[Elecraft] K2 PLL reference oscillator range is zero

[David Johnson]

Hello and Happy New Year to everyone.

  I have owned K2 serial no. 3980 for about five years and it has worked flawlessly.  A couple of days ago it stopped tuning properly.  Instead of tuning across a signal, it now acts like a crystal controlled radio that tunes in discrete steps of approximately 10 khz as the knob is rotated. 

 This same problem was described on this list just last month by Jon, KB1QBZ, and he said that he solved it by redoing the alignment and test steps, part II.  So I began by redoing the 4 MHz Oscillator Calibration, which gave me a frequency of 12.086 Mhz.

 The next step is the PLL Reference Oscillator Range test, but I got no change in the 12.086 Mhz indication by tapping either the Band + or Band - switch.

 So I measured the voltages on U6, and got 0 volts on pins 2, 3 and 4 at all times, with no change as I tapped the Band + and Band - switches.  I also got a steady 2.0 volts on pin 7.  I know these voltages are supposed to change as you tap the Band + and Band - switches, but is this something that you can see on a digital voltmeter, or is it something instantaneous that you need to view with an oscilloscope?

  I backed up and measured voltages on U5.  I got 5V on pin 1, and 0 volts on pins 3 and 4, all opposite of what the voltage table lists.  I also got 0 volts on pin 7 continuously even when I tapped the Band + and Band - switches.

  Finally I looked at the thermistor board, but I wasn't sure quite how to investigate it.  I have 1.2 volts at the junction of RA/Rt/RB and 0 volts at the junction of RB and RC.  

  Are my symptoms consistent with a bad thermistor, or is this more likely to be a failure within U5 or U6?

  Sorry for taking up all this bandwidth but I am not sure what my next step should be.

  Thank you for any and all suggestions on how to proceed.

  -- Dave  WB4JTT