getting rid of noise on car radio

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grant fair
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getting rid of noise on car radio

Post by grant fair » Mon Apr 26, 2004 9:43 pm

I just had a new car radio/CD player installed on my '93 Tercel. There is noise on the FM and AM bands. The old radio did not have this problem.<p>It's not alternator whine, more of a clicking noise which changes frequency slightly as the RPMs change.<p>The dealer replaced the antenna, and installed a choke filter in the power line. I added a filter to the antenna. None of these changes worked.<p>The dealer then noticed that when the unit was not screwed in to the car and was moved out away from the engine wall that the noise disappeared. When the radio is attached it is right in front of a wiring harness crossing the car horizontally along the top of the engine wall. He suggested the noise is coupling inductively, and I would have to figure out how to fix it; he was not willing to do more.<p>Ideas?<p>Grant
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cato
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Re: getting rid of noise on car radio

Post by cato » Tue Apr 27, 2004 3:06 am

I think there is a bad (dirty, corroded?) ground between the engine block and the battery. Therefore, spark current is traveling through your radio to get back to the battery and getting coupled into the audio circuits that way. Try adding a wire between the engine block and the negative terminal of the battery.....

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jwax
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Re: getting rid of noise on car radio

Post by jwax » Tue Apr 27, 2004 10:25 am

Contact the radio manufacturer and let them know about the faulty installation. Try a competitive installer for their opinion.
Then there's always grounded aluminum foil!
Good luck! I prefer the older radios over the fancy-dancy new crap anyway!

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haklesup
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Re: getting rid of noise on car radio

Post by haklesup » Tue Apr 27, 2004 3:07 pm

Does this radio interface to an amplifier (OEM or one you put in). If so, your noise is almost certainly a ground loop. This means that the ground voltage (noise) at the amp is different than that at the head unit. You are hearing the difference between grounds at two or more points <p>Trying to clean up the supply noise or create a central point ground will only work a little.<p>What you need is a ground loop isolator available at many places like Rad Shak. It is basically a 1:1 transformer with RCA jacks on both sides. Works like a charm every time.<p>If the speakers are directly connected to the head unit you may have a damaged radio. The ground in a car is notoriously noisy on all cars while the engine is running, the electronics in the radio should tolerate supply noise extremely well. Line filters are generally a waste of money and time.<p>By removing the radio from the car, you opened the ground loop and forced a more central ground at the antenna rather than the chassis.<p>Dirty or corroded ground clips would exaserbate the problem by increasing ground resistance and causing the differential in ground voltages (in a ground loop) to be bigger but it would not cause the noise.<p>I had the same problem and it is very common. Your installer must not be very experienced. Ground loops and the cure are usually learned early since it is so easy to create them in a car.

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Edd
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Re: getting rid of noise on car radio

Post by Edd » Tue Apr 27, 2004 3:19 pm

If the total unit is all in one case how about trying a batt powered AM pocket radio for “sniffing” for the noise source using the directional characteristics of its internal ferrite loopstick. Bring up close to your harnessed wiring bundles nearby or other suspect sources. If engine acoustical noise presents a problem….use headphones.<p>Addenda:
Needless to say with the unit tuned off station on a quiet spot on the band, with a slight advantage to the hi freq end of the dial.
To fully respect its potential sensitivity, place a foot long metal ruler (unlacquer coated) across the radios case with its length positioned at a right angle to the long dimension of the receivers internal loopstick and moisten your fingertips and grasp a penny as an interfacing electrode and drag it across the ruler making intermittent contacts. <p>73's de Edd
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grant fair
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Re: getting rid of noise on car radio

Post by grant fair » Wed Apr 28, 2004 9:52 pm

Thanks for the suggestions.<p>I did remove a ground lead from the battery's negative lead, clean the rust off it with a wire brush, and put Cramolin antioxidant on it. This lead goes to the car frame at the front of the car. I will try a direct ground to the engine block since this is simple and fast to try.<p>The unit includes amplifiers built in to the receiver/CD player, so I don't think the ground loop explanation fits.<p>I did contact Panasonic and they referred me to a service centre which I will contact, but I am not sure they will give me any advice for free. Will check it out.<p>OK Edd I give up. What does the ruler and penny do?<p>Grant
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jwax
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Re: getting rid of noise on car radio

Post by jwax » Thu Apr 29, 2004 4:05 am

It measures how much this will cost you, Grant!
hehe! :)

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jollyrgr
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Re: getting rid of noise on car radio

