouse and he foolishly tossed all of it, thereby losing the ball altogether.

His radio was particularily static-y after that, especially during thunderstorms. Since I felt he had to know that I was even, I freely admitted it, but he swears to this very day he didn't put the li'l fellow in my bucket. Thereafter he blamed me for years of lousy radio reception. I told him he should have saved the ball.

In retrospect, that is possible, that the poor mouse was only incredibly thirsty and fell in and drowned.

Wes
...

Perrysburg Dodgeboy wrote:
...as the RPM increases and decreases the frequency changes. Don

In inverse proportions?

Alt or plug wires, as the RPM increases and decreases the frequency changes. Adding extra grounding straps will help eliminate it. Also finding out if the plug wires or alt are going south would be in order. If your truck as 100K or more its time to change them and the plugs. BtW bad plugs will do this also.

Don

Thanks again for all the replies. I am still curious, though, why the frequency of the popping noise decreases as engine RPM increases. Any thoughts on that specific question?

The electric fuel pump is the major cause of noise in AM radios today. A capacitor can be placed across the power line to ground as close to the pump, usually on top of the gas tank.

Our '06 GMC 2500HD at work has a strap that attaches under the hood near the firewall and goes to the truck body at the firewall under the hood.

I've never paid any attention to this strap until it broke. The radio and all electronics worked fine with this strap broken but after an online search to learn of it's purpose I replaced it with a new one from NAPA.

Just another item to check.

Thanks for all the replies. I get the impression that the two most likely causes of this kind of interference are a)improper grounding of equipment and b)bad ignition wires. I will begin checking things out tomorrow.

45Ricochet wrote:

Old-Biscuit wrote:
Switch to an FM station :B

In-line filter..Radio Shack

LOL
or pop in a 8 track tape :B

BTW, I got a couple of chuckles out of these comments. I have never owned an 8-track player, but I do tend to listen primarily to FM stations in my pickup. Just looking to expand my horizons!:B