If this is just buzz frim the alternator it could be brushes getting short and arcing. Or the diodes are having some leakage and not smoothing the out the ac to dc.

I had a problem like that on my motor home with a GM alternator. AM band just whined with the speed of the engine.

I have a electronics buddy in my R.V. club. He installs radars and radios on boats. He said that the diodes are just crude large power diodes. Some of the auto people buy the cheapest than can get and they get worse with age.

He told me just install a capacitor to the positive wire output marked
"bat." on the alternator.

The capacitor has a positive and a negative leads. Positive on the positive and the negative to the frame of the alternator. Keep the leads as short as possible to help them not to become antennas themselves.

He told me to find a electrolytic capacitor with a value of at least about 2000mfd. @ 50 working volts or larger, if you have the room, out of an old power supply from a computer, t.v. anything that has one that big. Or buy one for about a dollar. It worked a quiet AM band!

FM is generally immune to the RFI that abuses AM broadcast signals

There's books about this subject. Special shielded distributor and ignition
wire shields, line filters, anti static powder for tires, etc
This is one reason auto manufacturers switched away from copper plug wires and moved to resistor plug wires (Ugh they are so flimsy) and resistor plugs.

You might get details at the ARRL website or get a link at
www.ac6v.com which is full of ham radio links and maybe find the mobile rfi
stuff you want.

Bill

I've been having this for a long time. I don't just have noise I have blindingly loud static, from the ignition system.

Why? not sure. It's all been replaced recently, but I don't really care about AM signal anyway.

When I get around to it I will install a power line conditioner - basically a serious of capacitors. It installs in-line with the power to your radio and *should* eliminate noise.

Doesn't treat the cause, but should help the symptom. I have a link around here somewhere.... the only critical bit is that you need to make sure it's rated to give out the peak power your radio requires and I don't know what that rating is. I should think that it wouldn't exceed the fuse ratings. So not more than 15A*12V... 180W peak power and ... 0.8*180 RMS = 144W. Unless you customarily run your stereo really loud, I would think that you would never need more than one that can accomodate 100W or 10Amps.
I think that you would only need to put a power conditioner on the feed for the radio, but you might need one (one closer to 140W) for the amplifier too.

AFAIK the antenna is internal and can't be cleaned. And I've had the symptoms through 3 radios (tape autoload breaks).

Good Luck,
Will

PS I found some that I was going to install for under 20$ a couple years back.
--
1990 740 Turbo, on its way to stock specs, maybe beyond

It has been a very long time since I had to deal with this kind of issue, so please forgive me as I attempt to answer this. Eventually it will come to me clearly, though it might take some back and forth in this forum, and maybe some memory-jogging from other posters.

Alternator whine comes to mind. What is the condition of the brushes, commutator, or whatever in your alternator? If I recall correctly you can shield from now until doomsday and you won't get rid of alternator whine, you have to fix it at the source.

Spark/ignition noise is usually distinct pops or snaps, so that is not likely it.
--
Scott Cook - 1991 745T, 1985 RX-7 GSL-SE, 1986 Toyota Tercel (Don't laugh, it is reliable, faithful AND gets 41 mpg!)