Completely dead AL-811

Discussion in 'Amateur Radio Amplifiers' started by WX3P, Feb 12, 2018.

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  1. WX3P

    WX3P Premium Subscriber QRZ Page


    Was working fine - shut off and came back a few hours later

    Won't power on - fuses good, switches good. AC to circuit board good

    I've looked over the circuit and can't see any other components that could be bad other than the transformer.



    Jack WX3P
  2. W1QJ

    W1QJ Ham Member QRZ Page

    Low level problem, you are missing something. Maybe a bad outlet or wire on plug bad.
  3. G3YRO

    G3YRO Ham Member QRZ Page

    Have you checked BOTH mains fuses? (never understood why these have a fuse in BOTH the mains leads !)

    What about the microswitch that interupts the mains feed when the case is off? I bet that has failed (gone open circuit)

    Do you have a soft-start circuit installed, which has gone wrong?

    Can you measure AC across the transformer input winding?

    If so, you wouldn't usually expect no output from ANY of the output windings on the transformer ! (even if one has gone open, which is unlikely)

    Transformers that fail usually go shorted-turn (and fry themsleves), not open circuit !

    Roger G3YRO
  4. K6BSU

    K6BSU Ham Member QRZ Page

    First thing I would do is measure resistance across the AC plug, with switch ON. Sudden failure would indicate broken wire or a fuse.
  5. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    The AL-811 has a high voltage indicating meter right on its front panel.

    Does it indicate "no voltage?"

    Are the tube filaments lit or dark?
  6. G3YRO

    G3YRO Ham Member QRZ Page

    As I said before, my money is on the Safety Microswitch that interupts the mains feed . . .it's located near the mains fuses on the back panel.

    Either it has gone open circuit . . . or the case is no longer pressing down on it properly!

    (if I'm working on one of these Amps I just solder a wire across it)

    Roger G3YRO
  7. WX3P

    WX3P Premium Subscriber QRZ Page

    Thanks all

    Ok - update - a previous owner had installed non-standard fuses it appears. Replacing with correct fuses, I get momentary power and then one fuse blows.

    I am going to remove the tubes and see what happens
  8. K0OKS

    K0OKS Ham Member QRZ Page

    Test the capacitors one by one out of circuit (after checking them with a meter to be certain they are discharged before touching. Ideally you test them with an LCR meter. However, if one is shorted then an ohm meter should show this as a low resistance that does not increase over time (from a good capacitor charging off the meter's power source).

    If one is bad it is best to replace them all, and the bleeder resistors while you are at it.
  9. WX3P

    WX3P Premium Subscriber QRZ Page

    Bad tube - Thanks all
  10. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    With 811As, this is no big surprise.

    If you replace them with 572Bs, they'll last forever (and fit the same sockets, no modifications required).

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