VHF Issue

[Rick D]

I have an older B&G (Virgil) VHF primary cockpit radio. I love this radio and do not want to replace it. It's a hunk of iron that weighs pounds, huge heat sink, detachable chunky mike, separate speaker, etc.

Anyhow, my transmissions were getting clipped a while ago. I took out the unit and cleaned up the power connections. I was getting within 1/2 volt of the battery voltage which seems right; the leads are large.

Recently, I found the same issue when the battery was reading 12.3 volts. Just fine above that. There is a transmit power graphic on the display. It showed a full power (on high) which then dropped way down a moment later.

So, if there is good power getting to the unit, it would appear it's the unit itself. Any ideas? Sounds like replacement time to me since it's going to cost a hundred to just crack the case and I doubt anything proprietary is available.

 

[Brian D]

I have an HF rig that does the same thing. I think they call that fold-back. If the SWR is detected as too high, the rig will reduce power to prevent too much current returning to the finals and damaging them. I would check your antenna and cable for possible replacement, unless they are new already. In that case, check the RF connector.

Just some thoughts.

 

[Rich Stidger]

Brian has a good suggestion. Try a different antenna first to eliminate that as a problem. If the antenna is found to be good, then I would crack the case an look for a visual connection problem- corrosion, loose, etc.

In addition to trying a different antenna on your radio, I would try another radio with your antenna. Somebody must have an extra one laying around.

 

[Rick D]

Great ideas!

I have two antennas on two radios. I'll buy some coax and a couple of connectors and try that out swapping antennas. Twenty year old masthead antenna and coax, so it wouldn't surprise me to find it needed something.

 

[Bill Roosa]

Are your sure it is getting enough current. while it reads battery voltage when no current is flowing (probe the power lead) what is the voltage to the unit when you key the mike on high power? if that does not check out then measure the voltage drop between the battery + and the unit + with the mike keyed and the battery - and the unit - with the mike also keyed. You should only get a fraction of a volt if the wiring is good. The one with the highest voltage drop is the offender.

 

[Rick D]

Dang, Bill...

Are your sure it is getting enough current. while it reads battery voltage when no current is flowing (probe the power lead) what is the voltage to the unit when you key the mike on high power? if that does not check out then measure the voltage drop between the battery + and the unit + with the mike keyed and the battery - and the unit - with the mike also keyed. You should only get a fraction of a volt if the wiring is good. The one with the highest voltage drop is the offender.

...another good point. In fact I chased a nav light issue whereby I was getting a full volt reading at the lead but when a load was applied it disappeared. Turned out to be internal corrosion in the leads.

 

[Brian D]

Rick, other than your RF power indicator reducing when you key, do the lights or any display items 'dim' also? I would think if you had a power issue anything on the display would dim when you key'd the mic. Kind of like car lights dim when they are on and you start the engine.

Just wondering.

 

[Rick D]

No Dim Lights, And,

Rick, other than your RF power indicator reducing when you key, do the lights or any display items 'dim' also? I would think if you had a power issue anything on the display would dim when you key'd the mic. Kind of like car lights dim when they are on and you start the engine.

Just wondering.

... thinking about it, no problem on low power. Still, could be either.

 

[Brian D]

Might want to invest in a 50 ohm dummy load for testing. The are cheap and something to have in the tool box when needed.

http://www.mfjenterprises.com/Product.php?productid=MFJ-262B

 

[Rick D]

Follow Up: It's Power, Not Antenna

... I let the house batteries drop to 12.3 volts (measured at the bank). Did my radio check, and sure enough, it was a clipped transmission. I tried it on 1 watt, and it checks fine, clear as a bell. So, I put my volt meter on the leads and found about a half volt drop at the power lead. When I keyed the mike on high power, the volts dropped to 10.6. It 'only' dropped to 11.3 on low power.

BTW, the radio check is Channel 27 with an automated replay. The antenna is about 8 miles away.

So, the nav station VHF which has it's own antenna on the radar pole works fine with the same 12.3 volts. I have not checked to see if it has a similar drop at the leads. So, my next step is to check the breaker panel itself, and maybe run a test lead out to the radio and try it.

It seems to me that a modern radio ought to put a full 25 watts at 10.5 volts anyway. I am going to check some specs to see. Wouldn't you think so?

 

[Brian D]

A modern radio should put a full 25 watts out at the PL-259. Beyond that, many factors come into play and I am sure you know what I mean. Cable attenuation, connector attenuation, coil attenuation, heat, corrosion, cold joints, and the list goes on...

However, what is causing such a drop in voltage is the question. A drop in voltage tells me that their is a strain for current to run the finals at their advertised power out levels.

If it were I, I would remove all electrical connections, clean with emery board or electronic spray, then reconnect.

Just a thought.