• You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
  • Click here to find out how to win free radios from Retevis!

Texas Star DX500 and high SWR

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...
 
Is it possible to replace the pot with a higher wattage part to make it a useable feature? If so, what specs should I look for in a suitable replacement? I'm passing my DX500 with a Galaxy 979, just tuned up by DTB.

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.
 
  • Like
Reactions: 1 person
If the point is to reduce the amount of power delivered to the amplifier, then why not do just exactly that? That power reduction device doesn't have to be made into, or be a part of the amplifier at at all...
- 'Doc


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.
 
  • Like
Reactions: 2 people
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! :D Best part of the whole ordeal.... he said he didn't fix it, so there was no charge.

73,
RT307
 
Did he test the amp using your Galaxy?
Did he test the radio being used for harmonic content?

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! :D Best part of the whole ordeal.... he said he didn't fix it, so there was no charge.

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.
 
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
 
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

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
 
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.
 
  • Like
Reactions: 1 person
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.



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.
 

dxChat
Help Users
  • No one is chatting at the moment.