[Elecraft] K2 PLL reference oscillator range is zero

[David Johnson]

Don,

  Thanks for the quick response!  

  I do have the 5 volts on pin 8 of U5, but a constant 0 volts on pin 7 even when tapping the Band + and Band - switches.  But I'm just looking at it with a Digital Voltmeter, not a scope so maybe I'm missing it.

  Pin 5 of U6 is 4 volts, so at least that data point is OK.

  Should I suspect U5 at this point?  What can I do to verify that U5 is being properly triggered at its input when I tap the Band + and Band - switches?

  Thanks again.

 -- Dave WB4JTT


--- On Wed, 12/31/08, Don Wilhelm <w3fpr at embarqmail.com> wrote:

> From: Don Wilhelm <w3fpr at embarqmail.com>
> Subject: Re: [Elecraft] K2 PLL reference oscillator range is zero
> To: wb4jtt at yahoo.com
> Cc: elecraft at mailman.qth.net
> Date: Wednesday, December 31, 2008, 2:56 AM
> Dave,
> 
> Yes, do check U5, but ignore the voltages on pins 1, 2 and
> 3 because they are digital and the active state may not
> correspond with the voltage measured with a DMM.
> U5 pin 8 must have 5 volts DC for the chip to work - if
> that is not true, stop (all other tests are futile).
> 
> Next monitor the voltage at U5 pin 7 while tapping BAND+
> and BAND-.  the voltage should go to 4 volts or more when
> BAND+ is tapped and to nearly zero volts when BAND- is
> tapped.  If that is true, then U5 and its driving digital
> signals are OK.
> 
> Next thing to check is U6 - check at pin 1.  The voltage
> when BAND+ is tapped should be near 8 volts and when BAND-
> is tapped should be near zero volts.  If U5 pin 7 was OK,
> but this test fails, the fault is likely with U6 or the
> thermistor board.
> 
> To give a quick check on the thermistor board, measure the
> voltage at U6 pin 5 - it should be 4 volts (or more
> precisely, 1/2 of the 8 volt regulator voltage.
> 
> If the U6 pin 1 output is correct, there is not much more
> to check.  R19 should have a similar voltage swing (between
> BAND+ and BAND-), and the voltage on varactors D16 and D17
> cathodes should be similar.
> 
> If all checks out so far, blame X1 or L31.
> 
> Remember that since this K2 worked properly before, there
> should be only a single failure.  Your challenge is to find
> the source.
> 
> 73,
> Don W3FPR
> 
> 
> David Johnson wrote:
> > 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
> >