adjusting quiescent currents in power amplifier of Klopik TRX

Discussion in 'Homebrew and Kit Projects' started by 4X5SD, Jul 23, 2021.

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  1. 4X5SD

    4X5SD Ham Member QRZ Page

    Hello,
    I am trying to set a proper quiescent current values for the tranistors in the power amplifier of Russian Klopik transceiver. I purchased a kit from https://www.rv3yf.com/ and assembled the amplifier. The circuit is attached below. I was able to adjust the collector currents of VT1 and VT2 by changing the resistances R4 and R9, respectfully. However, my problem is with the VT3: the drain current is very low (microamperes range). The current changes with changing the R13; however, it does not reach the required 250mA. This is very simple circuit but I don't understand what could be wrong with it. I do measure about +12V on primary winding of T2, and the bias voltage on the gate of VT3 is changing when I change the R13. I tested the MOSFET, it seems to be OK. I even replaced it with another one - the drain current is still low. The VT3 and VT4 of the output stage are currently disconnected and I did not apply a power for the output stage (+24V ) yet, just +12V for the initial 3 stages. Is there anything that I am missing?
    Any advice/suggestion would be appreciated.
     

    Attached Files:

  2. N7EKU

    N7EKU Ham Member QRZ Page

    Hi,

    Do you see +12V on the drain of VT3? Are you working with real IRF510's or Chinese/eBay/Amazon/etc imports? How are you measuring the current -- increase in power supply drain or by measuring voltage across R15?

    73.
     
  3. 4X5SD

    4X5SD Ham Member QRZ Page

    Hi,

    Yes, there is +12V on the drain of VT3. The IRF510 came with the kit, so I don't know what is their origin. To measure the current, I disconnected the drain of VT3 from the primary winding of T2 and put the multimeter there.

    Thanks.
     
  4. N7EKU

    N7EKU Ham Member QRZ Page

    Hi,

    What voltage do you measure on the gate of VT3 vs the gates of VT1 and VT2? This may give a clue, although gate voltages may vary a lot with each individual semiconductor.

    A better method of measuring the current of VT3 would be to measure the voltage drop across R15. 0.25A across a 1.6ohm resistor should measure as 0.4V

    73
     
  5. 4X5SD

    4X5SD Ham Member QRZ Page

    When I measure the voltage across R13, I can set the proper current. I adjusted the Value of R13 in such a way that the voltage across R15 is eactly 0.4V. The gate voltage was about 3.0V.
     
  6. N7EKU

    N7EKU Ham Member QRZ Page

    Great!

    Sounds like you are all set. The schematic has an asterix on the value of R13 and two others, which I expect is there to show you that you need to adjust it in order to obtain the desired idle current (just like you did!)

    73
     
  7. 4X5SD

    4X5SD Ham Member QRZ Page

    Right, The R13, as well as R4, R9 and the potentiometers R16 and R17 are used for idle current adjustment.

    Thanks!
     

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