Post by jollyrgr » Thu Apr 29, 2004 7:26 am

Some things to try....<p>Connect a wire from the frame of the radio to a metal surface on the car (such as the firewall). <p>Check the wiring. There are very good installers and there are wanna-be techs. In old radio setups the frame ground was used as one terminal of the speaker. The radio frame, negative power terminal, and negative speaker terminals were all connected together. Make sure that this is not the case with the Tercel. Modern radios use two wires to connect the speakers directly. If the negative speaker terminals are connected to ground and some idiot wired the negative of the stereo to the car frame, this would explain the problem.<p>
If the installation was free, you got what you paid for. If not, you should get your money back for the installation or have them fix the problem. If they refuse to do either HOPEFULLY you paid by credit card. You can do a STOP PAYMENT on the entire radio. (This usually gets the attention of the merchant.)
No trees were harmed in the creation of this message. But billions of electrons, photons, and electromagnetic waves were terribly inconvenienced!

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haklesup
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Re: getting rid of noise on car radio

Post by haklesup » Thu Apr 29, 2004 3:25 pm

I think Edd was saying that a steel rod (or ruler) will couple to the internal loop antenna if held close to it at right angles. The penny would form a spark gap powered by the ESD on your body and create tiny arcs (too small to percieve normally) that would be heard as popping on the AM band.<p>AM radios really are great at sniffing out electrical noise which by and large is very much like AM (though not modulated onto a periodic wave like radio). Just wave and twist the radio around in the vicinity of where you think the noise source is. The sound (static) will change, get louder as you get closer and the directionality of the loop antenna further helps to locate it (though not to a great precision unless you modify the antenna to optimize this response). You'll get the hang of it quickly once you try it. If the noise really is inductively coupling, you should be able to pick it up this way<p>Back to your description "It's not alternator whine, more of a clicking noise which changes frequency slightly as the RPMs change."<p>This could be something in the ignition or emission controls circuits. A clicking might be some other sensor or curcuit doing its job but with a bad filter cap that injects noise. Try describing it better or posting a WAV file. The sound may be a signature of the problem or at least a good clue.<p>Is this noise present with another radio (head unit)(not just your old one). Like I said before, the supply in the radio should accept quite a large noise level without letting it get to the amplifier. Most modern radios have a DC-DC converter for isolation (and to get the larger voltages necessary for loud volume) first thing. Very hard for noise to get through.<p>Try putting a capacitor close to the radio between the ACC and GND wires (or ground wire and nearby chassis or some combination of all 3) to act as an HF filter. 100uF, 16V would be a good place to start and go smaller (higher cutoff frequency). You want to filter the pop not the periodicity of the pop. One of those giant caps will not filter noise like this.<p>You said the noise was on AM and FM. Is it on DISC also. If not, the noise may be coming from a ground loop between the antenna and chassis. When grounded by antenna only , no noise. When grounded by the antenna and the ground wire (and maybe the chassis) you get the noise. Something is injecting current into the chassis near one of these ground points so that it locally has a slightly higher voltage, when this voltage causes a current to flow through a ground connection, you hear it as a pop. Try a capacitor in these areas or try to improve the ground on the offending module.<p>Do you have a RF modulator in series with the antenna, try removing it or if you had one with the old radio try replacing it.

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Edd
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Re: getting rid of noise on car radio

Post by Edd » Thu Apr 29, 2004 4:37 pm

Oh Boy! …hacklesup has already covered the ruler/1cent enigma in your mind. Relevancy being to the sensitivity of a battery powered portable AM transistor radio to random RFI and impulse type noise.
The idea specifically, however, was having two conductors at different RF levels (But also at electrostatically induced levels, if in the close proximity of household wiring or hi tension powerlines) intermittently make conductive contact between each other. The 1 ft conductor and the ~6ft “conductor” would be induced with different levels of RF in the low microvolt range continually being picked up by them. Upon contact between the two, you would hear a static pop produced coincident with their interconnect. The radios ferri-loopstick windings/core being the pick up medium.
I am hesitant to make any suggestions relevant to grounding/ground loops by virtue of the very uncertain nature of auto construction retention of metallic
infrastructure in their models nowadays. One is just as likely to find the use of plastics/composites/fiberglass in their place.
One thing that you do have is that the Panasonics basic electronics is enclosed within its own metal housing for provision of electrostatic and electromagnetic shielding.With that in mind and if I was confronted with your situation I would
initially have the unit totally disconnected/stripped of antenna/car batts 12VDC supply/ and only one speaker hooked up and the balance control shifted to that unit. Then you power up the radio via a separate 12V wet cell/ni cad pack/ gel cell and see if the unit is then void of those quirks that you were experiencing.
If so, then start individually phasing back in the different disconnects that you had initially enacted to pinpoint the offending source of your trouble..<p>73's de Edd
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