Looking for truck audio noise filter/supressor

Discussion in 'General Electronics Chat' started by Manfred Von Steinborn, Feb 25, 2009.

  1. Manfred Von Steinborn

    Thread Starter Active Member

    Feb 3, 2009
    38
    0
    Hi everybody,

    I am looking for an audio/CB/2 way radio alt/ign active noise filter-supressor diagram.
    Long time ago I saw one containing a toroid, 2 transistors, diode, 2 caps and a choke.
    I have a particular trouble with my truck Ken... digital tuner reception of distant AM stations. Reception is is perfect when I am close to the city. When traveling trough the boonies I get alt/ignition noise and (!) low freq. buzz when I hit the brakes.
    I thought about installing a large (1 F) cap @ the batt.

    Any help/ideas would be appreciated.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    That's one of the big problems with AM radio; it picks up and amplifies any noise. :p

    You can minimize ignition noise by making sure that your sparkplugs are clean, properly gapped, and that your secondary wires (and distributor cap/rotor if applicable) are in good shape. Try looking under the hood of your vehicle with the engine running when it's dark out; if you can see blue or purple flashes, you know it's time to replace your secondary wires. Even silicone wires need replacing every 50,000 miles or so.

    You can check them for resistance; about 1k Ohms per foot, no more than 10k Ohms per any one wire. Just make sure you don't take off more than one wire at a time; otherwise you might mix 'em up. :eek:

    Check to make certain that your chassis ground strap (usually tin plated copper flat braid with terminals on each end) is in good shape and properly connected. These frequently go bad in the northern states where salt is used on the roads and in coastal regions; and mechanics have been known to leave them disconnected. Generally, the chassis ground strap connects from a bolt on the engine to the firewall. Your mileage may vary. Bad chassis ground straps will cause all kinds of wierd problems. If you can't find yours, it's OK to add another one. Frequently, you'll find that the firewall connection has a good deal of corrosion. Use a wire wheel to clean it up really well, reinstall it, and put some paint on it to keep it from rusting again for a while.

    If you're getting alternator noise, you might have a bad diode trio or rectifier block inside the alternator (GM alternators). Some later GM alternators have switch-mode regulators that run at around 400Hz. You can "sort of" troubleshoot your alternator a bit by running the engine at night, and look to see if the alternator "idiot light" is glowing at all (even really dimly); if it is, it's time for a rebuild. A single bad rectifier in the bridge will wreak havoc with an AM radio, and a filter won't help. Southern states and other high-temp areas tend to kill alternators pretty quickly.

    Some auto parts places like Auto Zone will check your alternator for free. Some will test it in the car, some want it removed for bench testing. A few phone calls will save you some fuel.
     
  3. Manfred Von Steinborn

    Thread Starter Active Member

    Feb 3, 2009
    38
    0
    Alternator is Denso.
    Don't you think that 1f cap across the bat would help?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Not really. The battery itself is "sort of" like a large capacitor. A 1F capacitor will store quite a bit of energy, but even good ones have some leakage current, which will tend to drain the battery when the vehicle is not in use. Connecting a discharged 1F cap directly across a battery may cause the capacitor's leads to get melted right off. :eek: Initially, a very high current will flow into the capacitor; a current limiter such as a 12v light bulb in series should be used for the initial charging of such large capacitors. However, those large 1F caps weren't designed for the temps under the hood.

    A small capacitor at the output of the alternator may help somewhat, but it would need to be designed for the temperature ranges under the hood.
     
  5. wr8y

    Active Member

    Sep 16, 2008
    232
    1
    But what if the noise is RADIATED and not conducted? How do you KNOW that the noise is on the DC line coming in? (I know it probably is, but....)

    I had a similar problem with my 1998 E150 Ford Van. I hit a tree limb with the antenna going down a narrow driveway and it cleared up for about 3 months.... that got me to thinking.

    Anyway, went over a bumpy railroad track and the noise came back, it occured to me that I was hearing all kinds of noises - even the turn signals would "pop" in the receiver!

    I took the antenna apart and found the fingers under the antenna that made contact with the sheet metal (the "ground" connection) were not really contacting the metal. In effect, the shield of the coax really wasn't working as a shield.

    I replaced the entire antenna/cable assembly with one from AutoZone stores and made the mount TIGHT!

    Problem solved - I hear only LITTLE noise now, and then only when WAY out from the station I am listening to!

    Just my two cents. But remember, a noise problem can be conducted noise on the power going into the radio OR radiated noise getting into the receiver over the air. Putting a 'scope on the DC line at the radio or on the wiring at the fuseblock or battery will tell you what noise you have being conducted. Now, most auto receivers have a pretty good filter in them for conducted noise (with Ford being probably the least effective at it than others I have seen), not to say that adding another filter won't help.

    Hope this helps!
     
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