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Texas star 350dx high drive keeps burning input rf choke..

Discussion in 'Amplifiers' started by Myst-hic, Apr 5, 2021.

  1. Myst-hic

    Myst-hic New Member

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    I was given the unit from a friend that hadn't used it in 15 years, as with everything old I generally change electrolitics and any other checks I know before firing something..i did exactly that, pass thru swr remained the same 1/1.2 with dummy load engaged, bias voltage manually keyed was .54v, ohms at base 1.9..yes, I saw that as maybe a bit high? Anyhow when switching to antenna to test, immediately when I turned on power the input 10uh rf choke smoked..it has been replaced before I can see. Any ideas as to what to check next? I have the replacement chokes..antenna issue maybe? It's an Antron 99 grounded to copper pin to earth yet im.sure my metal roof is in its groundplane..still working to get that another 10 ft higher soon..so what do o check next? Any input is much appreciated, thanks!


     
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  2. Shockwave

    Shockwave Sr. Member

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    Unfortunately, your Texas Star has retired from amplifier service and converted itself into an oscillator. This is rather common for a TS amplifier and your case is a bit worse than most being that it's self oscillating (starts on power up with no drive applied) and sustained (burns choke up and probably holds relay keyed). The first thing to check is that all the screws are tight on the PC board grounds, chassis and SO-239 connectors.

    After that, you should probably take a picture and post it so we can see if there is anything that obviously stands out. How does the 10uh output choke look? Are the negative feedback resistors and caps intact? Sometimes these boards burns open traces on the bottom of the board. Especially if someone replaced chokes without removing the board, and soldered the wire too far into the board one time. Then, it can short out under the board.
     
  3. Myst-hic

    Myst-hic New Member

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    Thank you shockwave, the point you made that really stands out is the fact that whomever replaced the choke before obviously did not pull the board, as I have not either...ive been perplexed if there were traces on the backside because it looks like no board I have ever seen before..looked like top traces to me..all resistors look fine, I've tested about half of them..i don't believe it is staying keyed, powered up with no input or output is how I could take the bias readings..manually keying the key transistor..there are 3 in parallel that should be making 1.3 ohms or so, base to ground, it is reading 1.9..is this acceptable? If not I need to remove them out of circuit and test individually..i will see if I can pull the board and take pics of top and bottom today. All ceramics are intact, yet charred in that upper left around the choke, the first choke burned open, the replacement I spliced on the same as someone did before starts to smoke and I shut her down..i will pull my replacement and clean that area before taking a pic. One question I have today is, I'm just below the 9'ground counterpoise my Antron requires and have no choke balun, could that be an issue? I find it odd I can hook to my 25 watt homemade dummy load up plus reciever, turn it on, recieve and all, and it won't blow the choke..although I cannot key it without starting a fire on my bench I was going to try making an air choke balun today, but if your saying check the backside of board i will attack it that way instead, thank you!
     
  4. Shockwave

    Shockwave Sr. Member

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    I wouldn't pull the board unless I suspected blown traces. An amplifier that oscillates, typically behaves well on a dummy load because it is a purely resistive load. Because your bias voltage is only .54 volts, I would not think the resistors are contributing to this problem. The bias is a little low and would need to be on the high side to be an issue with this problem.
     
  5. Myst-hic

    Myst-hic New Member

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    Well all of that is good to know..i don't really want to pull the board and shotgun unless I had to. This is a horrible pic I took last night..after i replaced the choke..after a few "tests" the repkacement is now showing an swr of 3 from radio to amp, so i will take it out and clean it up, then take pic before replacing anything. Do you think it's worth a shot putting it in my truck with an old cobra and Wilson 500? Just to test if it's an antenna/ground rf issue?
     
  6. Myst-hic

    Myst-hic New Member

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  7. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    I would not run it with any radio that I liked with a 3:1 swr, it will not end well. If all the components check good, then I would pull the board out. I have heard of loose solder blobs and debris getting stuck under the board causing problems.
     
  8. Myst-hic

    Myst-hic New Member

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    Yes, exactly! I have a crap newer cobra 29 i use for the tests not going to hook my old Washington up to it till I have it thru multiple test runs! I totally understand the point of the ease it would take to bridge that 1/8 or less gap from board to chassis..i might try to put in my truck before dark and test, at 3 swr it still is testworthy, I have 3 more chokes left for tests, I hope I don't have to use them all thank you for your input shadetree!
     
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  9. Ranch55

    Ranch55 Sr. Member

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    If you know that the bias resistance is not correct, fix that first.
    Replace the 1.6 ohm, both 10 ohms, and check the 25 ohm.
    You need to get 1.3 ohm bias resistance, + or - .1 ohm.
    You need to get .63 volts bias, + or - .1~.2 v .....
    Using any more chokes will not find the problem.
     
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  10. Myst-hic

    Myst-hic New Member

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    T
    Thank you, that is some exacting info! Nowhere could I find the tolerances..unfortunatly I'm probably going to have to order the resistors if i find any out of spec, so this may take awhile, I will pull some legs right now and see what I can find.
     
    Shadetree Mechanic likes this.
  11. Myst-hic

    Myst-hic New Member

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    Update, same issue in the truck, I found my ohm meter to be .5 high testing on brand new resistors as I went to replace the resistors "out of spec". Bought a new meter, all is well, I am getting 1.2 ohms now on my bias when I hooked them all back on the board, so that's not the issue...what I cannot do at this time is recheck bias voltage because I have taken the rf choke out and cleaned the board. My next question is, looking at my photo, where I have the pointer, does that look like a broken/burned trace?
     

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  12. Ranch55

    Ranch55 Sr. Member

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    I can't make out the photo.
    Not a clear photo.
     
  13. Myst-hic

    Myst-hic New Member

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    Yeah, I have a hard time taking good shots with my phone..doesnt matter anyway..i scraped the sludge of with a pic, it wasn't a blown trace. I pulled the board, thought the old pin left from original choke had a cold solder joint, I replaced the caps in that section, new choke, put it together, same deal, I shit it down before it got too hot. The only issue I see if bias voltage related, still .54...im still researching what would cause that.
     
  14. 999

    999 Well-Known Member

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    upload_2021-4-7_19-31-35.png
     
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  15. 999

    999 Well-Known Member

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    upload_2021-4-7_19-32-31.png
     
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