TL-922A - no reported plate current!

Discussion in 'Amateur Radio Amplifiers' started by KK6VQK, Aug 19, 2018.

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  1. KK6VQK

    KK6VQK Ham Member QRZ Page


    I'm wading my way around repairing an old TL-922A. I baked in the tubes for a while and so far all I am getting is a nice yellow glow. And yes, all the relays go clicky clicky and I can engage the thing to transmit aaaaand - i get no plate current reading. I get grid current readings, but no plate current.

    HV reports fine for CW and SSB. I even see some output power - 10-20W in from my TS-440S gives around 300W out on 20m. But i see little to no deflection of the plate meter, let alone any idle current either in CW or SSB modes. If I vary plate/load then the output power also varies, so they're at least not totally shorted capacitors.

    I've checked the bias zener diode and the HV capacitor stack and the shunt board too, they all seem A-OK. So I'm wondering whether there's a biasing issue, or somehow plate/load is hilariously wrong.

    My first question - what else should I look at?

    My second question - just in case someone's cooked the plate current meter, does anyone know what the expected full deflection value should be for the plate current meter? Is it 100mA? Or something much much smaller? I'd like to at least verify that the plate current meter and shunt is all working A-OK before I move onto diagnosing more potentially complicated issues.


  2. W1QJ

    W1QJ Ham Member QRZ Page

    You could have an open plate meter which was blown out by a tube arc. This is typical. This is why I have "cloned" the plate current meters and sell them as replacements as you won't find a meter unless someone is parting out a 922. Aside from that I suggest that you first disconnect the wires from the plate meter at the meter and test the meter with an ohm meter on a high ohm scale about X1000 ohms and see if the meter jumps. Just ta the leads quickly to see if the meter responds. You should get some sort of movement of the needle. If not, try a lower scale and again just tap the meter to see if the meter needle responds. The movement is 500ua so it is quite small and an ohm meter will easily test the meter. If you can't get the meter to respond then the meter is blown out and yu can contact me at my PM and we cn talk about getting you a replacement meter. MOST of the time the meter blows out and the shunt (1 ohm) and the series resistor R7 (1500ohms) survive but you should check both anyhow. Both are up top on the meter board. So you have some checking to do.
    WA7PRC likes this.
  3. KK6VQK

    KK6VQK Ham Member QRZ Page


    So hm, should it measure around 500 ohms with a multimeter?

  4. KA9JLM

    KA9JLM Ham Member QRZ Page

    No. 500ua is full scale.

    You are using the ohm meter to provide low voltage and current to test if the meter movement moves.
    WA7PRC likes this.
  5. W1QJ

    W1QJ Ham Member QRZ Page

    Just see if the meter moves that’s all you are doing. Never mind the resistance for now. You have to determine if the meter is good or not. I don’t remember the resistance of the meter off hand.
    WA7PRC likes this.
  6. WA2AAR

    WA2AAR XML Subscriber QRZ Page

    I would do what Lou W1QJ tells you to do. He repaired my TL-922 and my drake L4 He know what he is talking about when it
    comes to amps.
  7. KK6VQK

    KK6VQK Ham Member QRZ Page

    Oh I'm going to listen to him. I just need some free time to go upstairs and open up the TL-922A again. :)
  8. KK6VQK

    KK6VQK Ham Member QRZ Page

    Oh another fun note - the max power seems to max out at about 500-600w out for 100w in. I can't seem to tune the amplifier to deliver anything more than that.

    Fun times.
  9. KK6VQK

    KK6VQK Ham Member QRZ Page

    So the ig/rf/hv meter quite happily full deflects with my multimeter in most resistance positions, but the ip meter measures like 8 ohms and barely deflects. Yes, this is out of circuit.
  10. WA7PRC

    WA7PRC Ham Member QRZ Page

    8Ω is mighty low for a milliammeter that uses an external shunt. I would expect to see something around 1KΩ or so.

    Alan Wolke @W2AEW (Field RF Application Engineer for Tektronix) has a yuuge number of very good tutorials, including these about panel meters:


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