[Elecraft] K2 Alignment & Test, Part II PLL problem, Continued

Wed Apr 19 04:00:48 EDT 2006

I'm back....

I got the new components (D16, D17, C84, C85, & Q19) from Scott over at Elecraft and installed them. I still got the same readings so I re-read the instructions and checked each and every component to make sure that I had everything installed correctly. 

I did find one mistake, I had soldered C88 to pins 11 & 12 of U4 instead of pins 12 & 13. I relocated the cap and the frequency went up slightly (reading 524 instead of 495). The cap measured ~58 picos (in circuit) so it's in the ballpark.

I also touched up a few soldering joints along the way, more for looks than anything else.

The voltage readings on pins 1, 2, & 3 of U6 are the same as before, and both variactors seem to be working just fine. 

Any ideas?

Thanks again for all of your help,

John

> 
> From: "Don Wilhelm" <w3fpr at earthlink.net>
> Date: 2006/04/11 Tue PM 08:18:46 CDT
> To: <rootboy at bellsouth.net>, 
> 	<elecraft at mailman.qth.net>
> Subject: RE: [Elecraft] K2 Alignment & Test, Part II PLL problem, Continued
> 
> John,
> 
> For the time being (until you solve the current problem) the only voltages
> at U6 that matter are pins 1, 2 and 3 - and those are correct.  The other
> pins will likely fall into line after you have this problem fixed and if not
> there is another problem with the VCO.
> 
> Right now, the voltages being fed out of U6 pin 1 are correct for the
> stimulus you provided (CAL FCTR and tapping BAND+ and BAND-).  Now check to
> see if that (proper) 0 to 8 volt swing between pressing BAND+ and BAND- is
> carried on through R19 and then through the RP2 sections and then onto the
> cathodes of D16 and D17.  If you get a decent voltage swing between BAND+
> and BAND- (you can expect a bit of voltage drop on the 8 volts end) at the
> varactor cathodes, then look for something else wrong in the PLL Reference
> oscillator (upper left of the schematic sheet - but only as far to the right
> as TP3), the PLL chip does not matter until the PLL Reference oscillator is
> functioning properly.
> 
> If you do get a good voltage swing at the varactors, then all I can say is
> either Q19 oscillations quit when the varactor capacitance is reduced, or
> there is something wrong at C84, C85, D17 or D16 which is stopping the
> oscillation.  If you suspect Q19 itself, you could try swapping it with one
> of the J310s on the Control Board at Q6 or Q7 (these in a less-critical
> audio circuit rather than being used as an RF Oscillator).
> 
> 73,
> Don W3FPR
> 
> > -----Original Message-----
> >
> > Thanks for the help Don. :)
> >
> > I tried your suggestions (and a few others as well) and L31
> > (which is a 12 µHz inductor on my unit) is the correct part.
> >
> > Diodes D16, D17, & D18, are in properly and pass their in-circuit
> > tests just fine.
> >
> > R19, RP2, R20, R21, RFC14, C84, C85, Q19, RFC14, Q19 and the
> > thermistor board are all installed properly and certainly look good.
> >
> >
> > Scott at Elecraft suggested holding the 4, 5, & 6 buttons while
> > turning the unit on (which showed "INFO 201") to see if that
> > corrected the problem but no luck there either.
> >
> > I took reading off of U6 while switching between "BAND -" and
> > "BAND +" in "CAL FCTR" mode, and here's what I got:
> >
> > BAND +, The display reads 488Hz  (Bad)
> >
> > U6 Readings:
> > Pin 1 = 7.63V
> > Pin 2 = 4.09V
> > Pin 3 = 4.09V
> > Pin 4 = 0V
> > Pin 5 = 3.96V
> > Pin 6 = 4.91V
> > Pin 7 = .02V
> > Pin 8 = 7.95V
> >
> >
> > BAND -, The display reads 12086Hz  (Good)
> >
> > U6 Readings:
> > Pin 1 = .00V
> > Pin 2 = .09V
> > Pin 3 = .02
> > Pin 4 = 0V
> > Pin 5 = 3.96V
> > Pin 6 = 3.96V
> > Pin 7 = 1.64V
> > Pin 8 = 7.95V
> >
> >
> > Pins 2, 3, 6, especially Pin 6, are out of tolerance.
> >
> > Taking resistance readings off of the thermistor board looks good.
> >
> > Pins 6/7 to Pin 8 reads about 5k ohms.
> > Pins 4/5 to Pin 8 reads about 295 ohms.
> > Pins 2/3 to Pins 4/5 reads about 1.96k ohms.
> > Pins 6/7 to Pin 1 reads about 8.76k ohms.
> > Pin 1 to Pins 2/3 reads about 1.8k ohms.
> >
> > All of these readings more or less mathematically work out to
> > what the meter says (with the exception of the second reading
> > which is 8V to Ground so no surprise there).
> >
> > The output from the DAC is 4.092 (4,092 counts), so it's near its
> > top end.
> >
> >
> > My partner suggests removing R19 and inserting a pot to swing the
> > voltage going into the oscillator circuit from 0 to 7-ish volts
> > to see if that stabilizes.
> >
> >
> > I got another suggestion to double check D36 for proper
> > installation. It's in correctly, and checks out (in circuit) okay.
> >
> > Possibly I left a component out, so I'll double check that I
> > didn't skip anything during the installation phase.
> >
> >
> > Thanks again for all the help,
> >
> > John
> >
> >
> 
> 