AL-80B taking out finals....

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

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  1. W1QJ

    W1QJ Ham Member QRZ Page

    If you have a dummy load connect amplifier to antenna and connect dummy load on the input. Key the amp into transmit with a foot switch or a shorted phono jack. You should get the xmt light coming on and no plate current or grid current. If anything happens get meter readings. You will safely be checking for oscillations and also if the electronic bias is bad. Report findings of this test.
    K8PG and W1TRY like this.
  2. AE1P

    AE1P XML Subscriber QRZ Page

    Will do...thank you, Lou
  3. AE1P

    AE1P XML Subscriber QRZ Page

    Ok, so as per instructions...when keying amp, no readings on plate or grid, no readings on reflected...
    HV looks ok, and alc set is working...
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    That's good.

    You might try the same test with the dummy load connected to the output and "nothing" connected to the input and tune the PLATE tuning and LOAD control through their ranges on each band to confirm the amp generates no plate current and no output power no matter where you set the tuning controls.

    I doubt "input tuning" has anything to do with blowing the "finals" in your rigs; if the input match is awful due to a failure on the input network board, those rigs should just fold back power and reduce output and not be damaged. I believe the problem described was caused by something else. I have an AL-80B and have intentionally driven it on the wrong band dozens of times (testing) where the input VSWR was nearly infinity and it certainly didn't bother any of my transceivers; they just produce very little or no output power, but nothing is damaged.

    If power generated by the tube becomes disconnected from the output connection somehow, bad things happen fast. I'd open 'er up (powered OFF, and AC unplugged) and use a good light and perhaps a magnifying glass to observe all connections from the tube anode cap/cooler through the plate tank components to the T/R relay in search of a bad or loose connection, any signs of arching, etc. Especially inspect the relay, which on older AL-80Bs is just a single 3PDT open-frame relay in the rear left-hand corner (there's another relay near it, which is the AC line step-start relay, and that would not be the problem here). Because 240Vac is "touchable" even with the amp OFF and discharged, when screwing around in that area be sure the amp is unplugged.

    If somehow that relay screwed up and could bridge the output contact back to the input contact -- boom. I've never seen that happen, but imagine "anything's possible."
  5. W1QJ

    W1QJ Ham Member QRZ Page

    Ok, well that means the electronic bias is ok. Still doesn’t mean the amp isn’t oscillating when the electronic bias turns on. That test basically the electronic bias. The only time I’ve ever seen an amp blow the finals in a radio is when a major oscillation occurs and even then it usually doesn’t but could. You certainly don’t want to keep hooking up radios. Most radios are well protected and will protect itself on either a short or an open in the antenna line. Damaged finals is usually an oscillation getting back into the radio. How does the parasitic choke resistor look? Had it been changed from the stock suppressor!
    KD2ACO likes this.
  6. W1QJ

    W1QJ Ham Member QRZ Page

    Steve, with electronic bias in the later models keying the amp with no drive power doesn’t bias the tube on like in your older model so your test wouldn’t show much with an open input like it would in yours.
  7. AE1P

    AE1P XML Subscriber QRZ Page

    You certainly don’t want to keep hooking up radios...

    You got that right , Lou...2 dead radio's is enough.:eek:

    I will pull cover shortly and do a detailed-thorough inspection.
    I bought this Amp brand new from Salem HRO,
    nothing has been changed..100% stock

    And thanks for your input, Steve, very much appreciated...
    KD2ACO likes this.
  8. AE1P

    AE1P XML Subscriber QRZ Page

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

    Resistance of 65 ohms...

    Checked tube with DMM...continuity from pin 1 to pin 5...
    open from pin 1 to all other pins...from pin 5 to all other pins...
    so I believe the tube is not shorted...
    Parasitic choke resister looks fine, I'm getting a reading of 100 ohms
    Not seeing anything obvious, nothing vaporized, no burn marks...
    I'm stumped...

    Neil, AE1P
  9. W1QJ

    W1QJ Ham Member QRZ Page

    I wonder if one of the mica caps on your suppressor opened up? Can you measure capacitance?
  10. AE1P

    AE1P XML Subscriber QRZ Page

    Not until Monday, have a fluke meter at work that will read capacitance...
    I'll check the mica caps then...

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