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where is the input tuning cap located

Discussion in 'Amplifiers' started by groundwire, Dec 27, 2017.

  1. groundwire

    groundwire Member

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    dx400v_dx500v_inter_connection_layout.gif hey guys, i have a new texas star dx500v and i want to replace the fixed value input tuning capacitor with a variable capacitor so i can tune the high input swr as flat as possible. i need to know which one it is. im not an amp guru so i need help with this one. i put a parts layout with numbers so you can see. is it c22 or c31 etc.. thank you for all your help...


     

  2. bob85

    bob85 Supporting Member

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    C20
     
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  3. Ranch55

    Ranch55 Well-Known Member

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    Yes, C20 ..........
     
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  4. loosecannon

    loosecannon Sr. Member

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    i would replace C20 with one of these: https://www.rfparts.com/468.html

    you don't have to get it from where i linked, as they should be available from many sources.

    why is your input SWR so high?
    LC
     
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  5. Onelasttime

    Onelasttime Well-Known Member

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    I use Arco variables in my builds but I think a lot of TS owners and repair places frown on leaving an Arco in place. I think they like to use them to find the exact value for that specific build than install fixed value silver dipped mica's in it's place once they have the value. Use a non-conductive sealant that can be removed later on.
     
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  6. groundwire

    groundwire Member

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    yup, thats the part i was gonna use. as far as the input swr goes, every texas star amp i have ever owned has had a moderate to very poor input tuning. they always have high input reflect for some reason. my dx500 is not real bad, if i remember right its like 1.4 swr (approx. 5w reflect) thats way to high for me.

    yea i have not decided yet what to do on that issue. i have seen quite a few amp circuits that use variable input tuning caps permanently without issues, (texas star dx65, palomar, messenger m4v etc) i dont see the problem. also like you said, i could find the correct value with a variable then put a fixed value cap in its place. i will probably go with scenario #2, find the value with then switch it out.
     
    #6 groundwire, Dec 28, 2017
    Last edited: Dec 28, 2017
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  7. loosecannon

    loosecannon Sr. Member

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    not sure why people frown on leaving the trimmer in place.
    It's not like there haven't been many linear amps designed and built with trimmers.
    I'm always open to learning something new though.

    one thing on using the trimmer; before installing it, use an ohmmeter to determine which solder lug is connected to the screw head that will be adjusted.
    This is the end of the trimmer that you should put towards ground.
    That way you won't detune the circuit by using a tool on the screw head.

    groundwire, you stated that you now have a 1.4 SWR going into the amp (which BTW, many of us find perfectly acceptable) and then you stated that this equaled a 5 watt reflect.
    exactly how many watts are you putting into this amp?

    one other thing to put out there just in case. don't forget to look at the power coming out of the amp while you are tuning the input.
    LC
     
  8. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    Agreed. I have a Mirage 2m amp that has trimmers on the input and output and if it is stable on 2m trimmers should be fine for 11m.
     
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  9. nomadradio

    nomadradio Analog Retentive

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    We got in the habit of putting a trimmer cap on wire legs, tack-soldering them to the input circuit. Removed the trimmer and measured the capacitance after it was set for minimum input-side SWR. This can be tricky with a mica compression trimmer. Moving the leads around can make the capacitance shift. But if the adjustment screw's setting is fairly tight when you unsolder it, the measured capacity should be fairly accurate. So long as we have a fixed capacitor with a value within about 10% of what the trimmer measured, it generally works just fine. Mica compression trimmers are okay, but an adjustable part is still a failure point down the line. Moving parts tend to fail first. If it will be run on a desk as a base amplifier, no problem. But if it's going into a dump truck, the fixed capacitor will take longer to become a problem than the variable one will.

    73
     
  10. groundwire

    groundwire Member

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    dont hold me to those numbers, im just going off the top of my head. rf input into the amplifier from the radio is 2w carrier 15w pep. just normal input drive
     
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  11. loosecannon

    loosecannon Sr. Member

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    gotcha.

    just for some perspective, a 1.4 SWR equates to a 2.8% power reflection.

    so with a 2 watt carrier you are looking at .056 watts reflected.
    LC
     
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  12. 338_MtRushmore

    338_MtRushmore Well-Known Member

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    LOL, Must be those cheap Chinese transistors. Pure junk I tell you, pure junk.
     
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  13. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    Hey guys, I just got a DX350hd and I am having the same input SWR issue. I was able to coil up some coax to make it work, but I would like to install the trimmer cap. I have compared the diagram of the 500 to the diagram of the 350 and it looks like I would need to replace C20 with the trimmer cap. Would this be correct? Also would I need the same one used above?
    https://www.rfparts.com/468.html
    Thanks!
     

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  14. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    Now its starting to look more like C8 would be the one to replace with the trim cap. I will keep studying it.
     
  15. Ranch55

    Ranch55 Well-Known Member

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    Yep, C8 would be the one......

    [​IMG]
     
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