AL-80B taking out finals....

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

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  1. AE1P

    AE1P XML Subscriber QRZ Page

    My trusty 15 yr old AL-80B needs to be put on the bench...
    during key up, it has fried not one, but 2 radio's finals...
    Antenna was resonant, load & plate were correct...
    and with about 60w of drive power...
    Instantly fried the finals...(dead shorts on both FETs, both radio's)
    Any thoughts on what to start looking for?...
    I would think that if a bad tube, I just wouldn't have any drive
    out of the amp?...
    Regards,
    Neil
    AE1P
     
  2. W1QJ

    W1QJ Ham Member QRZ Page

    I would think a severe oscillation is causing this. You may have a bad parasitic choke. Either that or somehow you are feeding RF back into the radio. Do you have the QSK mod in it?
     
    W1TRY likes this.
  3. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    I always place a low power Tuner between any transceiver or transmitter and the linear to determine the actual Forward and Reflected power.

    Was your AL-80B input filter ever adjusted to each midband?

    You didn't state your type or brand of transceiver but most have an internal ALC that folds back power in case the SWR goes up.


    Have you disconnected the coax and measured the input resistance? The resistance on the input connector should be infinite since the coupling is totally capacitive as per Schematic on Page 37.


    Pheel
     
  4. WG7X

    WG7X Platinum Subscriber Platinum Subscriber QRZ Page

    Maybe a relay problem? IOW hot switching? Or like Lou said, oscillations...
     
  5. AE1P

    AE1P XML Subscriber QRZ Page

    No, I do not have the QSK mod...
     
  6. AE1P

    AE1P XML Subscriber QRZ Page

    "You didn't state your type or brand of transceiver"

    Elecraft K3S, and Kenwood TS-590sg..
    Input filter never adjusted...

    Regards,
    Neil, AE1P
     
  7. AE1P

    AE1P XML Subscriber QRZ Page

    "feeding RF back into the radio"...
    Certainly something that has failed in the AL-80B then,
    as it has been inline, with no changes to cables and antenna
    and working fine until it blew finals in K3...thought maybe
    something wrong with radio...hooked up Kenwood... puff...
    same thing, instantly blown finals...
    possible cause?
    Thank you
    Neil, AE1P
     
  8. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page


    Again:

    Check the input resistance with a good DMM to the power Input SO-239.

    Also check the capacitors in the input Pi-net filter as described on Page 37 of the AL-80B manual.

    Please let us know what you find.


    Pheel
     
  9. W1QJ

    W1QJ Ham Member QRZ Page

    If you have a newer 80b it’s got electronic bias. Do you have a dummy load?
     
  10. AE1P

    AE1P XML Subscriber QRZ Page

    yes
     

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