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Ameritron AL-572 Dead

Discussion in 'Amateur Radio Amplifiers' started by KA1KA, Apr 26, 2021.

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  1. KA1KA

    KA1KA XML Subscriber QRZ Page

    I made an "oops" with my AL-572. Tuned 30 meters while band switch was on 40. Stupid. Didn't realize until after it wouldn't tune and the crackling started. Arcing was happening in what sounded like the rear of the amp near the tubes. Then stopped. Now, amp has no power output. Turns on, all tubes glow. When keying, grid current will rise proportionally with power level until it pins the meter. Plate current never moves. No output. I'm an automotive tech not an electronics tech :) I need advice on what to troubleshoot (or where to start) without killing myself in the process. I appreciate any advice in advance. 73, Adam
     
    K0UO likes this.
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Does the panel meter indicate presence of high voltage (should be around 2500Vdc or something)?

    That would be a pointer to exactly where to start looking for a problem.

    Funny part is, driving the amp on 30m with the bandswitch on 40m really should not have caused damage -- I'm trying to think why it would...the tuned input circuits should have created a big mismatch to your transceiver and caused the transceiver to reduce its output to something very low, thus not really "driving" the amp.

    "Switching" (turning) the bandswitch while transmitting can cause a lot of problems.
     
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  3. KA1KA

    KA1KA XML Subscriber QRZ Page

    Thanks for the reply! Ok yes there is indeed about 2500 VDC on the HV meter. Yeah I definitely didn’t turn the knob mid tune. I attached a photo of the only thing I could see that resembles a burn mark inside.
     

    Attached Files:

  4. N0NC

    N0NC Premium Subscriber QRZ Page

    The only indication of a burn mark I see is about 1/3 of the way up the plate choke. Kinda hard to see in between the tubes. make sure the power has been off a while, and the capacitor bank is discharged. then you your ohm meter to measure from the top to the bottom of the plate choke (that tall red coil in the center). It should read some low value of resistance. If it is open (very high resistance or infinity) then that removes the plate voltage from the tubes. DON'T attempt to measure the voltage on the top of the coil unless you have the proper equipment (high voltage probe). If you don't feel safe doing this, seek some help from a more knowledgeable ham. Be very careful, there are voltages in there that can KILL YOU.

    73, Chuck
    N0NC
     
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  5. W1QJ

    W1QJ Ham Member QRZ Page

    If you have no plate current that could indicate there is no HV getting to the tubes. You may have burned the plate choke open.
     
    K0UO likes this.
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    As already mentioned...just because the power supply is producing the 2.5 kV doesn't mean that's getting to the tubes.

    The meter measures before the tube connections.

    With the amp unplugged and fully discharged (HV meter should read "zero," which it should do a few minutes after turning it off), might use an Ohmmeter to measure from the HV power supply output connection on the power supply circuit board to one of the 572B anode caps. Resistance should be very small, maybe a couple of Ohms and indicate "nearly" a short circuit for that test.

    If it doesn't, something is disconnected, and quite possibly the big plate RF choke.

    I'm still trying to figure out why the amp could even be driven on 30 meters with the bandswitch set to 40 meters.:p
     
  7. KA1KA

    KA1KA XML Subscriber QRZ Page

    Ok, so with power disconnected and cap bank discharged I can perform resistance measurement? Looks like a replacement plate choke is readily available so it might be worth trying this before shipping to Ameritron and a long wait.
     
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Absolutely.

    The problem is likely simple and easy to fix. If it's the plate RF choke, yes Ameritron should have them and last I checked, RF Parts had the same one in their catalog as well.
     
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  9. KA1KA

    KA1KA XML Subscriber QRZ Page

    Ok, should I try to further discharge the unit in addition to unplugging and watching HV meter? Just want to make sure I properly discharge this (and not through my organs).

    It seems if there is a way to break it I'll find it :)

     
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    With the HV meter at "zero," any charge left in the filter capacitors should be non-lethal (a few volts), but shorting the B+ to chassis ground anyway isn't a bad idea.

    When one suspects the presence of HV, using a "shorting stick" that keeps your hand far away from everything is the norm. But for lower voltages that cannot contain much energy, I just use a screwdriver with a well insulated handle.:p

    Being more careful never hurts, even if that takes a bit longer.
     
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