[Elecraft] K2 PLL calibration

Wed Dec 17 14:00:01 2003

Bob,

I just remembered something - you said your K2 serial number was 147 - that
is shortly after the field test version, although I am certain you have the
version A board.  Anyway, check the value of RP2 on the RF board.  If it is
10k (marked xxxx103x), all is fine, but if it is 100k (marked xxxx104x), you
should have added a 10K resistor across pins 1 and 2 and another across pins
3 and 4 (it also would not hurt anything to solder another 10k across pins 7
and 8).  These added resistors are necessary when doing the PLL upgrade
unless RP2 is already 10k.

One other thing - if your board originally had the 100k RP2, and you changed
it to the10k which was removed at RP3 (like I did), be aware that the
resistor from the gate of Q19 to ground may have also been changed because
at one time it was a part of RP2 - well, this resistor must be 100k.  In the
very early K2s, this resistor was a part of RP2, but later was changed to a
discrete resistor R21.

Any of the above items can keep the PLL from working properly, and things
will not lock up.
Could this possibly be your problem?

73,
Don W3FPR

----- Original Message -----
From: "Masleid, Michael A." <[email protected]>
To: <[email protected]>
Cc: <[email protected]>
Sent: Wednesday, December 17, 2003 1:05 PM
Subject: RE: [Elecraft] K2 PLL calibration

Hi Bob,

The voltage that you see at U5 seem reasonable.  The 3 volts at pin 4
is odd, but since this represents a high speed digital signal, it might
show up as any value between 0 and 5 for a short time.

As to how the circuit works:

The LTC1451 contains a shift register.  "Ones" and "Zero" are sent to pin 2,
and clocked into the shift register with clock pulses on pin 1.  Other data
and clock is sent to other chips the same way, so you will see a lot of junk
on pin 1 and pin 2 that is being used elsewhere.  The LTC1451 only responds
to stuff on pin 1 and pin 2 when pin 3 is held low, that is, DAC2CS is at
zero volts.  This happens once in CAL FCTR each time you hit BAND+ or BAND-.
The data that is shifted into the LTC1451 is latched on the output when
DAC2CS goes high again.  The data will be "remembered" as long the 5 Volt
power supply voltage is not disturbed, and as long as there are no
"glitches"
on pin 3.  The quickest way to get a glitch on pin 3 is to touch the VFO
knob.  It turns out that the LTC1451 gets loaded with new data when you
change the VCO setting.  If you spin the knob, you should see a very steady
pulse on pin 3, and you can see the data pattern counting through values
on pin 2 (using a 'scope of course).  So.... if there is something wrong
with your shaft encoder or VFO knob felt washer, that could cause some of
the trouble you are seeing.

If you put the rig in CAL FCTR with the probe at TP3, you should be able to
see the frequency cycling through a short range of frequencies as you
rotate the VFO knob.  You should also see a slight dip in voltage at
pin 3 if you spin the knob fast....

Do you have access to a scope with triggered sweep capability, or
access to a digital pulse catcher?  You need to see if there is one and
only one pulse at pin 3 for each push of BAND+ and for each push of BAND-.

Good luck and 73, Michael, AB9GV
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