[Elecraft] K2: PLL Temperature OVERcompensation?

[John, KI6WX]

Bill;
The problem you describe is not how the circuit would fail if the
compensation rate was incorrect.  However, I suspect that for some reason
you are getting either noise at the output of U6, pin 1 or the voltage is
not running in the correct range.  It should be in the range of 2 to 8 volts
and a wide temperature variation will only change it by less than 0.5 volts.
You may also want to check for shorts/opens in the board or in the pins
connecting it to the RF board.  You can also check the frequency of the PLL
reference by connecting the counter to TP3.  This will normally be around
12095 kHz and vary by 2 kHz on 20 meters and 5 kHz on 40 meters as you tune
over a 5 kHz range.

You can remove the circuit board and replace it with four 10K resistors.
These resistors will need to be placed in pins 1&2, 3&4, 5&6, and 7&8.  This
will give you exactly the same functionality as RP3.

-John
 KI6WX

----- Original Message ----- 
From: "Bill Coleman" <aa4lr@arrl.net>


>
> OK, I have a rather weird problem with my K2 #2548.
>
> This K2 has the A->B mods, the SSB, NB, 160m and KAF2. Back in March, I
> used it in the WPX SSB contest, and it was fine.
>
> In May, I ordered the Temperature Compensated PLL Reference upgrade. I
> built and installed it per the instructions, aligned the K2, and it
> seemed fine.
>
> To be honest (and this will sound like heresy to K2 owners), I haven't
> had much time to actually USE the K2 since then, until late July. I
> dropped into the shack, turned it on 20m. What I discovered can only be
> described as "weird".
>
> The SSB signals sounded sort of watery, and some of them where hard to
> impossible to tune in properly. 40m seemed to be OK, but 20m (and
> higher?) frequencies, the K2 just wasn't working correctly.
>
> I recently had a chance to diagnose this a bit, and what I uncovered is
> strange:
>
> 1) I checked the PLL Temp Compensation board, to see if all the resistors
> were in the right place. They were. (Although I haven't checked their
> values with an ohmmeter)
>
> 2) Thinking that the PLL calibration might be off, I ran CAL PLL on the
> cold K2. When it finished, the weird sounding, hard to impossible to tune
> signals on 20m were gone. Ah, problem fixed.
>
> 3) I put the covers back on, and left the K2 on overnight, to allow the
> internal temperatures to stabilize. Then I ran CAL PLL again. Again, 20m
> sounds good. I turned the K2 off.
>
> 4) Came back later with the K2 cold. Turned it on and the weird 20m
> tuning problem exists again. Ran CAL PLL on the cold K2, and the problem
> is fixed.
>
> 5) Checked the K2 later (thinking the CAL PLL might have warmed the K2
> up), and the problem is still fixed. I assume if I leave the K2 on
> overnight again, the weird 20m signals will return.
>
> Since CAL PLL fixes the problem, it is reasonable to suspect something in
> the PLL -- something introduced with the temperature compensation
> upgrade. It's almost as if the compensation is going too far, or
> something else is changing with temperature.
>
> The really odd thing is that 40m sounds nearly normal, but 20m is
> definitely weird. Perhaps it has something to do with the higher PLL
> frequencies used on 20m.
>
> I'd really like to use the K2 in the NAQP SSB this weekend, so I'd
> appreciate any suggestions or ideas in diagnosing this. Heck, I almost
> wish I had a replacement RP3 -- my K2 wasn't THAT drifty before I put the
> PLL temp comp mod in.
>
> Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr@arrl.net
> Quote: "Not within a thousand years will man ever fly!"
>             -- Wilbur Wright, 1901
>