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Author Topic: 811-H and RF feedback?  (Read 5945 times)
KB3BTO
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Posts: 41




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« on: February 24, 2011, 04:40:42 PM »

  During a recent ice storm, I started to work a DX station without checking for ice build-up on my 20 meter dipole and without checking the SWR. When using the amp, I usually run it at 400 watts output driven by 22 watts as measured by a Bird 43 with a 100 watt element installed.
  While transmitting, I suddenly noticed that the Bird watt meter had pegged to max deflection. I immediately ceased transmitting, checked my settings, and then tried transmitting again and had the same result: max deflection of the watt meter.
  I ran the rig, a TS-940, barefoot and it's fine. I've been told that I might have fried something in the amp due to possible excessive SWR.
  Questions: Has anyone experience a similar problem? If so, what components were effected? If not, can anyone provide me with some constructive input as to what to start checking first?
  My "test equipment" is limited to a DMM and an analog VOM.

   Charlie Shaw
   KB3BTO
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W1QJ
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Posts: 2979




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« Reply #1 on: February 24, 2011, 04:54:33 PM »

Go back and re-read what you have written.  It does not make any sense, or maybe it does?  You say you have a Bird wattmeter with a 100 watt slug, you read 22 watts of drive from the 940, then when you run the amp the meter pins.  Sure it pins.  You are running 400 watts into a Bird 43 with a 100 watt slug, of course it is going to pin.
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K7KBN
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Posts: 3693




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« Reply #2 on: February 24, 2011, 05:51:49 PM »

I think he meant that the 22 watts drive was being measured by the Bird.  How he was reading the 400 watts, he doesn't say.

So why would a Bird 43 peg full scale on a 100 watt slug if it was measuring 22 watts?  Hmmm...

Maybe I was reading the OP wrong, but that's what I came up with.
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73
Pat K7KBN
CWO4 USNR Ret.
W8JX
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Posts: 13268




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« Reply #3 on: February 24, 2011, 07:10:39 PM »

Getting back to question, power reading is relative to impedance of load as it is a function of voltage and resistance. If the antenna was shorted out and had a very low input impedance, it could cause watt meter to read artificially high.  I see no mention of a SWR check when problem occurred.
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Ham since 1969....  Old School 20wpm REAL Extra Class..
W1QJ
Member

Posts: 2979




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« Reply #4 on: February 24, 2011, 08:00:01 PM »

Here is a quote of just what he said (  While transmitting, I suddenly noticed that the Bird watt meter had pegged to max deflection).  Now assuming he has the Bird meter after the amp, if he is running 400 watts from the amp, the meter sure as hell would peg.  Sure, he was reading 22 watts from the exciter but when he puts the amp on and goes to 400 watts. you had better have a 500 or 1000 watt slug.  I went back and read the post MANY times and I can't read anything else into it other than the way I see it.  Now if he has the Bird meter before the amp then that is a different story.  Why he would put it there is beyond me since there is a pretty accurate power meter in the 940.  I think we might have a case of operator error here?
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KH6AQ
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Posts: 7982




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« Reply #5 on: February 24, 2011, 08:21:14 PM »

It is the exciter wattmeter - the Bird - that pegged. Yes RF could make the exciter go to full power. Is the amp damaged? Probably not. A test into a dummy load or an antenna on another band will tell the tale.
« Last Edit: February 25, 2011, 02:58:20 AM by WX7G » Logged
W1QJ
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Posts: 2979




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« Reply #6 on: February 24, 2011, 08:34:52 PM »

It is the exciter wattmeter - the Bird - that pegged........HUH?

Did you mean to say....Is it the exciter watt meter OR the Bird that pegged?
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W8JX
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Posts: 13268




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« Reply #7 on: February 24, 2011, 08:37:08 PM »

I read it as the exciter watt meter pegged because he does not say bird meter was installed after amp. If a output tube or two shorted out it could short out input too and make watt meter peg.
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Ham since 1969....  Old School 20wpm REAL Extra Class..
W1QJ
Member

Posts: 2979




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« Reply #8 on: February 25, 2011, 05:10:25 AM »

If there is a problem with the RF load to the exciter, would not the exciter fold back it's power and the power meter go down instead of up?  I think so.  Besides his explaination is vauge and not very well thought out and leaves too many questions as to just what he means.  Its like this....THROW THE COW OVER THE FENCE SOME HAY.  Can you help me lift that cow and throw him over the fence?  Thanks
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KF5AHV
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Posts: 56




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« Reply #9 on: February 25, 2011, 05:45:32 AM »

i would check the amps input. if you had a meter reading reflected power between the amp and rig and it suddenly jumped up the first thing i would check is coax and connections. wiggle them while you transmit.
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KB3BTO
Member

Posts: 41




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« Reply #10 on: February 25, 2011, 07:23:56 AM »

