[Elecraft] PLL Reference Oscillator Range and VCO Test

[Don Wilhelm]

Dick,

If your internal probe works in TP2, then it is probably OK.

Good on the 3.9 volts at U6 pin 5, that part is encouraging, but it does 
not entirely rule out a problem with the Thermistor board.
Skip that for now and first check to see if the problem is in the 
digital side or the analog side of the DAC.  Try the following tests in 
order.

Monitor the voltage at U5 pin 7 while in CAL FCTR.  When you tap BAND+ 
it should go to 4 volts, and when you tap BAND-, it should go to zero 
volts.  If it does not do that, you probably have a problem on the 
digital inputs to U5.  Those come from the control board - check the 
soldering at U5 pins 1, 2, and 3, and CB U6 pins 19 and 24 as well as U8 
pin 10.

If RF U5 pin 7 tests good, move on to monitor U6 pin 1.  It should 
change from zero volts with BAND- to near 8 volts with BAND+.  If it 
does not check all the soldering on the back of the Thermistor board, 
especially checking for solder bridges.

If U6 pin 1 tests good, take a big step and monitor the voltage at the 
anodes of D17 and D17.  You should see the same voltage change as you 
did at U6 pin 1.  If not, check soldering and values for R19 and RP2.

If no problem was found in the tests above, then you have a problem with 
the PLL oscillator itself (Q19 circuit).  Check resistance to ground 
from the source of Q19.  If you do not see near 270 ohms, check the 
soldering of R20 and the lead stripping and tinning of RFC14.

No problem solved so far?  Check capacitors C84 and C85 to be absolutely 
certain they are 120 pF (marked 121).  The makings on those small blue 
bodied capacitors is very difficult to read and you may have to remove 
them and check under high magnification and a strong light.

If all the above is good, it should work.

73,
Don W3FPR

On 4/8/2013 6:37 AM, Dickie M0AUW wrote:
> Hi,
> I’m currently assembling my K2 but am stuck on the PLL reference Oscillator
> test.
>
> In the frequency counter mode with the probe attached to TP3, I get a
> reading of 12099.03 kHz, when pressing band + I get 1298.99, Pressing band –
> I only get 00.467 to 00.475 changing rapidly. (Perhaps the probe is playing
> up but seems to work with other tests?)
>
> However, with an external frequency counter attached I get 12099.00 and no
> change when pressing band +, but when pressing the – I get 11500 varying to
> 11700 at high speed.
> I’ve checked solder joints around that area in particular at R19, RP2, L31,
> D16 and D17 and at U6 pin 5 am getting 3.90 volts.
>
> Also on the VCO test I am getting a reading of 7738.64 but I suspect this
> will be out due to the PPL test?
>
> Any help would be much appreciated – 73s Dick M0AUW
>
>