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Texas Star DX500 and high SWR

Discussion in 'Amplifiers' started by 2RT307, Oct 12, 2012.

  1. 2RT307

    2RT307 Sr. Member

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    I have a Texas Star DX500 (non variable) amp that I have been running for about 6 years now, trouble free. Recently, the amplfier would throw the SWR's off the scale when turned on. I have checked all grounds, changed coax jumpers and coax to antenna, checked the antenna mount, etc, and found no issues. I also bought a new meter, and it reads high as well. Took the amp to a local tech, and he saw the same thing on his bench.



    He does not have a spectrum analyzer, but said it looked to be throwing harmonics out and this was causing the high SWR's. He could not find any caps or resistors that were blown or out of spec. Nothing appears heated/burned inside the amp. The matching network in the amp tested ok. Not sure where else to look, and I am trying to have this fixed locally to avoid shipping charges. Any ideas where else to look for the problem causing the high SWR's?

    73,
    RT307
     

  2. AudioShockwav

    AudioShockwav Extraterrestrial Admin
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    You may need to replace of the 10uf chokes and the 25 ohm 5 watt sandbar resistors in your amp.
    It seems that Texas Star has recently made a change in these because after time they start to go high in resistance.

    Contact Mike At Home Page



    He Is a great tech on these amps, and can help you troubleshoot many common problems.
    You should check the bias on you amp with the amp keyed, with no drive.
    He has instructions on his troubleshooting page that tells you how to do this.


    This subject came up on Bennies board @ cbtricks.com




    The problem was the same

    Much thanks go out to Mad Scientist for sharing the cause and fix, and to Bennie for all of his support in providing radio and Amp tech info.


    The whole thread :
    Login


    73
    Jeff

    ( you need to be a member there to view the whole thread )
     
    2 people like this.
  3. 2RT307

    2RT307 Sr. Member

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    Thanks Jeff, that is great information.

    73,
    Brett
     
  4. Shockwave

    Shockwave Sr. Member

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    It sounds to me like the problem described on CB tricks is lack of bias current as a result of resistor values drifting upwards. While too much bias can contribute to self oscillations, a lack of bias pushes things the other way and would not be the problem here. We should be thinking about the negative feedback loop and if the emitter connections of those output transistors are still at RF ground.

    As I recall Texas Star may be using those small glass caps that look like diodes for the negative feedback with the large resistors across each transistor. Check the values of those caps. If in doubt, replace them with .001 or 102 ceramic disk caps.

    To insure the transistor emitter connections are at RF ground, check all of the screws holding the board to the heatsink to be sure they are all tight. I think your amp was designed to work on negative or positive ground vehicles and that would mean that those screws are not connected directly to the board DC ground.

    If that is the case and the amp is only used in a negative ground system, jump across the caps with a wire from the traces around the screws in the heatsink to the larger ground plane connection on the board that has the negative wire attached. This will reduce the inductance the emitters see to ground and help stabilize the amp.
     
  5. AudioShockwav

    AudioShockwav Extraterrestrial Admin
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    Here is what he was seeing:
    So Donald, you are correct that the Idle current was dropping.




    The Radio was also a contributing factor as well:
    In the end, cleaning the radio up, replacing the sandbar resistors, and the chokes got the amp/radio back were he wanted it to be to run properly on Ten.
    If you get a chance, it was a good read.

    73
    Jeff
     
    #5 AudioShockwav, Oct 14, 2012
    Last edited: Jul 17, 2014
  6. 2RT307

    2RT307 Sr. Member

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    Sorry to take so long to get back on this one... I have been out of the country for the last couple of weeks.

    Jeff, I did email Mike via the link you provided. Thanks for that! He replied quickly, and gave me some suggestions. He didn't like the input pad resistor behind the power switch, and suggested replacing that with a 75 ohm 6 watt resistor.

    I will try to get my local tech to tackle that soon. I love this amp, and want it working right!

    73,
    Brett
     
  7. 9C1Driver

    9C1Driver Sr. Member

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    Yea, the 350 & 500 are darn good amps.
     
  8. 2RT307

    2RT307 Sr. Member

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    Ok, so just to throw a curve ball, I hooked up my other TS DX500 today. It has been my primary amp for the base. Guess what? Same problem! So, I figure it must be a ground issue on my mount. I changed out to a bee hive Wilson mount a few months ago, and suspect that may be causing the problem. Strange thing is, my SWR's with just the Magnum S45HP are good at1.2:1 through the band. I would not have thought I would see such a drastic increase by using the amp.

    73,
    RT307
     
    1 person likes this.
  9. dxing440

    dxing440 Active Member

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    This is the same radio, and amp, i use in the mobile. S45HP and DX500V. And i have the same problem, but Ive used 3 diff antennas and mounts and another radio. SAME thing high swr amp on, 1.2:1 amp off:bdh: I feel your pain. ONLY way to bring the SWR down is to add coax between amp and antenna. BUT everyone says IF the antenna is tuned properly the SWR shouldn`t change. IDK
     
  10. The Mad Scientist

    The Mad Scientist W9WDX Amateur Radio Club Member

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    Hmm ,another one.

