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Texas Star DX 667V Issues

Discussion in 'Amplifiers' started by skiman1, Jul 13, 2019.

  1. skiman1

    skiman1 Member

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    I have an older nice and clean DX 667V with Toshiba's and no output, but everything else seemingly "works" as it should otherwise. I followed some of the steps here in this other thread and got as far as I could in a short amount of time;

    https://www.worldwidedx.com/threads/repairing-a-texas-star-667.164712/



    Observations:

    - Receive preamp works without issue, yay!!!! The one thing I don't care about works like a charm lol!!!!;)

    - When radio keyed the relay engages, and bulb behind the meter works

    - SSB Delay engages like normal when the button is engaged and when I key up on SSB and talk into the mic it acts like it should.

    - SWR and power output with the amp off/bypass is fine (currently 2w deadkey) but when the amp engaged, power output drops to about 1/8 to 1/4 watt with Dial A Watt not engaged, and with it engaged, power is about half, as one would expect the circuit to do, putting out around an 1/8 of a watt, and when I turn the variable with D-A-W engaged and off, it affects the output as expected and with smooth taper, so don't "think" the pot is fried.

    - Per the transistor testing in that thread, I have proper B+ on all 5 transistors, but some leakage of 0.21 (less than a 1/4 of a volt) on the front two 2879's and the 2290 driver, but show 0.0 on the back two, so wouldn't consider it true leakage, but since all 3 of the front ones have the same exact leakage amount, I would think its from whatever the issue is more than the transistors being the issue?

    - I didn't get to the point of floating the transistors as stated further on since I didn't have the time and not actually sure if I had to if they had proper B+ voltage in the previous step.

    - In one of the other steps it advises to to to test the relay by shorting the resistor behind the relay to ground, and all other testing it seems OK, and of course relay engages as it should when I did this step.

    Again its super clean inside, nothing obviously burnt or smell. Any suggestions where to go next?

    Gut shots attached, thank you for any help you can share, hopefully one day through the help I receive here I can be as helpful as some of you guys, pretty green on this amp stuff!:D

    Thanks, Skiman1
     

    Attached Files:

    #1 skiman1, Jul 13, 2019
    Last edited: Jul 13, 2019

  2. sp5it

    sp5it Master of puppets

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    Remove driver and finals and measure them with multimeter. Mike
     
  3. skiman1

    skiman1 Member

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    From what I've see on how to do it, its beyond my skills and I don't trust myself to do it cleanly, looks fairly involved for a newbie, if that's what it comes to I will have to send it out for repair.
     
  4. nomadradio

    nomadradio Analog Retentive

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    I would not recommend this as a 'first-time' linear project.

    This amplifier is not very forgiving if you get it "almost" fixed. It can blow out a brand-new set of the expensive RF transistors if you miss one bad part.

    You'll never guess how I learned this.

    Now I check that resistor before applying power with new parts installed.

    73
     
    AudioShockwav and skiman1 like this.
  5. 999

    999 Well-Known Member

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    The variable on the 667 is always on. The power is dropping when you push that button in because there is a resistor connected to it that drops the amp down in power. (that button should read AM).

     
    skiman1 likes this.
  6. nomadradio

    nomadradio Analog Retentive

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    If you're lucky, only the driver transistor is blown.

    But the way the driver is wired up, getting the transistor's base and collector leads unhooked for testing is kinda gymnastic. Putting them back is a PITA.

    The output combiner in the pics looks shiny and unburned. That's a good sign for the four 2SC2879 final transistors. Most of the time one pair will fail before the other pair. When that happens, the combiner will overheat big time. Burns it to a crisp if you keep using the amplifier that way.

    73
     
    #6 nomadradio, Jul 14, 2019
    Last edited: Jul 14, 2019
    AudioShockwav and skiman1 like this.
  7. 999

    999 Well-Known Member

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    And I would get that 2 watt dead key from your radio down closer to 1/2 a watt. Your amp will live much longer.
     
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  8. nomadradio

    nomadradio Analog Retentive

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    Did I read this wrong?

    73
     
  9. skiman1

    skiman1 Member

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    Nope, there is no output from the amp.
     
  10. skiman1

    skiman1 Member

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    If running the 667, normally the radio (Taiwan Cobra 148) would be down to 1.25w, but the old Bilinear Non-Variable DX 600 with mrf454's that I'm currently using won't even trigger with less than 2w's. Runs great with AM button depressed with a 100w carrier and swings up to about 400-425.
     
  11. skiman1

    skiman1 Member

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    I know, I've watched a video of taking them out and to me its just not something comfortable doing, and the amp looks clean, nothing fried or abused looking.
     
  12. nomadradio

    nomadradio Analog Retentive

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    RF Power transistors tend to look perfect after they break down inside. The white ceramic body is far tougher than the circuits inside.

    I have a box containing hundreds of blown RF transistors. All but a couple of them looked just fine, but tested blown.

    What we do with this model is to first hot-wire the radio drive directly to the splitter feeding the 4 final transistors.

    Doesn't exactly turn it into a DX500, but if the big transistors are okay you'll get power and the output combiner won't get hot and smoke.

    Gotta look and see if we have any pics of this procedure.

    Troubleshooting tends to start by identifying which parts are working, until you find the ones that are not.

    73
     
    #12 nomadradio, Jul 15, 2019
    Last edited: Jul 15, 2019
    skiman1 likes this.

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