Kenwood TL 922 help needed

Discussion in 'Amateur Radio Amplifiers' started by WD8BWW, May 22, 2017.

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  1. W1QJ

    W1QJ Ham Member QRZ Page

    The 1 ohm resistor is the shunt and the 1.5k is a series resistor to the meter. A big current went through there for sure. Lucky the plate meter isn't damaged, hopefully it didn't change resistance either. That remains to be seen. Fortunately I've cloned the plate meter in those amps and have them available. My meter only requires changing R7. If someone has a bad multimeter the fix is to switch the meter scale on a good plate meter for the multimeter and then replace the plate meter with mine. As far as your tube, well the filament obviously let go and something happened during the event and it baked those resistors. A quick check of the zener should be done. If that's ok a new tube should get you up and running again.
     
  2. KA9JLM

    KA9JLM Ham Member QRZ Page

    I zener is a bad diode with a low break over voltage. o_O

    They conduct in both directions.
     
  3. WD8BWW

    WD8BWW XML Subscriber QRZ Page

    I think ill replace the zener just to be safe.
     
  4. KA8NCR

    KA8NCR Ham Member QRZ Page

  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Except...the 3-500Z is actually a "zero bias" triode. Amp manufacturers mostly use a 5-9V zener or something to cut down on wasted power and heat while maintaining reasonable linearlity...but the tube actually doesn't need any bias. At zero bias and 3000Vdc on the anode, its idling current is still entirely reasonable and wouldn't pin a 1Adc meter or anything close to that.

    Many 3-500Z amps operate at zero bias by design.
     
  6. KA9JLM

    KA9JLM Ham Member QRZ Page

    I guess they had a few extra zener diodes that they needed to make use of.

    As long as the extra parts do not rattle in the case, All is good.
     
  7. W1QJ

    W1QJ Ham Member QRZ Page

    The original Eimac amplifier circuit they released years back when the 3-500 replaced the 3-400 operated at a plate voltage of 2500 volts. That's all that was needed for legal limit of one KW DC input. True, they showed no operating bias and was run at zero bias. However at 3kv and over a bit of operating bias is desirable especially on CW. The 922 runs about 3100v on plate do a bit of operating bias was added.
     
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Sure. And the Henry amps using 3-500Zs had some models that ran 4000V on the anodes, so they added bias, too.

    But at 3.1 kV, bias saves power and heat but isn't truly required; idling current will be higher, but it sure won't be 1 Amp!:p
     
  9. WD8BWW

    WD8BWW XML Subscriber QRZ Page

    is there a failure in the one tube who's filament will not light that could have burned out the 1 ohm voltage drop resistor? I'm sure the 1.5k gave up as the weaker of the two, meter/resistor. Is there a way to verify the meter was not damaged and is still accurate.
     
  10. KD0CAC

    KD0CAC Ham Member QRZ Page

    Allans video is good for testing meters .
    A couple quick checks , hard to do with meter in heavy object - rotate , tap on - to see if needle is free to move & spring return ,
    using your multi-meter on ohm test - with resistor in series - put probes on either side of meter contacts - needle should move up - disconnect & should return .
    Depending on meter rating - if you quick tough & release very quickly - meter should move BE CAREFUL - THIS CAN DAMGE METER IF LEFT CONNECTED FOR LONGER THAN 1/4-1/2 SECOND !
     

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