  Wow. Sorry guys, I thought that my original posting was clear enough to provide a brief outline the problem but apparently not. I'll elaborate.
  I choose to use a Bird 43 watt meter with a 100 watt, 2-30 MHz element  to obtain a more exact measurement of my TS-940's output power. It is connected after the transmitter but before the 811-H amplifier. In between my amplifier and my tuner, I have an MFJ-868 watt meter (the one with the BIG meter face) to display a peek meter reading of my amplifier output power.
  During the latest bout of freezing rain, I noticed that the Bird 43 watt meter was at maximum deflection and not at 22 watts when I pushed the PTT button to transmit. This happened each time that I attempted to transmit, so I broke contact and began to check for the cause. Finding nothing readily apparent, I posted a inquiry as to the possible cause and was told that "it sounds like an RF feedback problem to me, " or words to that effect. It was the only response, so I pursued that line of reasoning.
  I then posted the question asking if anyone else had experienced this problem with the 811-H amplifier.
  So, here we are.
  Does that help to clarify the situation any? If not, please let me know what areas are still gray.

  73
  Charlie Shaw
  KB3BTO
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KF7CG
Member

Posts: 1215




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« Reply #11 on: February 25, 2011, 11:09:14 AM »

If you are using ALC, the mismatch to the AL811H is causing its ALC circuits to call for more drive. This could be combined with the effects of new antenna reactances affecting the nuetralization circuitry and thereby feeding power to the input in such a phase as to change the reading on th Bird.

Remember, grounded grid amplifiers have power transferrence from the input to the output and whith a large mismatch this could go the other way while not causing a parasitic (or not).

KF7CG
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W1QJ
Member

Posts: 2979




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« Reply #12 on: February 25, 2011, 02:04:33 PM »

You really need a dummy load to make tests because the ice may have damaged you antenna.  That said, I would reduce the amount of variables.  You relaize that you have a tremendous amount of variables in your set up.  Think about it.  You have a coax from the 940 to the bird meter (1) then from the bird to the amp (2) then from the amp to the MFJ meter (3) then from the MFJ meter to the tuner (4).  That is a total of 8 connections that can go bad or be at fault, not to mention your antenna.  I would use a dummy load and start with the exciter into the dummy load and see what happens, then one by one ad the respective components and see where the problem lies.  If you don't have a dummy load (you should!) then go from the 940 to the antenna and check the swr on the internal swr meter on the rig, this will see if the antenna is at fault.  if the antenna tests good then you can add each unit until the bad thing shows up.  I am not so sure if the ALC is your problem, but I would disconnect it to see if that might be your problem.  I don't think the ALC can override the setting of the RF control on the radio, but I might be wrong.
The funny thing is if it was an SWR problem I would think that the 940 would fold back in power instead of increase output.  SO you have some testing to do.
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KH6AQ
Member

Posts: 7982




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« Reply #13 on: February 25, 2011, 02:21:11 PM »

Wow you guys can make a simple troubleshooting exercise so complicated.

He says the transceiver is ok.

1) Is the antenna VSWR below 2:1? If so the amp will have no trouble loading into it. If the VSWR is excessive fix the antenna.

2) Connect the transceiver only to the antenna. Running CW slowly increase power to maximum. If no problems are encountered continue.

3) Connect the amp. Start with very little RF drive from the transceiver. Increase the transceiver power (CW mode sending DITs so you don't cook the amp) until the amp is putting out rated power or something strange happens. If the transceiver power pegs all of a sudden you almost certainly have an RF feedback issue. The amp is not broken.

Troubleshoot from that point.

« Last Edit: February 25, 2011, 02:25:48 PM by WX7G » Logged
KB3BTO
Member

Posts: 41




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« Reply #14 on: February 25, 2011, 05:12:57 PM »

  Okay, boys, put your thinking caps on because I'm more stumped now that I was at first. Here goes...
  I opened the amp and all "looked" okay: no signs of arcing, carbon deposits, whatever. (I realize that visual inspection is not a exact science, so don't beat me to death about it.) I checked the 51 ohm resistors on top of the tubes and they all met specs.
  I checked all coax connections and found zero shorts. I even changed the cable from the "Remote" connection in the back to the TS-940 in case there was a short in the plug.
  I went through the tune-up procedure from scratch, into a dummy load (yeah, I have one) and all was well. Once again, I was a happy ham but that didn't last long.
  The amp was pumping out 400 watts and was being driven by 22 watts with a 1.5:1 SWR on my 20 meter dipole just like it "used to," when all of a sudden the same symptoms surfaced: the Bird 43 going to max deflection when I keyed the mic and the ALC level pegged as well. Please note that this happened "after" the rig and amp had been in the operational status for 20 plus minutes.
  This has me thinking that there's a thermal connection to this situation. I just plain don't know what it might be right now.
   I'm going to try and run the rig barefoot again tomorrow, if time permits; however, I anticipate that it will work just fine like it did the other day.
   Happy hamming.

   73
   Charlie
   
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