    Hello gentelman,

    I just cought this thread on the T.S. 500 swr issue. I have to say that that one was a real pita to get a handle on. There were many issues contributing to that problem, however the amplifier was half to blame. I would recommend changing out those sandbar resistors and those 10uf chokes. they were an immeadiate help. Changing jumpers, grounds, and antennae will not. Changing jumpers will just "fool" the meter into seeing a lower swr, and the "actual" swr WILL remain the same.

    Those replacement chokes that T.S. is using now are wound with a thicker wire and thus bleed off more of the harmonics, It does sound strange but it does. those sandbar resistors are a wear and tear item and spend the majourity of their life in a heated condition and eventually change resistance.

    I am happy to hear that Mike got back to you rather quickly. Mike and Ed are good friends of mine and I have been conversering with Mike about various problems for years.


    I do not recomment adding any padding resistance on the input of those amplifiers as a means of lowering input swr. It is a much better alternative to go after the heart of the issue which would be with the radio...specifically the filter network before the first mixer in most situatuions as the veractor tuned circuits that these radios utilize is not perfect and the filter network may need to be adjusted for the primary set of frequencys that are being used.


    I thank all of you for the positive comments reguarding that problem and I hope the information that I provided will help all with similar issues.

    the updated chokes should be available from maycomm electronics. and the sandbars can be had at most major distributors.

    If you try this and have any further problems please feel free to contact me and i will do my best to help you resolve these issues. good luck to you,


    Bandit35
    AKA
    Mad Scientist
     
  11. 2RT307

    2RT307 Sr. Member

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    Hey MS, you know me from another forum as T.C. :) Thanks for the information. I am really considering shipping both amps to Mike so that I can get them back up and running properly.

    This has been the weirdest thing, since both TS500's have worked flawlessly for me over about 6 years now, with little to no change in SWR's from barefoot to amp powered on.

    BTW, I have several mobile rigs that I have tried with the same setup. Also have several antennas that I have tried. Rigs include President Lincoln, Uniden Grant XL, Stryker 955HPC, and Magnum S45HP. Antennas are Firestik II 5', Skipshooter 4', Everhardt 4', Predator 10K 22" shaft (coil above the roofline of my pickup), and a Wilson 5000 (I know, don't get you started on that one). With any of these rigs/antennas, the SWR's go off the scale when the amp is turned on. Yes, this is with resetting the meter (quickly)!

    I appreciate you tuning in to this thread, and will keep you posted as things progress. Work has been very busy lately, and I have not had the time to really address this issue.

    On edit, forgot to mention that I also tried my Bencher TVI filter in line to see if it might kill harmonics. This did not affect the SWR reading, either.

    Thanks and 73,
    Brett
     
  12. The Mad Scientist

    The Mad Scientist W9WDX Amateur Radio Club Member

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    No problem Brett, I really haven't been working on anything since that problems was adressed.

    Not to supprising that the tvi filter didn't work. Most of them are junk and the ones that do work usually have a minimal effect. It is also hard to get rid of the harmonic problem when there is so much of it being driven into those transistors via the bias devider network.

    Oh, and that's ok. I burried the wilson thing. I still have that thing lying around like some kind of sick war trophy...LOL

    Make sure you shoot Mike an email before you send them over to make sure he has the updated chokes. The last time i talked with him i don't think he had them yet.

    Keep me posted and nice to hear from you again. I hope all is well with you and yours,

    Eric
     
  13. 2RT307

    2RT307 Sr. Member

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    Hey Eric, finally getting around to trying to have these two amps repaired. Can you give me, (and the other readers of this post with the same problem) specific part numbers to order for the DX500? I want to order them myself and take them to my local tech to have him install them. He'll be a lot less grumpy that way. :whistle:

    And if this doesn't work, I may approach GooseRider to see if he'll build a class A w/SSB delay 4 pill amplifier just for grins.

    73,
    Brett
     
  14. The Mad Scientist

    The Mad Scientist W9WDX Amateur Radio Club Member

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    Sure no problem Brett, they are

    R21 25 ohm 5 watt sandbar
    R28 25 ohm 5 watt sandbar
    L1 10uh 1 watt choke(updated version)
    L2 10uh 1 watt choke(updated version)

    You may have to get these directly from Texas Star as I don't know if warious dealers have them yet. Or perhaps you could find these values from a local parts supplier.

    I have been thinking of replacing that bias devider network in favor of a regulated bias supply using the LM1914 regulator. Since you can't use one of these for each transistor I will have to use one for each pair. Doing it this way will keep the harmonic infiltration the lowest it can be.

    73's
    Eric
     
    1 person likes this.
  15. 2RT307

    2RT307 Sr. Member

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    And how do you get ahold of Texas Star? I don't find anything on the web, and lost my warranty cards in the move.

    73,
    Brett
     

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