Odd temporary car audio problem

[kranky]

Any thoughts on what the problem is? Not that it's serious, just seems very strange to me.

Radio fine when car first starts. After a couple of minutes if I'm listening to one particular station, it will fade into static and hum as if it was slightly off-frequency. Over a minute or two, the hum goes from low to high slowly as if you were tuning a shortwave radio. Then it fades back into a solid clear signal and does not reoccur until the next time the car is started.

- Only ONE AM station, around 1000 khz, is affected. I can tune to the next station up and down from there and it's fine. Other stations are fine, FM stations are fine. FWIW the affected station has a very strong signal - 50,000 watts.
- The colder it is outside, the longer it takes until the static/hum starts and the longer it lasts. When cold, it might take 4-5 minutes until the hum/static starts and and 2-3 minutes to stop.

What could cause this - on just one station?

 

[DominionSeraph]

Best guess would be something to do with the feedback loop for the local oscillator.

 

[kranky]

Do you have any idea if it's the kind of thing that will get worse over time? I can live with it as it is now, but I might get it serviced if the problem will only get worse down the road.

 

[Doublejr]

Do you have any idea if it's the kind of thing that will get worse over time? I can live with it as it is now, but I might get it serviced if the problem will only get worse down the road.

It would depend on what is causing the issue, it could be many things.

Next time it happens see if the hum changes if you change the rpms of the engine. You could also grab a portable radio and tune it to that station and see if it cuts out also, this might tell you if the noise if coming into the radio via wire vs electromagnetic that would affect anything on that frequency.

The only thing I had similar to this happen was when IIRC a voltage regulator was going on my alternator and it made all kinds of weird noises in the radio.

 

[DominionSeraph]

Do you have any idea if it's the kind of thing that will get worse over time? I can live with it as it is now, but I might get it serviced if the problem will only get worse down the road.

Impossible to say. It could go any way. It could be the initial condition is limited to that frequency/received power and that it get worse and lose its current ability to lock back on as it warms, so you would lose just that one station. It could become sensitive at other frequencies and start to drift there as well. Or it could fix itself.

 

[nerp]

Someone in the area is screwing around with some waves or something. That's often the source of strange AM problems.

 

[The Boston Dangler]

years ago, the prescription would have been a capacitor across the alternator, but that hasn't been a common issue in a long time.

if you have a portable radio, use it to verify it's an issue with the car before spending any time/money.

 

[SSSnail]

Just turn the volume down when you shutoff the car - problem solved.

 

[kranky]

Doesn't matter where I am so it's not specific to a location. There is a barely noticeable change in the sound if I flip the turn signal on that pulses with the turn signal, but it doesn't change with engine revs.

It doesn't seem to get any worse with time so I think I'll live with it.