[Elecraft] K2 VCO Alignment only 0.02V @ R30

[Don Wilhelm]

Jeremy,

U7 pin 2 and U4 pin 5 are both the SDO signal which comes from the 
microprocessor over on the control board.
If you have 300k resistance between those two points there is a break 
between them.

I don't know why you mention U7 pin 5 (unless it was a typo) because 
that pin is connected to ground.

U7 feeds U6A which in turn controls the voltage on the PLL varactors. 
Your PLL is working just fine, so there is no problem there.
Recheck the resistance between U4 pin 5 and U7 pin 2, or better yet 
between U4 pin 5 and pin 24 of U6 over on the Control Board - you should 
have close to zero ohms.

73,
Don W3FPR

On 2/3/2013 10:51 AM, JeremyJones wrote:
> So this morning I started rechecking everything.  I've found that the 5V on
> U4 pin 5 is intermittent.  Not in a way that would indicate a loose or poor
> connection, but either that it is there, or it isn't (mostly isn't) when
> powering up the unit.  U7 always has the 5V on pin2.  I checked the
> resistance between U7 pin2 and U4 pin 5 and found about 300k.  Looking at
> the schematics however, it looks like U4 pin5 is actually connected to U7
> pin5 (SDO).  There is less than 1 ohm of resistance between U4 pin5 and U7
> pin5, so it would appear that U7 may have an issue.
>
> The other odd thing is that even when there is 5V coming into U4 pin5, the
> symptoms at R30 (0.020V) and the low band frequencies are still there.  It
> looks like U7 may provide U4 a clock signal as well.
>
> I was seeing the 8V at R30 after reflowing all of the joints near the PLL
> Reference.  Could the increase in temperature in the are have triggered the
> thermistor?
>
> The only thing changed between having the 8V at R30 last night and then
> losing it was the rewinding of T5 to bring it back up to 16 turns of red.
> The continuity checks are good through it, and L30 still changes the
> frequency when adjusted.
>
>