I have a Galaxy DX 979 that variable power, but DTB set it at 2W min, which is perfect for DKing my TS DX500 at ~125W, but sometimes I want a little less, in which case I guess I could press the Dial-A-Watt button with the knob still wide open to get it down further...
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Texas Star DX500 and high SWR
- Thread starter 2RT307
- Start date
It already is a useable feature. When you are running a proper input wattage you can dial the peak power down to say 200 watts if you just want to coast.
The variable pot IS NOT intended to let you compensate for over driving the input wattage.
Ding ding ding ding.....we have a winner. What he is saying is adjust the wattage on your radio in the first place and you won't be burning the pot up.
I never got around to shipping these off to Galaxy... but took them to my local tech today to have another look see. He put one on his spectrum analyzer, and reported that it had a pretty nasty third harmonic (12 db down?) at 80 Mghz. Tried to put a tuned choke on it to kill it, but it still had the problem. He was perplexed, said he had that work before on other amps. He and local amp builder Goose Rider both reported they had never heard of anyone having this type of problem with Texas Star. (Neither of them participate in online radio forums). He told me that the harmonic was what was throwing my SWR's off, and that output was normal on the amplifier, nothing getting hot, etc. I showed him the parts list that Mad Scientist gave me, and he said he could not see how those would help. I wish I'd had more info wiith me to explain at the time, but didn't print up this whole thread! 
Best part of the whole ordeal.... he said he didn't fix it, so there was no charge.
73,
RT307
Best part of the whole ordeal.... he said he didn't fix it, so there was no charge. 73,
RT307
Hi Robb, he tested it on his Cobra 25 shop radio, which he said he was aware of it having some harmonics. I have ran several different rigs on this setup including a President Lincoln, Omegaforce S45HP, 257HP, Uniden Grant XL, and RCI 2950, all with the same results of 3.5:1 SWR with the amp on. SWR's with the amp on are 1.5:1 or less across the band with any of the above listed rigs, whether running a Firestick 5' or Predator 10K.
73,
RT307
I never got around to shipping these off to Galaxy... but took them to my local tech today to have another look see. He put one on his spectrum analyzer, and reported that it had a pretty nasty third harmonic (12 db down?) at 80 Mghz. Tried to put a tuned choke on it to kill it, but it still had the problem. He was perplexed, said he had that work before on other amps. He and local amp builder Goose Rider both reported they had never heard of anyone having this type of problem with Texas Star. (Neither of them participate in online radio forums). He told me that the harmonic was what was throwing my SWR's off, and that output was normal on the amplifier, nothing getting hot, etc. I showed him the parts list that Mad Scientist gave me, and he said he could not see how those would help. I wish I'd had more info wiith me to explain at the time, but didn't print up this whole thread!
73,
RT307
Sorry to hear that Brett, but it isn't supprising. Most tech's that I have mentioned this problem to pretty much said the same thing..."Good luck with that"!. The fact the he said that he couldn't see how it would help is pretty much self explanitory. I would send Ed one of them as I suggested to you earlier and let him have a poke at it. Give him the benefit of the dought at least once and see what happens. As you now have some concrete evidence that there is a harmonic issue as was proven by the s/a testing. I would mention this to him in the note that will accompany this unit.
Please contact me via phone and let me know what happens.
73's
Eric
If the 3.5:1 SWR is really being caused by a third harmonic, it couldn't be any easier to confirm. Just install a low pass TVI filter capable of handling the amp between it and your output watt / SWR meter. Even a cheap one should knock down a third harmonic on this band by 60 db and quickly confirm if the info you got was correct. I doubt this is the problem or the tech would have pointed out this simple solution to handle a common harmonic problem.
I mentioned in a previous post that I had previously put my Bencher TVI filter in line with no effect. However, I purchased new 3' coax yesterday to make sure the pig tails were good. Will try to test again this weekend.
73,
RT307
I think you are right, Eric. Just need to get one of the amps off to them so they can look at it.
73,
Brett
Excellent choice! If the first "Tech" told you the trouble was a 3rd harmonic only 12db down from the fundamental when your Bencher TVI filter had no impact at all......run like hell to the next tech in line. That one was clueless and may not even know how to operate a spectrum analyzer. When an honest tech hits a problem he can't figure out, he'll tell you he's stumped. When a "fly by night" guy gets stumped, he'll baffle you with BS until you're just as confused as him. For instance, claiming a self oscillation is a 3rd harmonic.
There are other issues that could be contributing to this problem as well. I also beleive that if the bias devider network was replaced with the voltage regulated bias source it would help a good deal. I have seen issues with self oscillation with them also.