Cobra 29LX with RFX75, supply voltage problem

[Larry-b]

I purchased an RFX75 and istalled it on the 29LX and it works like a champ, also installed variable power adjustment (works great). I can adjust DK from 1/2 watt to 30 watts and it swings to 117 PEP. It's been working for about a week and I've gotten many compliments on the increase in power and modulation etc. I have it set up as my base rig and use a standard car battery and an automatic battery charger, battery maintains roughly 12.8 to 14.3 Volts (standing) depending on charge rate. I deceided to check the voltage while transmitting and found the battery voltage dropping to 6(six) volts. The battery is out of my antique truck and starts it with no problem. I do not understand how or why the voltage would drop to 6 volts with less than a 10 amp load. Sounds like a bad battery to me but ??
I ordered a 30 amp power supply to use in place of the battery but wondering what may happen with 13.8 constant volts????
Any thoughts?
larry-b

 

[GnG8d]

I ordered a 30 amp power supply to use in place of the battery but wondering what may happen with 13.8 constant volts????
Any thoughts?
larry-b

Use the supply and forget that you ever used a battery and charger. Having worked with a guy who lost an eye to a battery explosion, you'll never hear me say it's okay ....... even temporarily. I've also heard stories of people becoming ill due the gasses being given off of a charging battery in a not so ventilated area.

As cheap as supplies are, it's safe insurance to buy one.

 

[9C1Driver]

Agreed. And don't expect any more power from your RFX radio, it's being all it can be. In fact I would actually be careful on how high you run the dead key.

 

[Robb]

Wouldn't use a charger on a battery because it will probably add a 60 cycle hum on transmit and receive. Just get a real 20 amp power supply so that you don't have to deal with inhaling dangerous acid fumes, or be concerned of the battery exploding from the hydrogen gas from an errant spark. That is all it takes. Seen it happen; it wasn't pretty.

If you run that RFX that high, it won't last long. 75w will last indefinitely and nobody can ever tell the difference at the receiving end.
Promise.

Are you using small gauge wire over a long run? If so, that could be starving the radio. That can make quite a difference.

 

[222DBFL]

The the rfx 75 should have more than 15 watts max dead key. Pep of 80-100 max. And with it cranked up like that, a good fan on the rfx 75 is a good idea too. I would lower both the dead key and total output to around 80 watts. JMO. The unit will last longer and no one on the other end will notice a difference in the slight decease in power. This is JMO. Good luck and god bless

 

[KD2GOE]

the RFX75 need 13.8~14 volts to operate correctly.. and is vary voltage sensitive..

you need to run it with no more then 15 watts DK or you will burn it up quick...

 

[Larry-b]

Thanks for the input guys.
Just for the purpose of not having a lengthy thread, I did not include other information regarding the operation and installation of my set-up.
I have been running radios and amps from batteries and chargers since the early seventies but with precautions taken. The battery is in a metal box and ventilated to the outside. All connections are using srew terminals to eliminate the chance of stray sparks. I have had a few Older chargers put out a humm but that is easily fixed and the charger I use now has no humm. I read the instructions upon installing the RFX75 and keep the DK below 15 watts, I only mentioned the 30 watts to describe the variable power adjustment. The 117 watts PEP is with a 10 watt DK. I have a digital thermometer probe mounted inside the fins of the heatsink at the transistor location and on very lengthy QSO's the temperature has not gone over 115 *F.
My original question was "why would there be a voltage drop of 6V at the battery" and what may happen with a constant 13.8V from a power supply but I suppose no one else runs radios and amps from a battery so my question will remain unanswered.
Thanks for the replies
Larry-B

 

[Larry-b]

Follow-up

As it turns out the voltage drop problem turned out to be the wire feeding the radio/RFX75 was TOO small. It was a 16 AWG and I had a 12 AWG on order that arrived today. Man what a difference. Voltage only drops about .4 volts and the power jumped way up and is working fine.
Thanks again for the input.
Larry-B

 

[Larry-b]

Follow-up II

OK I took everyones advice and bought a 30 amp. power supply. I connected the power supply and checked output, 13.8V. The (adjusted to max) DK has jumped up up from 30W to 50 watts (I still run it at 10 watts DK). The PEP watts has jumped up from 117 to 165+- and the temp is running around 122*F as would be expected at that watts output. I checked the voltage at the power supply at DK and it drops to 5-6V, same problem as with the small power wire but it is now a 12AWG and the load is less than 10 amps. The RFX75 and radio seem to be working very well, I just do not understand the voltage drop?? Any thoughts? Surely I do not need a #10AWG power wire to the radio/RFX75???

