[Elecraft] K2 #4156 VCO test

[James T. "Jim" Rogers]

Denny wrote:
I am ex - WB8LKL and am studying to re-license.
I am building K2 # 4156. Everything has gone well until getting to the VCO
test. Freq. reading is unstable at about 3.9 Mhz. Voltage tests revealed the
following.
RF board
U4; pin 5 - 0V (should be 5)
    pin 7 - 5V (should be 0)
    occasionally while testing, pin 7 goes to 5V also. Power down and it
returns to 0V.
    pin 13 - 0V

I am not an expert but while helping another Dennie, I had the opportunity
to trouble shoot the PLL circuitry.

1) Verify U4 is the correct component (I am sure it is) and is properly
oriented in its socket ( I am equally sure you have done that).
1) Verify the +5V VDD thru RFC15 to U4. And verify pin 12 is actually at
ground.
2) Make sure the reference oscillator is working. You can verify this at TP3
with the built in frequency counter function.
3) Pins 5 and 7 would be difficult to check since they are the serial data
in and the clock signal for the data to the PLL chip. If you are not
spinning the dial there will not likely be a lot of activity here. And it
would take a really fast scope to see the data and/or the clock.
4) Pin 13 is the control voltage to the VCO thru the low pass filter U6.
Make sure there is nothing pulling this line to 0V as you report, i.e. a
solder bridge etc. The higher the reverse voltage applied to the varactors
the lower capcitance they represent and the VCO will move higher in
frequency.
5) By adjusting L30 you should be able to change the VCO frequency with or
without the control voltage.

I think from your voltage checks the most probable candidate is the
circuitry around the PLL chip U4. Of course there is the possibility of a
bad chip but I would think this would be unlikely.

Hope this all helps. Some of the real gurus will be on here later and may
have more to add.

73, Jim W4ATK
K2 #4028