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Author Topic: Fresh Alt whine on new antenna install.  (Read 835 times)
KC5TEK
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« on: April 01, 2004, 01:03:57 AM »

I finished up an install of a newish antennna for my IC-2100H in my 98 S-10.  My previous antenna was a magmount on one of the wheelhumps in the bed with the coax running under the third door, a 'temporary' solution that was never fixed, so to speak.  After the cable was bound in the third door's hinge over the weekend and nearly cut in two, I started gathering parts at work for a new install.

As it stands now, I have my old MFJ 5/8th wave antenna on a NMO mount that's attached to a steel bracket that's screwed into the back driver's side corner of my truck's bed.  Between that and the radio is approx 20' of LMR-240 that we had laying around the shop.  I was bad and used a Rat Shack 259 connector that I had laying around at home.  The power leads are hooked directly to the battery and both pos and neg are fused.  Nothing else has changed and now I can hear alternator whine, rising and falling with RPMs, on recieve.

I have yet to get any signal checks so I dont know if I am radiating the whine or not.  The antenna itself is a few years old but looks fine.  The NMO is brass/teflon/aluminium and there is only one tight bend in the coax where it enters the cab, but not bad enough to kink things, I hope.  

Where should I start looking for my whine?  It is not very loud, but I can hear it.  My battery turned up dead one morning a few weeks ago but came right up after a few hours on a charger, I have no whine on AM, nor have I ever had beforehand.
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K0BG
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« Reply #1 on: April 01, 2004, 10:34:06 AM »

GM Products are the worse for alternator whine which is typically caused by a leaky diode or two. It could very well be conincidental with the failure of the battery. The fact you don't hear it on AM is probably because the background noise level is masking it. In any case, you should make sure there is a good RF ground on the truck bed.

If it were mine, I've would have mounted the antenna in the middle of the cab roof even if it required using a 1/4 wave to clear the garage.

Alan, KØBG
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K5LXP
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« Reply #2 on: April 01, 2004, 12:07:17 PM »

I'm going to differ with Alan on this one, while he's right about GM alternators (and they're easy enough to check) I would also suspect the attachment of the negative lead of the radio to the negative lead of the battery.  The whine you're hearing may be caused by a ground loop, current going through the radio's negative power lead, through the radio and down the shield of the coax.  One experiment that might prove this is to try a "floating" (ungrounded) antenna like your mag mount per your previous installation and see if the whine goes away.  In the commercial radio world the radio's negative lead is never connected to the battery, it's always connected to the nearest practical point of the vehicle chassis to where the radio is mounted.  There is usually less voltage drop doing it this way too, and less hazardous to the radio should the vehicle develop a ground fault.

Mark K5LXP
Albuquerque, NM

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KC5TEK
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« Reply #3 on: April 01, 2004, 07:52:50 PM »

I bummed a good multimeter from my engineer buddy at work today and found that I've got 29-40 mVAC coming out of the alternator, not terribly dependent on engine RPM.  I installed a line filter choke in the pos line and I am still getting the 29-40 mVAC at the radio.  Is it possible to change out the diodes on the alternator without buying a completely new unit?

I refuse to drill a hole in the cab roof for an antenna.    

On checking for RF ground, how would I go about doing that with my handy-dandy multimeter?  Just clip one lead to the mount and the other to the chassis or battery?  

I put my tape measure beam on the radio and tuned it to a quiet signal around here and heard no whine, so I guess I've got a ground loop happening.  I will try hooking the neg lead to the chassis of the truck and see what that'll do.
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KC5TEK
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« Reply #4 on: April 01, 2004, 10:21:35 PM »

Ok, I've done a bit more 'work' on this problem tonight.  I put a magmount on the truck and had no whine.  I hooked the new coax back to the rig and even with no antenna connected, I still get the whine with no change after putting the antenna back on.  I'll try putting the neg lead to chassis ground instead of battery and see what that does.  I'll also try hooking into another battery to see if I still get whine with the motor running.
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AA4PB
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« Reply #5 on: April 01, 2004, 11:33:04 PM »

It sounds like you have pretty well identified it as a ground loop problem with current flowing between the negative lead of the battery thru the negative lead, radio, and coax shield to ground at the antenna. I'd try moving the negative lead from the battery to another chassis ground somewhere. Also a good heavy ground lead from the radio chassis to the nearest vehicle ground may help.
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K0BG
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« Reply #6 on: April 02, 2004, 12:01:36 AM »

Mark and Bob are most likely correct about the ground loop. I have always stated that grounding various parts of the radio setup to different parts of the car can and do cause ground loops. They are the most difficult to detect and to repair. That said, if you're getting that much ripple you may have two problems; a ground loop AND an alternator problem. Rather than use a DVM, use a good scope and loop at the wave form to see what it is.

Alan, KØBG
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K5LXP
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« Reply #7 on: April 02, 2004, 01:04:59 AM »

In this instance, one problem may be begetting the next.   To your alternator, your radio is presenting a better ground than it's own ground.  I would be checking the chassis grounds under the hood for the battery, the alternator and the engine block.  Use that DVM to check for voltage from each of those components' grounds to the chassis- there should be zero volts, a couple mV at most.  Any more than that and you have to fix or add your own grounds until these potentials are the same.  If the alternator is running into a compromised chassis ground it can't possibly regulate correctly, which will also explain the ripple.  I've seen vehicles with as much as 2V difference between alternator ground and chassis ground, with all sorts of resultant wierdness.  The hazard this presents to the radio when it becomes a "better ground than ground" isn't when there's a catastrophic ground fault, that will just blow the radio's negative fuse.  It's the "bad" grounds that will allow a few amps worth of current to go through the radio and antenna coax, causing operational problems or even circuit damage.

