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   Home   Help Search  
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Author Topic: New to HF, FT-757GXII Help  (Read 1362 times)
N0ZCB
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Posts: 5




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« on: December 23, 2012, 09:20:10 PM »

Any thoughts on why the ALC meter would not move at all when I have an auto tuner attached that has matched the antenna?  My first thought was that the FWD REV switch on the back of the radio was set to REV,  but that was not the case, it is set to FWD.  If I remove the antenna tuner the SWR is about 2.5,  when I transmit I get the expected operation of the ALC meter. 

This is my second attempt at getting started in HF, so I am relatively new.  The radio was purchased used, the tuner is new.  I don't know what to think about the ALC meter. 

Thanks for your help. 
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KA4POL
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Posts: 1966




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« Reply #1 on: December 23, 2012, 11:35:59 PM »

The SWR shows you how well the antenna is matched impedance wise. The antenna tuner makes the TX see a pretty well matched antenna impedance. So any mismatch will show between antenna and AT. This mismatch is still the same if you measure SWR after the AT. To monitor the SWR you would need a meter between antenna and the AT.
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N0ZCB
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« Reply #2 on: December 24, 2012, 06:52:42 AM »

Thank you for the reply.  The issue that I am trying to figure out is why the ALC meter deflects/moves when transmitting without the tuner attached, but when I attach the tuner I do not see any movement when transmitting of the ALC meter. 
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AA4PB
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« Reply #3 on: December 24, 2012, 07:49:12 AM »

The ALC is often affected by the load impedance placed on the transmitter. A high SWR may cause an increase in ALC activity. It may be that a problem in the transmitter is preventing it from putting out enough power into a 50 Ohm load to generate the specified ALC activity.

The first thing I'd do is hook up an external wattmeter and a 50 Ohm dummy load to the transceiver. Then you can see if you get the ALC activity with a proper load and determine if the transmitter is able to put out a full 100W. Then you'll know if it is a power output issue or an ALC circuit issue. With the dummy load you can also check the operation on other bands.

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AC2EU
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« Reply #4 on: December 24, 2012, 07:54:36 AM »

AlC has to do more with audio distortion than anything else. ( Automatic Limit Control) It is a form of audio compression but with high levels you will have distortion and harmonic splatter. The cleanest audio has little or no ALC deflection, but some is OK on audio peaks. Adjust your mic eq and gain for best results.
Monitor your SWR and Power meter while speaking in SSB mode. These will show you if you are getting full power out on peaks.
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N0ZCB
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« Reply #5 on: December 24, 2012, 08:21:26 AM »

Thank you both for the replies.  I do have an external RF / SWR meter,  it was included with the radio.  It is a Tempo RBF-1.  I understand how to use it to measure SWR, and the measurement seems to agree with the SWR meter on the radio.  However the RF reading only seems to be 6 watts as indicated on this meter with an SWR of about 1.5.  Here is an image of the SWR meter. http://www.eham.net/data/classifieds/images/387144.jpg  I do not have a manual for this meter, but am guessing that this isn't a true RF meter.  I do see the SWR changing when I speak into the microphone so that is probably a good sign.  I will either build, or borrow a dummy load, seems like something that would be useful for troubleshooting this.   Thanks for your patience and the help. 

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AC2EU
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« Reply #6 on: December 24, 2012, 08:55:53 AM »

6 watts? Is that confirmed  by the radio's power meter as well? Is the power control at max?
If so , you may have blown finals or serious cable/antenna issues.
Using a dummy load will be an excellent diagnostic!
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N0ZCB
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« Reply #7 on: December 24, 2012, 09:13:48 AM »

The radio's power meter will hit the max on the meter when transmitting, so that is probably good.  I think I will be able to tell a lot when I get a hold of a dummy load and can accurately measure the power output.

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AA4PB
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« Reply #8 on: December 24, 2012, 09:17:23 AM »

6 watts output shouldn't provide any ALC activity. ALC should only activate (with a 50 Ohm load) when the power output approaches maximum.
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N0ZCB
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« Reply #9 on: December 26, 2012, 10:00:50 PM »

Thank you all for the advice.  I followed the advice of AA4PB, and built a dummy load and did some additional testing. My radio will output 100 watts to a dummy load, so at least it looks like I have not damaged anything yet.

As it turns out, when my external auto tuner is going through it's routine in tuning, the SWR spikes at infinity.  When this happens the transmitter reduces it's output and the tuner is not tuned correctly for the frequency, meaning the SWR is still high enough where the transmitter is cutting back on the power and I do not see the ALC moving.  So the big mistake it looks like I am making is having the transmitter power set too high when I activate the tuner.  (be nice, I'm learning)  A local HAM loaned me an MFJ-949D and walked me though how to use it.  I still think I have an antenna feed line problem, or an antenna problem, but at least I now feel like I have the tools to help diagnose the problem and know it is only a matter of time before I get it right.   Thanks again for your help.  :-)
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