Larry-b

 

[Robb]

OK I took everyones advice and bought a 30 amp. power supply. I connected the power supply and checked output, 13.8V. The (adjusted to max) DK has jumped up up from 30W to 50 watts (I still run it at 10 watts DK). The PEP watts has jumped up from 117 to 165+- and the temp is running around 122*F as would be expected at that watts output. I checked the voltage at the power supply at DK and it drops to 5-6V, same problem as with the small power wire but it is now a 12AWG and the load is less than 10 amps. The RFX75 and radio seem to be working very well, I just do not understand the voltage drop?? Any thoughts? Surely I do not need a #10AWG power wire to the radio/RFX75???

Larry-b

Surely, you do.
If you want to hear it again, that RFX won't last long @ 120w; neither were they built to handle it. Even if you added a fan to keep it cool, it is still too much output. MOSFET transistors will suddenly pop even to the point where they produce max output. You're walking on real thin ice; just a heads up . . .

 

[Larry-b]

Thanks Robb

I understand your point about the RFX75 putting out that much power and I agree. I'm just stating what it is doing and the only thing I have done is change the power cord and the output power keeps going up. I can install resistors to lower the max output power but still do not get the Voltage dropping to 5 to 6V at the power supply?? I could understand the voltage dropping that low and the watts decreasing but not increasing. I put the original #16AWG power wire back on it and the DK and PEP watts dropped back to where they were in the beginning but the voltage at the power supply still drops to 5-6V and I would believe that says the power cord was too small. So here I am looking at the #12AWG power cord being too small. So If I cchange the power cord to a #10AWG to stop the voltage problem, the output watts will surely go up. I will change the power cord to a#10AWG and cut the output back accordingly. I do not understand why a 75watt transistor would need a #10 wire.
Thanks
Larry-b

 

[Robb]

I have to wonder about the accuracy of your power meter too. If that RFX put out 165w, it would have popped a long time ago.

On the power wire deal, run 10ga wire on it. You can also change the dead key to 6-8w and the peak watts will also come down a bit back to the safety zone.

 

[9C1Driver]

I have to wonder about the accuracy of your power meter too. If that RFX put out 165w, it would have popped a long time ago.

On the power wire deal, run 10ga wire on it. You can also change the dead key to 6-8w and the peak watts will also come down a bit back to the safety zone.

LOL that was my next comment. Looks like a better watt meter will be the next purchase. I thought 117 watts peak was hard to see, and now 165? Not. At the rate he is going the RFX will be toast before a new watt meter could arrive anyway.

 

[Larry-b]

Back to the POINT of the question

OK my meters are OLD and are crap. I get that.
The original question was regarding a "Voltage drop problem" at the power source.
When I was using a car battery and charger (heaven forbid) I had the same voltage drop problem and changing the power cord to the radio/RFX75 from a 16AWG to a 12AWG solved that problem and let's just say the total power output increased significantly. I then changed to a 30 amp regulated power supply (to get away from the battery safety problem) and now have the same voltage drop problem and to me that would indicate that the #12AWG power cord is too small. however the total power output increased sigignificantly again. Regarding the comment "at the rate he is going the RFX75 will be toast before a new meter arrives" I am "NOT" trying to increase the power output of the RFX75, I am simply trying to solve a "VOLTAGE DROP PROBLEM". I do not understand why a 75 watt transistor would require a #10AWG wire. I will deal with the power output problem when the voltage drop problem is solved.
At this point I think I will forget about the voltage drop and just go back to the battery and charger before the whole thing fries.
Thanks anyway guys

Larry-b

 

[CbRadiosPRONTO]

Here's a thought. I don't think you're getting a voltage drop. I think you're using a digital multimeter that is being skewed by the rf output when you key up the rfx75. If you try another digital meter, it'll probably do the same thing. When I have this problem, I rely on my inline analog meter to give me an accurate reading. Sometimes it's not as sensitive when using a battery as the battery acts as a big filter (capacitor). Before you poo-poo this idea, try running a separate post/neg wire 10 feet away from the bench from the radio or power supply. Then hook up the meter to it. Hopefully it won't be affected by leaky RF. See if you're getting a good reading.

As far as 165 watts goes, that's way off. Coax, antenna, swr's will affect both a digital multimeter reading as well as the wattmeter reading. Also, if your voltage actually dropped to 6-8 volts, the rfx would only be putting out about 17 watts max pep.


Sent from my iPhone using Tapatalk