[Elecraft] K2 -- loss of output power control

[Brian Lloyd]

On Thu, Dec 11, 2008 at 10:47 AM, Don Wilhelm <w3fpr at embarqmail.com> wrote:
> Brian,
>
> Yes, that is not enough voltage from D16.  The K2 power control circuits
> think that there is not enough power output and ramps up the power in an
> effort to see more voltage on the VRFDET line.

That is what I am thinking also.

> I would recommend that you 'bite the bullet' and remove the KPA100 board
> from the heat sink and check T4 carefully.

I did that but now I am going to do it again. Grr.

> Replace D16 and D17 both.  They
> may have been damaged by a lightning surge, but they usually just short and
> fail to work at all - your failure is a bit different in that you do get
> some voltage (and that is why you should check T4 sand the other components
> in the wattmeter sensing area.

I have checked T4 and it shows proper continuity to the board so I
believe that the soldering is good. (The solder joints look good under
a magnifier too.) D16 and D17 both show the same forward and reverse
resistance once they conduct which makes them suspect to me. They do
show open at low voltage so they are not shorted. Seems they are the
most likely culprits.

> If you do not care a lot about power accuracy at low power levels, you can
> use 1N4148 diodes in place of the 1N5711 type.  The 1N4148s are a little
> more 'hardy'.

But they are standard silicon small signal diodes. They would not work
well at low power levels due to their greater forward drop. Can I sub
in some other small-signal schottky diode? If I can't find a 1N5711?

>
> 73,
> Don W3FPR

73 de Brian, WB6RQN