Need help understanding transistor circuit

Discussion in 'General Electronics Chat' started by tonydix, Aug 27, 2017.

  1. tonydix

    Thread Starter Member

    May 4, 2010
    27
    1
    I am trying to troubleshoot a no transmit problem om an icom IC-746. Part of the circuit is attached.
    My understanding of its function is as follows but please correct me. I may have it wrong :-
    Q1222 normally has 5 volts on its base, its collector is held high by 14 volts via R1222. This means that Q1221 has 14 volts on its base and collector via R1221, so does nothing. When the ptt button is pressed TXS is pulled to ground and so Q1222 collector is pulled to ground and so Q1221 is now switched on and thus 8V is now connected to T8V

    I hope I have this correct !.. Sadly TXS is being grounded but T8V is not 8 volts so something is wrong !!
    Any help much appreciated

    Tony
     
  2. LesJones

    Well-Known Member

    Jan 8, 2017
    2,050
    604
    I hope you mean that normally the right hand end of R1223 has + 5 volts on it (With respect to ground.) If The base of Q1222 has + 5V on it's base then it's base emitter junction is open circuit. R1223 should limit the current to it's base so the base is only about 0.6 to 0.7 volts positive. In that condition Q1222 collector will be conducting so there will only be about + 0.2 volts on it's collector. (This is the opposite to what you state.)

    Les.
     
  3. ericgibbs

    Moderator

    Jan 29, 2010
    7,539
    1,520
    Hi Tony,
    The 14v T8V is clamped at ~8V by the diode to the 8V supply.
    E
     
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  4. tonydix

    Thread Starter Member

    May 4, 2010
    27
    1
    Thanks for the help. I did mean TXS is normally 5 volts but is pulled to ground when pressing ptt.
    ie the rhs of R1223 is 5v / 0v.
     
  5. ericgibbs

    Moderator

    Jan 29, 2010
    7,539
    1,520
    hi,
    If the circuit is still giving a problem post the TX On/Off voltages for those two transistors.
    E
     
  6. tonydix

    Thread Starter Member

    May 4, 2010
    27
    1
    Many thank ericgibbs. I have identified an open resistor and a damaged track. Oh its all so tiny !!
    Which simulation software were you using ? Looks very helpfull. I owe you a beer !
     
  7. ericgibbs

    Moderator

    Jan 29, 2010
    7,539
    1,520
    hi tony,
    It is LTSpice , free download also a Yahoo LTS user group, has lots of examples etc.

    Beer sounds tempting, weather is just over 28C here, which is hot for the UK.!

    http://www.linear.com/designtools/software/

    Eric
     
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