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Author Topic: 2 Meter Mobile Power Concern  (Read 1101 times)
KF6GRI
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« on: February 20, 2010, 02:25:48 PM »

I recently installed a Yaesu FT-1900R in my '04 Honda Accord (Thanks to K0BG for his how to site on mobile installs).

When I start the car with the radio off, I notice that the LCD display backlight briefly flashes. My thought is that the voltage from the battery dips significantly as the starter motor cranks the engine. After the car is running, I power on the radio and all seems well.

Is this just the radio sensing low voltage and protecting itself by checking its 'on' state? Will this cause any problems with the radio long-term? Are any corrective measures needed?

Thanks!

Jim
KF6GRI
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KJ4KKI
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« Reply #1 on: February 21, 2010, 02:25:23 AM »

I'm not sure of the technical reason, but I had a Cobra CB radio that would squawk from the off state when I started my truck.  If I remember correctly, the weather radio was what came on for about 1/2 second.  Is it a continual power plug, or power when on only?  That might be a consideration.
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K0BG
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« Reply #2 on: February 21, 2010, 07:29:58 AM »

Well, you've discovered a ground loop!

The power cable negative lead in most amateur transceivers (if not all) is grounded to the chassis of the radio. It is also worthy to note, that the finals are always connected, even when the radio is turned off.

One scenario is when the radio is wired directly to the battery, and the ground lead and/or the radio chassis is grounded to the vehicle's chassis. Therefore, when the voltage drop through the ground lead from the battery to the engine is more than the voltage drop through the radio's wiring, you create a ground loop. This is one reason there is a fuse in the negative lead.

The solution should be obvious, in that both power leads should be directly connected to the battery. This said, it is also an indication that the starter ground lead has more resistance than it should have, for whatever reason. This should be checked.

Also note, that there are two positive, and two negative leads from the battery. One positive lead goes to the engine block as a return for the starter, and the other is the accessory power source. Same for the ground leads. One is for the starter, the other the accessory ground. In passenger vehicles, they are typically #4 and #8 respectively.

Lastly (and an important caveat), if you ever blow the negative lead fuse, remove the positive lead fuse, and then find out what part of the vehicle's wiring failed, before you replace them.
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KF6GRI
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« Reply #3 on: February 22, 2010, 09:43:36 AM »

A ground loop! I'm a lucky guy! Thanks for your detailed response.

I'm a little puzzled about what to do next. Here's a sketch of my set up and what I *think* I need to do.

The radio is connected directly to the battery (pos + neg leads fused). Radio chassis is mounted in a plastic box, essentially insulated.

I'm not seeing a clear ground loop here. Maybe I'm not as smart as I think I am. Could the antenna coax shield attached to the roof be causing the loop? Seems like the lead to the battery would be a much more efficient path.

Am I right in assuming that this momentary 'flash' on the radio display during engine start is a low voltage condition?

If that is correct, then it could be that the vehicle battery is undersized and that the starter motor is drawing more current than the battery can provide. Again, assuming that I'm not missing the boat on the ground loop.

Thanks again for your help.

Jim
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K0BG
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« Reply #4 on: February 22, 2010, 11:08:27 AM »

Your Honda is six model years old. Depending on the climate, and vehicle use, SLI batteries last from 2 to 6 years. Therefore, I have to assume the battery has been replaced at least once. Batteries do start to deteriorate before they just quit!

Battery wiring lasts a lot longer, but here too, poor climate and/or maintenance shorten their lives. It is always best to check all of the connections when things like this occur. This is not limited to just the battery wiring, no matter how new things are.

And yes, it is possible to have ground loop current returned via the coax.

Ground loops are a nemesis to even well-seasoned amateurs. So don't get discouraged, and just start looking at everything in the current path, especially the leads to the starter.

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VO1GXG
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« Reply #5 on: February 27, 2010, 07:14:56 AM »

If you have a Ground Loop coming on your Coax try disconnecting the radio from the coax, start your car and if the same thing happens then it is not the coax as it is no longer connected.

I could be wrong but it sounds correct!!
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KF6IIU
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« Reply #6 on: March 02, 2010, 03:28:53 PM »

My FT-1802 gives a little noise through the speaker when I start the car, even with the radio off. It is the same noise as happens when voltage is applied to the power leads, so I presume it is just the battery voltage dropping below what the radio likes. This doesn't cause me any concern.

I have an Optima Yellow Top battery - my guess is since its internal resistance is slightly higher than a starting battery the voltage drop while cranking the engine is greater than it would be otherwise.
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