Batboard

I have 2 Motorola CDM series radios in stalled in my truck for several years.

Today on my VHF radio is making a weird hum in the backround (over the air that others hear not me) while transmitting, ONLY if the truck is running.
The only recent change was I added a channel to one of the Zones.
All wire checked, solid good connections and a clean good ground.


Any help / input ??

Does it change with engine RPM? If so its probably an alternator whine. Check your connections, antenna, power, ground. Check for corrosion on battery and alternator connections, Check capacitor on alternator...rinse repeat...YMMV

You might also check your battery. If its failing more strain on your alternator. If all this checks time to check your alternator. You may also install a whine filter inline with your radio power lead. Try this.....
http://www.worldwidedx.com/home-brew/31 ... ilter.html

Good luck

And.......check your mike......try a different one.....

Been a number of threads on this same issue.

My money is in the battery is 3 years or older. Probably needs to be replaced before you get stranded.

You also need to do a test on your alternator. Need a digital voltmeter. I am not going to type it all again on how to test the alternator. go do a search for it.

Jim

Also, remember the battery IS the filter for the alternator, an AC generator.

Where is your radio grounded? Is the main power lead from the radio direct thru a fuse to the battery positive? Or the battery positive in the under hood fuse block.

abbylind wrote:And.......check your mike......try a different one.....

Tried. No change

Jim202 wrote:Been a number of threads on this same issue.

My money is in the battery is 3 years or older. Probably needs to be replaced before you get stranded.

You also need to do a test on your alternator. Need a digital voltmeter. I am not going to type it all again on how to test the alternator. go do a search for it.

Jim

Battery less than one year old.

Alternator test ok.

Will wrote:Also, remember the battery IS the filter for the alternator, an AC generator.

Where is your radio grounded? Is the main power lead from the radio direct thru a fuse to the battery positive? Or the battery positive in the under hood fuse block.

Grounded directly to the chassis.

Main power directly from junction under hood (where battery connects to the alternator and starter)

Ignition connected to relay off of ignition switch

Whining noise smells like a bad diode in the alternator

rescue52 wrote:

Jim202 wrote:Been a number of threads on this same issue.

My money is in the battery is 3 years or older. Probably needs to be replaced before you get stranded.

You also need to do a test on your alternator. Need a digital voltmeter. I am not going to type it all again on how to test the alternator. go do a search for it.

Jim

Battery less than one year old.

Alternator test ok.

Have you read the other threads about this same subject. Just because someone says the alternator is good, doesn't mean it doesn't have one or more bad diodes. You really need to take a digital voltmeter and put it on the AC scale and see just how much AC voltage is in the system when the engine is running. Don't forget to add load while your doing the testing.

Jim

This all started when you "added a channel" to the VHF. Did you take the radio out of the truck to have it programmed, and then put it back in? If so, is there a chance that the radio mounting bracket is drilled into a good, solid piece of steel in the dash? If that is true too, then you may have picked up a ground loop between the dash, the antenna shield, and the battery minus cable. It ran quiet for years because the bracket was new and you were careful putting it in. You didn't scrub the paint on the bracket. But, it's been a couple years, the blush has worn from the rose, you man the thing out, scratch some paint, get it programmed, shove it back in, tighten it down, and Bingo! - ground loop. A teeny tiny hum only heard over the air when you pull current during transmit. Pull it out of the bracket and see if the hum goes away. Make it easy on yourself - get a portable and listen to it.

Bill_G wrote:This all started when you "added a channel" to the VHF. Did you take the radio out of the truck to have it programmed, and then put it back in? If so, is there a chance that the radio mounting bracket is drilled into a good, solid piece of steel in the dash? If that is true too, then you may have picked up a ground loop between the dash, the antenna shield, and the battery minus cable. It ran quiet for years because the bracket was new and you were careful putting it in. You didn't scrub the paint on the bracket. But, it's been a couple years, the blush has worn from the rose, you man the thing out, scratch some paint, get it programmed, shove it back in, tighten it down, and Bingo! - ground loop. A teeny tiny hum only heard over the air when you pull current during transmit. Pull it out of the bracket and see if the hum goes away. Make it easy on yourself - get a portable and listen to it.

I did NOT remove to program.

I used a laptop with a rib box (being that it is remote head mounted.)