Mark K5LXP
Albuquerque, NM
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KC5TEK
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« Reply #8 on: April 02, 2004, 01:24:38 AM »

I think my problem is solved.

I played around with grounding various parts of the circuit, antenna base to chassis, radio chassis to vehicle chassis and finally neg cable to chassis.  

Neg to chassis fixed it.  I verified this by using a set of jumper cables as clip-leads to vary the ground from between battery and chassis.  With the squelch open and volume down, I got no whine out of the speaker with the neg clipped to a nearby chassis bolt.  With the neg cable clipped to the battery, I got the whine.

I've not verified this with a signal report as I couldnt raise anyone on the drive home at this late hour.
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K7IHC
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« Reply #9 on: April 02, 2004, 05:50:39 AM »

Good job.  I always use a chassis ground for all my mobile radio installations, with a good quality *black box* type noise filter in the (+) lead.  Bonding items like the trunk lid, hood, and exhaust system to the body are things I like to do, too.
Virtually all commercial/public safety mobile radio installations use chassis grounds, so why is it many hams still use (and recommend) the battery (-) post for a transceiver ground?
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AA4PB
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« Reply #10 on: April 02, 2004, 10:39:07 AM »

I agree that it is best NOT to connect the radio ground directly to the battery. I think people have gotten into the negative battery terminal thing because some chassis grounds can be unreliable if you aren't careful about picking them. Most newer vehicles have a good chassis ground located somewhere near the battery. This is most often a bolt threaded into a nut that is permanently fixed to the chassis. It is the point where most of the vehicle equipment grounds are returned and usually has a heavy wire that runs from the bolt to the battery negative terminal.

Another thing I've found with some radios is that the negative lead passes thru the case and connects to the PC board inside. This gives a path for the ground loop currents to flow directly into the radio's PC board, thru the board to the case ground, thru the antenna coax and back to chassis ground at the antenna. Sometimes a fix can be to provide a good external connection between the ground lead near the radio and the radio's case. This stops the current from flowing into the PC board where it can get into the audio circuits.

The way to analyze ground loops is to follow the current. Most of the current takes the path of least resistance thru the heavy battery ground lead provided by the vehicle mfg. No matter how heavy it is, it always has some resistance. That permits a little current to flow thru any other parallel ground paths. If one of the parallel paths happens to be thru your radio you may wind up with alternator whine. If you have a ground loop, all the filters in the world on the positive lead will not stop it.
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KC2MMI
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« Reply #11 on: April 03, 2004, 01:37:17 AM »

Jeremy-
 With the whine solved, the alternator may no longer be a concern. But yes, you can usually replace the "diode frame" in a Delcotron. Delco, Delphi, they get upset if you call it an "alternator", they're proud of using PWM since the 70's<G>.

 Wholesale alternator parts suppliers are on the web, Wetherill Associates (if I spelled that right) aka WAI are one national OEM that supply almost anything, if your business opens an account (free) with them. A diode frame for a 60A alternator can be under $20. No doubt your truck is a little more, but this is probably 1/4 of what GM would charge at the dealership.

 There used to be a lot of hams at Delco, who would take the time to talk electronics on the phone with customers. Might be worth a call o ask them directly.
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K5LXP
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« Reply #12 on: April 03, 2004, 01:30:23 PM »

The last time I bought a "diode pack" was from JC Whitney, though it's been a long time.  I've also bought alternator rebuild kits from Pep Boys that included the "diode pack".

Mark K5LXP
Albuquerque, NM
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K1CJS
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« Reply #13 on: April 05, 2004, 11:47:55 AM »

First, I always bond the various body components together using ground straps. The box and bed of your pick-up are clamped/bolted to the frame, and as time goes by resistance between the box and the frame increases due to corrosion. The strap between the engine and the frame also corrodes, and since most GM products (most vehicles, come to think of it) have the ground cable from the battery connected directly to the engine, ground potentials between body components and the battery ground change radically.  Also, remember to check the extra wire from the negative battery terminal to the interior engine compartment wall.  Its amazing how ofter that wire is corroded or disconnected--when you see a vehicle with dimmed yellowish headlights, that wire is more than likely the culprit.

If you re-do the bonding straps on the vehicle, you would more than likely find the alternator hum gone.

As far as replacing the diodes, yes it is possible to do so, and if you get a quality aftermarket set of diodes and a new brush assembly, your alternator would also likely quiet down considerably noise wise. I have done this to a few GM charging systems to get rid of noise. I have used the Standard line of aftermarket parts--the better grade ones which are or used to be their "Blue Streak" brand.
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KD6AAJ
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« Reply #14 on: April 21, 2004, 11:24:33 PM »

I worked in a 2-way shop for about 1-1/2 years. We ALWAYS hooked both leads DIRECTLY to the battery because the shortest path to the battery with the cables prevents a voltage drop, whereas the body may or may not have as good of a ground.

Also, if the ground fuse were to blow, and the radio case was not grounded, then the coax would carry the current and possibly catch fire.

I always fuse the positive lead at the battery, and another fuse inside at the radio.

Use rubber grommets to prevent shorts. If you use zip-ties to secure the wires to non-metalic surfaces, then they will not vibrate as easily or rub on sharp metal edges.

I over-size my power leads from the battery to inside the passenger area, then use molex-plugs to attach my radio's power cables.

My IC-2100H has a 7 amp fuse. I ran #12 wire from the battery with a 10 Amp plastic ATO fuse at the battery.

I wish I could post some photos!

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