I picked up a Fatboy 1x4 amp with a variable on the front. The amp is working great putting about close to 800 watts when all the way open. However I'm have an issue adjusting the dead key either with the amp's variable or from the radio. The dead key is stable up to about 75 watts with 1.3 SWR's. However once I turn it up any higher it will jump from a 75 watt dead key to a 200 watt dead key. The SWR's will go over 3. I want the dead key to be around 100, but I just can't do it. I can't do anything more than 75 without it jumping to a 200+ dead key and high SWR's. I was thinking maybe a resistor on the variable is bad? Any other suggestions what the problem may be? I already tried a different radio, a different antenna and different jumpers, same issue. I'm pretty sure it is something going on in the amp.
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High SWR's, trouble with carrier on amp
- Thread starter bassman21
- Start date
Sounds like your overdriving it to me. Open the amps variable all the way and adjust the drive from the radio. find the point it starts to take off then back it back a bit.Check that its stable, then thats your max( not what the manufacturer claims) .
No this isn't the issue, regardless where the variable on the amp is, it is the same issue. I said it will do 800 watt, not that I was running it like that. This is the dead key, not the swing. I can't get the dead key over 75 without it jumping to 200+ with high SWR's. I should be able to smoothly go to whatever dead key I want and SWR's should not be jumping like that.
Most likely have some RF coming back into the amp. Reflect. Possibly adding some ferrite chokes to the antennas feed point would help with this as well as getting the VSWR readings down as low as possible. Also it could be the variable itself causing the amp to go into oscillation. Not an amp guru, but have heard some not so good things about Fatboy amps. Impedance mismatches and also sub par workmanship. I am not trying to bash the amp maker in any way. May not be the amp. But did you not say you were running a different amp before this one?? If that one was operating correctly then it might very well be the amp having the issues. Also what kind of radio are you using? And export or AM only radio and has it been peaked and tuned??
A clean signal in will help to produce a clean signal out. Not saying your radio is the issue either, just asking so as to try to get as much info as possible. Hope you get it resloved. Pull the amp out of line and see if you can work on getting the VSWR down some more on the antenna. Have you trimmed the whip yet? If not you may want to do this now. Not saying it will cure anything either, but it won't hurt anything. And antenna analyzer would be a great tool at the moment.
Last but not least, you can try the bandage move and try placing the amp away from the radio if it's close. Use different length coax jumpers to VSWR meter and such. Again, just from past experiences this is some of what you might try. Like I said, no amp expert here that is for 110% sure LOL! Just trying to help. There seems to be some issues with the amp, but it's hard to tell at the moment. Post up some pics of the board and you might get some better answers as well. Pics are worth a thousand words!! Might be a wrong part value or something. Ya never know. Again, hope you get it fixed. Be safe and have a good one.
A clean signal in will help to produce a clean signal out. Not saying your radio is the issue either, just asking so as to try to get as much info as possible. Hope you get it resloved. Pull the amp out of line and see if you can work on getting the VSWR down some more on the antenna. Have you trimmed the whip yet? If not you may want to do this now. Not saying it will cure anything either, but it won't hurt anything. And antenna analyzer would be a great tool at the moment.
Last but not least, you can try the bandage move and try placing the amp away from the radio if it's close. Use different length coax jumpers to VSWR meter and such. Again, just from past experiences this is some of what you might try. Like I said, no amp expert here that is for 110% sure LOL! Just trying to help. There seems to be some issues with the amp, but it's hard to tell at the moment. Post up some pics of the board and you might get some better answers as well. Pics are worth a thousand words!! Might be a wrong part value or something. Ya never know. Again, hope you get it fixed. Be safe and have a good one.
Thanks for the feedback. I tried it with my Cobra 25 and my Lincoln II. No dirty mods have been done, just a basic supermod on the 25 and modulation turned up a bit on the Lincoln. I have ferrite chokes all over already. I removed them just in case and it made no difference with this. I'm going to test the resistors on the variable and test a few other things. I'm also going to try another antenna all together as I used the same mag mount and coax with the two antenna's I tried. My other amps are working fine, but that doesn't always mean it is the amp though it seems more likely. I'll post an update and welcome any other feedback.
So the amplifier is oscillating?
This is caused by too much input drive?
Can an SWR issue cause this oscillation problem?
Or rather, is the High SWR present because of the oscillation produces harmonics way beyond 27Mhz?
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SWR is always the same low or high power.
Bad SWR can "trigger" oscillation, that is why @OP should test it on dummy load too.
Oscillation can be on harmonics or any other frequency than carrier is. That can be a reason of high SWR read on meter. That can be checked on spectrum analyser or measure SWR on dummy load through 30MHz lowpass filter.
One more thing to check is output relay in PA. maybe it is defective and can handle low carrier only.
Mike
Bad SWR can "trigger" oscillation, that is why @OP should test it on dummy load too.
Oscillation can be on harmonics or any other frequency than carrier is. That can be a reason of high SWR read on meter. That can be checked on spectrum analyser or measure SWR on dummy load through 30MHz lowpass filter.
One more thing to check is output relay in PA. maybe it is defective and can handle low carrier only.
Mike
Then my station must have a serious issue, 40 watt carrier on ten meters gives an SWR of 2.5:1, & with a 10 watt carrier the SWR was 1.5:1
I trust the meter, in so far as the reading is an indication of a problem comparing the SWR reading at both power levels.... or your meter is fooling you
I'm leaning towards it being a grounding issue or even c.m.c. or that the amplifier is going into oscillation and producing harmonics throwing only God knows what kind of signal at the antenna just to have it reflect back into the meter/radio/p.a. because of course the antenna is only (really) resonant at the target frequency.
( meaning my own setup here, not o.p.'s )
Is this something you've been using that started having this issue? What brand of trsnsistors? These types of amifiers can be picky about jumper lengths. If you put it on a dummy load I'll bet everything appears to be fine. If you know the antenna is right the amp needs some repair.
Remove and test the gain of all transistors. The 4 finals should be closely matched. Test all caps and resistors and replace anything mismatched or out of tolerance. Retune it if needed. That's the only way to know for sure the amp isn't the problem.
Remove and test the gain of all transistors. The 4 finals should be closely matched. Test all caps and resistors and replace anything mismatched or out of tolerance. Retune it if needed. That's the only way to know for sure the amp isn't the problem.
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