Ameritron AL80B Troubleshooting

Discussion in 'Amateur Radio Amplifiers' started by N6HA, Apr 4, 2019.

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  1. N6HA

    N6HA XML Subscriber QRZ Page

    I have an Ameritron AL80B that was working. I started noticing some sporadic issues last year but has finally stopped working.
    • Turns on, tube gets hot and glows
    • xmit light turns on and I hear a click from the relay
    • high/max SWR on my radio when switch on OPR with radio set to lowest output
    • very low SWR when switch set to STBY
    • HV reading 2800
    • Plate lg 125ish
    • absolutely 0 RF output
    I've taken it apart and there are no obvious signs of something being burned up. I pulled the tube and it looks like it's in great shape. Nothing looks scorched anywhere. Do my symptoms above indicate anything obvious? What could cause this behavior? What else should I test?
  2. KM3F

    KM3F Ham Member QRZ Page

    Is this condition on all other bands the amp is used on?
    Is the plate current level displayed with no mike gain?
    These are important signs for diagnostics.
    A. The input circuit in key up should present a reasonably low SWR.
    If not the input circuit has a fault or it could be the tube has a short between grid and heater.
    B. If only on one band could be the input circuit, band switch, faulty jumper/connector etc.
    C. If plate current shows without any mike gain, the Auto bias circuit has gone into fault.
    This may be a secondary result of the input fault, therefore two issues could be present at the same time.
    The auto bias circuit should keep the tube plate current cut off with no input signal.
    Good luck.
  3. AA5CT

    AA5CT Ham Member QRZ Page

    Try a different band? Each band has a different input matching network.
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    "Plate Ig 125ish" doesn't make much sense. Plate current is Ip and without RF drive, on the AL-80B it should always be zero until you actually start driving the amp with RF. Ig is grid current, and ditto; should always be zero without drive.

    If you mean Ip (plate current), and it's 125 mA "with" RF drive applied, that's very low. As I'm sure you know, with full RF drive it's normally in the 400-500 mA range. Ig will vary with drive and loading but should also rise and fall with more or less drive.

    My first thought when this amp is being driven normally but produces no output power is the T/R antenna switching/bypass relay contacts being bad, dirty, oxidized, or misaligned. That could allow you to drive the amp all you want and the tube will get hotter, but there won't be any output power. The relay also switches the input to the amp (as well as the output) and bad or misaligned contacts can cause your rig to be looking into "nothing," which would create a high SWR reading at the rig.

    Just because everything is normal in STBY doesn't mean the relay is good; the STBY contact set uses different physical contacts than the XMT contact set.

    That's just first guess.

    You can't tell if the tube is good by looking at it. Most "failed" 3-500Zs didn't fail due to overheating, they fail due to an internal arc created by ionization of the gas inside which shouldn't be there.:p However, a common demonstration of that failure is blown fuses.:)
  5. N6HA

    N6HA XML Subscriber QRZ Page

    • Same on all bands
    • Fuses aren't blown
    This is with the switch on "hv" and in standby mode with no power


    When I get home after work I'll test all the relay contacts.
  6. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    The grid meter is also pinned back against its stop... looking at the scheme and thinking about that one.
    Last edited: Apr 4, 2019
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Based on this new information, I doubt it's relay contacts.

    This is from the guy who designed that amplifier:

    2.) Grid meter moves backward, and Ip meter reads forward, with no drive and amplifier in standby

    • Tube has a filament-grid short from shipping damage or tube manufacturing defects
      • A shorted grid-filament shows as little or no RF output power, and high transceiver SWR
    • Tube has or had an internal arc from gas or air leakage
      • If a tube arced, a filament circuit component may have shorted to the chassis. This could be a MOV on the filament winding of the transformer, or a gas protection clamp tube under the sockets
      • A defective bypass capacitor may be shorting the filament or the filament center tap line to ground
    • A backwards moving grid meter that never goes forward is always an indication of a filament to ground shorts, either inside or outside the tube
  8. N6HA

    N6HA XML Subscriber QRZ Page

    Definitely matches the symptoms. Where is that text from? Seems like that would be a useful document to have.
  9. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

  10. VE2GCE

    VE2GCE Ham Member QRZ Page

    Chris, thanks for posting the W8JI link.
    I did not know MFJ purchased Ameritron.
    That explains that cross-needle meters on the AL80B which I like from the MFJ antenna tuners.

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