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Texas Star DX 400 Input SWR

Discussion in 'Amplifiers' started by Shadetree Mechanic, Sep 3, 2018.

  1. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    Hey guys, I just picked up a well used DX400 and the input SWR is 2:1. I put a 460 trim cap in my DX350 and it did really well. With this one, the trim cap only made it worse. I think the layout for the DX500 is the same as the DX400 but I don't know if a 460 is the right thing to use. The cap I am replacing with the trim cap is C20, can someone let me know if that is the correct one? Here is the diagram I used:
    dx400v_dx500v_inter_connection_layout.gif



    Any help is greatly appreciated, Thanks.
    Chris
     

  2. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    This is the cap that was in the C20 position. Is this 330 microfarads?
    magnifier_20180902_165656.png
     
  3. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    And a shot of the guts.
    IMG_20180902_124502490.jpg
     
  4. nomadradio

    nomadradio Analog Retentive

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    That capacitor is 33pf, not 330. The marking at the top is like the 3 color bands on a resistor. 33 followed by zero zeroes. Like orange-orange-black.

    Under that it says 33, but the "pf" is smeared.

    The DX400 (not the "V" version) uses 120pf for C20.

    Is yours a DX400V? Or the plain DX400 without the variable control?

    The diagram you uploaded above is for the "V" version.

    73
     
    Tallman and Shadetree Mechanic like this.
  5. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    Thanks for the reply Nomad, it is just the regular DX.
    IMG_20180902_124438434.jpg
     
  6. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    I looked on CB Tricks and found the right diagram I think. This unit is the Bi-lateral version what ever that means. CB Tricks only had a picture of the Bi-lateral so here is the diagram of the regular DX400:
     

    Attached Files:

  7. Tallman

    Tallman W9WDX Amateur Radio Member, KW4YJ EXTRA class

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    #7 Tallman, Sep 3, 2018
    Last edited: Sep 3, 2018
    Shadetree Mechanic likes this.
  8. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    CB Tricks has the component list and shows C20 as 120pf for the 400 and 330pf for the 500. Maybe the Bi-lateral is different in some way because it uses a 33pf? As shown in my picture, someone has done some work to this and improved the ground around the transistors so I am not sure what else was done. The amp does work and the guy I got it from was using it in his car with the input swr at 2:1. Maybe I need a different value trim cap or maybe replace a different cap?
     
  9. TheRealPorkchop

    TheRealPorkchop Certified Sith Pimp

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    Well I’m certainly not balking at Nomad, no way, hands down but that 33 looks like a 331 with the 1 being smeared or distorted. That would be 330pf and the 00v at the bottom could be 100v or 500v but it’s distorted as well and barely legible.

    Then it clearly says 330 at the top but that’s skewed by extra numbers we don’t know the meaning of. A capacitor tester would be a good thing to have here since it’d tell you what that parts value is currently, though not what it should be.

    Maybe the ground improvements are someone’s attempt to fine tune the amp? And maybe said fine tuning is what’s causing the issue of the 465 not giving you a good value to pick for replacement? I call that having koolaid in it. Maybe the ground improvement has nothing to do with it but maybe there additional koolaid in that thing that you’ve not found yet?

    Seems only two possible choices, try both and see what happens. Better yet, that DX400 schematic is probably exactly like the amp you have, look over the whole thing and see if other values are the same. Probably some jack leg has been in it fine tuning and tweaking with a golden screwdriver.
     
    Shadetree Mechanic likes this.
  10. Tallman

    Tallman W9WDX Amateur Radio Member, KW4YJ EXTRA class

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    I'M sorry I posted the wrong schematic the one I edited is now the correct one. Yours did not have the dial a watt function.

    C20 is listed as 120pf on the schematic for the DX400. Once the cover come off there is no telling what was changed.
     
    #10 Tallman, Sep 3, 2018
    Last edited: Sep 3, 2018
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  11. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    The trim cap I used was a 468 not a 460, sorry if that threw anyone off. The range is 175pf to 680pf.

    https://www.rfparts.com/468.html
    468_trimmer_1.jpg

    I also have a 462 that is 10pf to 80pf. Maybe I will try it next.
     
  12. TheRealPorkchop

    TheRealPorkchop Certified Sith Pimp

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    Well between myself and Nomad, I’d take his advice first and try his suggestion. All I was saying is it’s so distorted that it’s really hard to tell what it is in a picture.

    Try both and see what happens. How are you checking the input match?
     
  13. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    I am checking it with my radioshack SWR / power meter.
     
  14. TheRealPorkchop

    TheRealPorkchop Certified Sith Pimp

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    Anything connected to the output side, antenna or dummy load?
     
  15. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    I am running it with my antenna Sirio Vector 4k. I have it with my Uniden Madison with 2w dead key.
     
: Texas Star, DX400, SWR

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