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Old 02-06-2012, 05:19   #1
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VHF Will Not Transmit on High Power. Only Low

Need help. I purchased a new standard horizon VHF 2 years ago. I also ran all new rg-8 coax cable. My soldering isn't the best but I gave it a whirl. Last year about half way through the season I could no longer transmit on high power but low power was fine. I have great reception (10mi) but poor transmitting. I replaced my antenna and still no luck . I guessing it's my connections but does anyone have any suggestions before I drop my mast?

Jason
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Lake Huron
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Old 02-06-2012, 05:24   #2
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Re: Vhf will not transmit on high power. Only low

Very likely to be connections, i.e., the PL-259's. Connecting them properly to RG-8 coax -- and sealing out the moisture -- is not a trivial undertaking.

You could eliminate the radio as a problem by attaching an alternate antenna and an swr/power meter to see if it's putting out full power. And, be sure to check that the power wires to the radio are adequately connected to a good source of 12VDC power.

But, as you surmised, the problem is very likely to be connections.

Suggest you practice a bit on some spare PL-259s and coax, then renew the connections on your coax to the radio and antenna.

Good luck. Let us know what you find.

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Old 02-06-2012, 05:30   #3
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Re: Vhf will not transmit on high power. Only low

It is either the power connections or the radio. If it is working on low power, you have eliminated a problem with coax / antenna. High probability it is the power connection. In high power, typical vhf radios draw about 6 amps.
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Old 02-06-2012, 05:57   #4
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Re: Vhf will not transmit on high power. Only low

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Originally Posted by BocaIII View Post
It is either the power connections or the radio. If it is working on low power, you have eliminated a problem with coax / antenna. .......
Not necessarily. The difference between low power (1 watt) and high power (25 watts) is significant. With a fault in the transmission line (coax) and/or it's connections, it's perfectly possible that the radio could receive/transmit on low power but give problems at the high power level.

But, as you suggest and I suggested above, I'd start with a test to eliminate the radio as a source of the problem. Given the OP's original post, though, my hunch is that it's a connections and/or coax problem.....maybe a stray wire strand, maybe water ingress, etc.

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Old 02-06-2012, 06:52   #5
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Re: VHF Will Not Transmit on High Power. Only Low

I agree with others that it could be marginal power or antenna connection, but given that the radio receives well and works on low power I would also suspect the problem is a failed component internally, possibly a power transistor or internal power supply issue.

Troubleshooting antenna cable:
1) disconnect both ends, do continuity test between core and shield. It should fail (no connection).
2) connect a lead with test clips between core and shield at one end, at the other end do continuity test between core and shield. It should pass.

Troubleshooting input voltage:
1) Figure out a way to measure voltage at the input to the radio by exposing the leads. (Probably the best place is at the inline fuse where you connected it to the house power, but you may need to get creative using extra crimp connectors in a Y cable.)
2) measure voltage while receiving
3) measure voltage while transmitting at low power
4) measure voltage while transmitting at high power

You should expect to see a voltage sag under load, but even at step 4, your input voltage should be at least 11.9v If not, add a parallel connection to the house power and see if it works better.

If these things don't find your issue, try replacing the radio. Last time I had a VHF issue I replaced it, sent the original for factory repair then sold it on eBay when it came back. Good luck.
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Old 02-06-2012, 07:03   #6
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Re: VHF Will Not Transmit on High Power. Only Low

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Originally Posted by Panacea1 View Post
snip
I have great reception (10mi) but poor transmitting. snip
I would expect greater than 10 mile reception. Small boats should be 10-15 (depending on height of antenna) and land stations such as CG should be 30+
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Old 02-06-2012, 09:46   #7
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Re: VHF Will Not Transmit on High Power. Only Low

What Bill said. It is possible that the antenna line has a problem which is causing a high reflected power (SWR) ratio, and most modern radios will cut their output power in that situation, so they don't burn out from it.

A test with an SWR meter will show this. Or a test with an alternate antenna (an "emergency" antenna isn't expensive) can give you a fast comparison.

For UHF coax fittings, I find liquid solder paste (Solder-It) works very well, as you can put it on the coax before you insert the coax into the plug, and that helps gets thorough solder penetration with less heat damage to the insulation. You also can't use a pencil iron for these plugs, you need a real soldering gun to give you enough heat to melt the solder, without heat transferring into the plastic and melting it.

Dielectric grease in the connection, sealed over with butyl or silicon tape to keep the water out, ensures it will stay dry.
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Old 02-06-2012, 10:36   #8
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Re: VHF Will Not Transmit on High Power. Only Low

Or silicon grease to displace any water which could cause oxidation. It also does not negatively affect rubber.
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Old 09-06-2012, 10:40   #9
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Wow thanks to all who took time to reply. I will let you know the out come. If anyone gets up to the North Channel cocktails on me.
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Old 09-06-2012, 10:58   #10
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Re: VHF Will Not Transmit on High Power. Only Low

I agree with the Coax as most likely problem.

When soldering or otherwise putting cable inside the sleeve on the connector, the tinyest wire hair which is thinner than most human hair, could be shorting out. Unless you look hard, it is easy to miss.
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Old 10-06-2012, 13:21   #11
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I had same problem with my standard horizon gx 1600 it was the high power side of the radio called them and they replaced it free of charge
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