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Another Faulty Ameritron AL-11H

Discussion in 'Amateur Radio Amplifiers' started by G3YRO, Aug 5, 2017.

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  1. G3YRO

    G3YRO Ham Member QRZ Page

    Well as my other thread has been hi-jacked, let's post the symptoms, in the hope that somebody can help !

    Filaments are all on OK, and there is HT on the Anodes . . .

    But I have NO standing Anode Current on Transmit . . .

    And no backwards Grid Current on Standby (like when you have a filament short somewhere) . . .

    I have confirmed that Relay 1 IS going over OK . . .

    However, applying 10 watts of Drive, both Anode and Grid Current Meters stay at zero . . . and a there's a high SWR into the Amp.

    Applying 50 watts of drive, there is a very slight indication of Anode current (25mA?) and the Grid Meter goes backwards . . .

    I'm at rather a loss ! I'd really appreciate any suggestions from anyone.

    Roger G3YRO
     
    AF6LJ likes this.
  2. KA9JLM

    KA9JLM Ham Member QRZ Page

    Did someone cut the wrong green wire ?

    Good Luck.
     
  3. W1QJ

    W1QJ Ham Member QRZ Page

    The tubes are not being biased on so that leaves problem in the EBS circuit. Maybe the 3904 or 3055 transistors.
     
  4. AF6LJ

    AF6LJ Ham Member QRZ Page

    This should be a simple find acording to the schematic there is a simple stick of diodes for bias a meter shunt and a set of relay contacts.
    Looks like from what Lou is saying the MFJ manual is not up to date.
     
  5. AF6LJ

    AF6LJ Ham Member QRZ Page

    Helps to download the right manual. :)
     
  6. KA9JLM

    KA9JLM Ham Member QRZ Page

    My manual has errors, But they are fairly easy to spot.

    I wonder if there is a correct schematic available ?
     
    AF6LJ likes this.
  7. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Hi Rog,
    The schematic currently on the MFJ website may not be for the version of amp you have in front of you. If you take a look at the grid connections, is there an RC network to ground or are the pins directly grounded? Knowing which version it is, we can dig up the correct schemo. It may be at W8JI's website or at least, he gives a thorough walk through of the changes that happened over the years to that amp.

    http://www.w8ji.com/ameritron_811h_572_amplifier_trouble_shooting.htm

    http://www.w8ji.com/al811_important_modifications_changes.htm

    etc.

    Lou's explanation sounds like it's in the ballpark.
     
    Last edited: Aug 5, 2017
    AF6LJ likes this.
  8. G3YRO

    G3YRO Ham Member QRZ Page

    The circuit seems to be as per the latest MFJ circuit (this is quite a new amp, only a year or two old) . . .

    There is an RC Network from the Grids to Ground . . . the amp hasn't been modified

    In fact everything seems identical to the other one I just finished repairing (but different faults!)
    As I posted previously, Relay 1 IS going over when you switch the Amp to Transmit (and connects the input antenna socket to the input circuitry), so the 2N3904 is OK.

    Is it possible that the other contacts on this relay have welded . . . so that the Valves are still biased off when you switch to transmit?

    If so, would this produce the symptoms described? ie with 50 watts of drive, hardly any increase in indicated Anode current, and the Grid current going backwards? (don't see why that would happen!)

    Roger G3YRO
     
    AF6LJ likes this.
  9. KA9JLM

    KA9JLM Ham Member QRZ Page

    :)

    How much power out do you get in standby ?
     
    Last edited: Aug 5, 2017
    AF6LJ likes this.
  10. G3YRO

    G3YRO Ham Member QRZ Page

    It's fine straight through (on Standby) LOL !

    I've just disconnected the white wire from the bias diode string that goes to the filament transformer centre-tap . . . and no change !

    Ie still no standing current . . . and when you apply a LOT of drive, the grid current meter still goes backwards.

    So guess that's not the problem.

    Any other ideas? (this really has me stumped!)

    Roger G3YRO
     
    AF6LJ likes this.

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