K2 VCO Alignment only 0.02V @ R30

[Don Wilhelm-4]

Jeremy,

Do I recall correctly that you had a good PLL range?  If so, I have
trouble believing there would be 300k resistance from CB U6 pin 24 to RF
U7 pin 2 while you have zero ohms to U4 pin 5.  Are you certain you
counted the pins correctly?  Pin 1 has a circular solder pad and the
rest are rectangular.

If you want to check the operation of RF board U7, monitor the DC
voltage at pin 7 while rotating the VFO knob - the voltage should change
gradually while tuning through a 10 kHz range and then jump back to its
lowest (or highest depending on which way you rotate the knob) at the
end of each 10 kHz interval.

Did you remove the Control Board when you made the measurement on the
collector of Q1?  If not, then somewhere in the vicinity of 5 k is not
unreasonable.

73,
Don W3FPR

On 2/3/2013 12:41 PM, JeremyJones wrote:

> Looking over the board I did find that I had U8 C45 connected to pin7 instead
> of pin6.  That has been corrected.
>
> U6 pin 24 to U4 pin5 is less than 1 ohm, however U6 pin24 to U7 pin2 is 300k
> ohm.  I don't see any damage on the board.
>
> I also noticed going back to the beginning for the resistance checks on the
> control board that Q1 collector should be great than 1M, but is reading 5k.
>
>
>
> --
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[JeremyJones]

Rechecked the resistance, and indeed there is 300k from U6 pin 24 to U7 pin2.  Looking at the schematic in the manual, it shows U6 pin24 connected to U7 pin5 and U8 pin11.  At this point I do have less than 1 ohm.

The schematic shows U6 pin23 connected to U7 pin2 which is reading less than 1 ohm.

The control board was removed when checking Q1, and it is showing 4.2k to ground.

[Don Wilhelm-4]

Jeremy,

I am sorry, I had the wrong IC,  RF board U5 pin 2 should be connected
to Control Board U6 pin 24.  Yes, it also connects to Control Board U7
pin 5 and Control Board U8 pin 11, as well as RF board U4 pin 5.

73,
Don W3FPR

On 2/3/2013 3:45 PM, JeremyJones wrote:

> Rechecked the resistance, and indeed there is 300k from U6 pin 24 to U7 pin2.
> Looking at the schematic in the manual, it shows U6 pin24 connected to U7
> pin5 and U8 pin11.  At this point I do have less than 1 ohm.
>
> The schematic shows U6 pin23 connected to U7 pin2 which is reading less than
> 1 ohm.
>
> The control board was removed when checking Q1, and it is showing 4.2k to
> ground.
>
>
>
> --
> View this message in context: http://elecraft.365791.n2.nabble.com/K2-VCO-Alignment-only-0-02V-R30-tp7569523p7569582.html
> Sent from the Elecraft mailing list archive at Nabble.com.
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[JeremyJones]

No problem Don, you've been a huge help.

So I went back to the beginning and did all resistance checks in the manual up to this point and everything is good.

PLL range test @ 7100.00khz gives me 12099.13 high and 12086.33 low.
VCO @ 4000.10 gives me 7840.33

I do still have some bad voltages on U4 and U6 which I'm tracing now to see where everything goes to and comes from.

[JeremyJones]

So I really thing that U4 is faulty. It's putting out 5.029V on pin13 when that should only be 4V. All the components surrounding it seem to have the correct voltages according to the manual.  I think I'll replace that and see how things look after.

[Don Wilhelm-4]

Jeremy,

5.029 volts at U4 pin 13 means that the PLL synthesizer IC is working.  
Remember that the manual voltages are for a properly working K2 that has
the PLL IC in lock with the VFO frequency.  Your voltage only indicates
an out of lock condition.

What is the R30 voltage now with that 5.029 volts at U4 pin 13?  You
should now have some voltage there.  Can you adjust that voltage with
L30?  If so, you should now be able to get everything within the normal
range.

If you are still seeing zero volts at the left side of R30, then U6B or
its components is at fault, not U4.

73,
Don W3FPR


On 2/3/2013 8:31 PM, JeremyJones wrote:
> So I really thing that U4 is faulty. It's putting out 5.029V on pin13 when
> that should only be 4V. All the components surrounding it seem to have the
> correct voltages according to the manual.  I think I'll replace that and see
> how things look after.
>

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[JeremyJones]

So Gary from Elecraft gave me some things to check today.  Heres where I am at for the moment.
T5 is back to 15 turns of red.  This brings the frequency close to spec @ TP1.
Pulled C72 and measured its value at 273pF.
U6 has 4V @ pin5 and pin1 goes from 7.5 to 0V when doing the PLL range test.
T5 lead 3 is showing 2.4V
TP1 is reading 0.686V with the RF probe.
TP1 is reading 8179.89 @ 4000.10
There is 5V on both sides of RFC15.
Cal PLL gives me info 232 even with TP1 @11915.60 tuned to 7000.00

Seems I'm going in a bit of a circle, but thats how things are at the moment.  I'm letting the unit sit powered up for a while before I try Cal PLL again.

[JeremyJones]

So Cal PLL is still not working.
The 5V from U4 pin13 makes it all the way to U6 pin 6, but is blocked by C94 on the other branch.
Still only 0.020V at R30.

[JeremyJones]

After some more trouble shooting with Gary from Elecraft, he decided to send me a new U4.  After installing it, all tests and alignment for step 2 went off without any issues at all.

[Bill Frantz]

I'm glad to see you solved the problem.

Exchanges like this one between Jeremy, Don and Gary are one of
the reasons I read everything on this list, and not just the
items for the equipment I own.

Cheers - Bill, AE6JV

On 2/13/13 at 4:50 AM, [hidden email] (JeremyJones) wrote:

>After some more trouble shooting with Gary from Elecraft, he decided to send
>me a new U4.  After installing it, all tests and alignment for step 2 went
>off without any issues at all.

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