Ok guys I noticed on my 71 that I am getting some kind of electrical interference through my stereo. I hear is mostly from my rear speakers and its like a whirling sound that will increase with engine RPM's. It goes away when I turn the stereo off. I didnt install my aftermarket stereo and speakers as they came this way with the car. The car has 2 kicker panel speakers, rear deck 6x9's and a amp powering them. The engine is running an HEI ignition. So what's my best place to start to get rid of this? Are the noise filters I should be running somewhere?

Thanks
Magna

 

You want to make sure you have eliminated all of the possible causes before you start adding noise suppressors.

Since you have an amplifier you want to make sure to have a good solid ground at the head unit, the amp, the antenna, and the battery. A different voltage potential between any of these electrical points is called a ground loop and can generate noise. Also make sure the engine to body ground strap is tight and not corroded. The automobile chassis is not a great electronics grade conductor so you want to clean all ground points, no paint or rust under them. Next, make sure the power wires and the RCA cords are on opposite sides of the vehicle. Leaky diodes in the alternator can let enough A/C through to induce noise from the power wires into the signal leads but not cause problems with the rest of the electrical system. The type of distributor shoud not make a difference unless you are running solid plug wires. Depending on the amplifier, you can develop a ground loop if the amp is mounted metal to metal. Use a wooden isolater to mount it. This can be customized to match the interior or trunk. Last but possibly most important, make sure the speaker leads do not touch chassis ground. This can take a keen eye and lots of time with an ohm meter to find if you do have a speaker short. If you have kick panel mounted speakers make sure the speaker wires don't have screws through them, check under the threshold strips. You would be shocked at how many times that happens. Make sure the speaker wires do not touch chassis metal behind the speakers as well. Also surprisingly common.

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Could it be a bad or missing Voltage Regulator Capacitor?

 

The regulator capacitor is there to reduce arcing across the points inside the regulator and smooth out the transition from one level of charging to another, not to smooth out any ripple induced noise.

 

I may have not used the correct terminology but I believe the Radio Capacitor (condenser) is designed to prevent interference and divert unwanted engine noise from transferring through the radio. They may be mounted to the ignition coil and/or voltage Regulator. Is this not correct?

 

I believe there is a filter capacitor attached to the ignition module on an HEI.

 

Still incorrect. The "condenser" on the ignition is also there to reduce arcing across the points as they open and close. The noise created by the ignition and regulator both can cause RF interference in AM radios. If the ignition and regulator get bad enough it can induce noise into an amplifier system but by that time your car will barely be running. Engine noise coming through a stereo amplifier is almost always one of the problems listed above. Corvettes and Avantes create a unique problem because they are fiberglass and do not shield the engine compartment. With those vehicles the ignition noise can be virtually impossible to get rid of. Typically ignition noise is absent with the volume all the way down and increases as the volume is turned up. Grounding issues are always present and get worse as the volume is increased. You still have the same issues to resolve no matter the cause of the noise. Simply replacing a condenser will not usually effect stereo ignition noise but it may make your car run better.

 

Sorry I have to disagree, on points ignitions there is a filter cap on the ignition coil in addition to the one inside the distributor. Imho its not put there for no reason. On the older Vettes in addition to the filter cap connected to the positive side of the coil, there was a metal shroud over the distributor and all the spark plug wires were encapsulated in weaved wire shielding grounded on one side similar to coax cable for noise suppression.

 

The capacitor/filter/condenser systems were designed and put in use before FM radio became widely available. Likewise the ignition shielding on Corvettes. They were designed with AM radios in mind. Ignition systems that degrade will produce RF interference that can be heard many car lengths away. A good quality electronic stereo will have filters inside it to reduce such interference if not eliminate it completely. Engine Noise in a stereo system will almost always be caused by the problems I detailed in my first post of this thread. I am trying to help the OP diagnose a noise problem that I am intimately familiar with. If you insist that the condenser on your coil will do it, by all means go install it. I have spent 15 years of my adult life installing, repairing, and diagnosing stereo problems so I am completely familiar with what the myriad capacitors on a motor vehicle do and will not do. Again, by all means, go right on with your bad self and install more of them.

 

I appreciate all the info on tracking my problem down. I wont be able to dig into for a bit thanks to work keeping me out of the garage. I have a windshield antenna so that shouldnt have any gorunding issues and I know the amp is just screwed to the trunk shelf.

 

Windshield antennas can be culprits because they do have a grounding point under the dash. The "snap" on the windshield connects to a coax that is screwed to the metal dash and then runs down to the radio. Check it by simply unplugging at the radio. They are the devil itself to repair so if it is bad it is easiest to put a fender mount antenna on.

 

The whine that you are getting is most likely from the alternator, and it can be difficult to remove that whine. In the mid 1970's, Chevrolet Division released a 15 page booklet "radio noise diagnosis and suppression update", that goes step by step in how to remedy these type of problems. I can't locate my copy right now, so I can't help with a remedy for your specific problem, but one of the things that I do remember most, was that clean tight grounds were the most important part of eliminating problems, along with adding capacitors or replacing defective ones was one of the most often suggested solution. Also check the capacitors grounding to make sure that it is secure and clean, so the capacitor can do its job. They also pointed out that 90% of the noise would come from the antenna system. The other 10% came through the back end, i.e., the power supply to the system.
I would start by checking, cleaning, and tightening grounds to be certain that the alternator, and other "motors" have a clean and tight ground, and work your way to the engine block, to the firewall, to the frame, etc.. It is going to take time to chase this gremlin, but in the end, it will be worth the effort.