K2 question regarding Pin 24 U6 control board operation

[Will-69]

Hi all,
  This question is more to fulfill my curiosity, but maybe it has to
do with the problem I'm experiencing with the PPL reference oscillator in
the synthesiser section.

  While trying to debug why I was getting a reading of 190.00 -/+
2KHz in the TP3 PLL reference oscillator test point, I decided to do some
voltage checks.  I discovered that pin 2 of U5 on the RF board wasn't
getting the 5V I was expecting according to the voltage tables.  I traced
this all the way back to the control board, U6 pin 24.  the voltage
reading was 0V, so thinking this is a digital output, I hooked-up a logic
probe and confirmed the voltage reading.  I conirmed this same reading at
pin 5 (SD0) of U4 on the RF board.

  Now, U5 on the RF board shows 4.06V on pin 7 so maybe this is OK
and I'm just making myself worry over a red herring...but I thought I'd
ask.

  On a lark, I took the boards apart and powered up the control
board alone.  First time, pin 24 showed the correct voltage.  On second
power-up, still outside the RF board, the pin showed 0V....maybe normal
behaviour?

  Anyways, I'm still looking through the RF board (redid RFC14 - one
winding too many - and re-seated the thermister board) until I can find
where my PLL ref problem lies.

Thanks,

Will

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[Don Wilhelm-3]

Will,

Yes, the voltage on U6 24 is a 'red herring'.  That is a serial data
line, and while you might observe the +5 volts active signal at times
with a 'scope, much of the time it is inactive and at zero volts.

You can check the operation the data into the DAC which controls the PLL
reference oscillator most easily by observing the DAC output - set the
K2 to CAL FCTR and observe the voltage at RF Board U5 pin 7 as you tap
the BAND+ and BAND- buttons alternately.  You should see the voltage
change between close to zero volts to about 4 volts.  If you do see a
change, then you can conclude that the SDO signal is being properly
received by the DAC (U5).

It would appear that your PLL reference oscillator is not oscillating  -
you should see a signal at approximately the PLL crystal frequency of
12096 mHz.  Check the voltages at Q19, check to be certain the leads of
RFC14 are well stripped, be certain D18 is oriented correctly.  If that
does not solve the problem, look at the schematic for RF Board sheet 1
and find the PLL Reference Oscillator in the upper left corner.
Identify all the components associated with that part of the circuit and
check them for correct values and proper soldering.

73,
Don W3FPR

Will wrote:

>
> Hi all,
>     This question is more to fulfill my curiosity, but maybe it has to
> do with the problem I'm experiencing with the PPL reference oscillator
> in the synthesiser section.
>
>     While trying to debug why I was getting a reading of 190.00 -/+ 2KHz
> in the TP3 PLL reference oscillator test point, I decided to do some
> voltage checks.  I discovered that pin 2 of U5 on the RF board wasn't
> getting the 5V I was expecting according to the voltage tables.  I
> traced this all the way back to the control board, U6 pin 24.  the
> voltage reading was 0V, so thinking this is a digital output, I
> hooked-up a logic probe and confirmed the voltage reading.  I conirmed
> this same reading at pin 5 (SD0) of U4 on the RF board.
>

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