AL-80B taking out finals....

Discussion in 'Amateur Radio Amplifiers' started by AE1P, Jul 3, 2020.

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  1. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    I downloaded and zoomed your latest pic and the mica caps are 100 pF caps 500V 5% SM (Silver Mica) types.

    On my computer the resistor Bands were rendered Red-Black-Red.


  2. W1QJ

    W1QJ Ham Member QRZ Page

    Neil, if you are listening, if either of those mica caps are open circuit it will render the resistor which is 100 ohms out of circuit. That would make the parasitic suppressor useless and the amp will definitely “ take off”. If you can lift one leg of each cap and measure it that would tell a lot.
    K4AGO and AE1P like this.
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Even my dog does that (lifts one leg).

    He's a male.:)
    AC0OB likes this.
  4. AE1P

    AE1P XML Subscriber QRZ Page

    Thank you Lou...
    So...C1 measured 100 pf... C5 measurement was erratic,
    it would read between 30 to 120 pf, not stable like C1.
    I'm going to replace them both seeing as I have them both lifted...

  5. W1QJ

    W1QJ Ham Member QRZ Page

    Ok, if C5 is flaky that could cause problems indeed. At one point you said the tube was getting red and then it stopped. Something is changing and it could be C5. I would most definitely change C5 for sure. As was previously mentioned you could easily fashion an old type suppressor out of a 2 watt 100 ohm carbon comp resistor with the right amount of turns around it. Since old carbon comp resistors are hard to get these days and are expensive if you can find them, obviously it was cheaper for Ameritron to use the resistor they use now and add the caps t null the inductive reactance of the resistor which is what those caps do. People who have a stash of old 2 watt carbon resistors want between 2-5 dollars for a single resistor. Now you can why Ameritron wet this route. It cost them less than that to fashion that little PC board with the parts.
    AG5CK and AE1P like this.
  6. KM3F

    KM3F Ham Member QRZ Page

    w1qj, you just piqued my interest in testing some different type resistors on a VNA to see what/if the reactace shows up vs frequency sweep and by how much.
    If so, that should offer a clue to the series value of the caps that would be needed for such an application.
  7. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    AE1P likes this.
  8. AE1P

    AE1P XML Subscriber QRZ Page

    "re-measure the resistor value while the caps are out"

    Which I did... 99.9 ohms...

    AC0OB likes this.
  9. W4JFA

    W4JFA Ham Member QRZ Page

    Looks brown black brown to me.
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    To me, too.

    Remember home monitors are not color corrected.

    You can buy a "studio grade" monitor which is color corrected, but they cost a lot more. I have four of them at work, used for video post-production including color correction.

    Anyway, it seems the resistor is fine and something else is wrong.
    KW6LA likes this.

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