[Elecraft] K2: V RFDET odd behaviour

[Don Wilhelm]

Iain,

Does that happen with the KPA100 completely removed from the base K2 (no 
ribbon cable plugged in)?

If so, then you have a problem with the path for the VRFDET signal that 
originates from D9 on the RF board.

When the KPA100 is connected, the wattmeter in the KPA100 is used as the 
source for VRFDET and must be working properly to achieve proper power 
control.

If you are working with the base K2 alone, try removing the KSB2 option 
- it also uses the VRFDET line.  You will have to put the leads of a 
.001uF capacitor between pins 7 and 12 of J11 to transmit and if you 
want to receive, jumper between pins 1 and 3 of both J9 and J10.

If you have proper power control with the KSB2 removed, it is likely 
that KSB2 U1 is damaged and possibly Q1 as well.

73,
Don W3FPR

On 12/7/2014 7:19 AM, Iain Kelly wrote:
> Good afternoon,
>
> I've been working on K2 s/n 5484 and have some off behaviour that has me
> stumped.
>
> The background is that during a KPA100 build the PC control cable was wired
> up backwards and then connected to the K2. Some smoke escaped from the
> control board area. The MCU has been replaced as we had been able to prove
> there was damage there.
>
> I've now got to the stage where I have 15W out again, trouble is it's all
> the time when I turn the power adjust pot up while in transmit.
>
> The pot is working ok and the K2 display shows 0.1 to 15W when adjusting
> that control in receive mode. In transmit the power output shows up to 0.4W
> on the K2 but 15W on an external meter. Turning the ower control to 0
> reduces the transmit output.
>
> I've measured the voltage on the cathode of D9 and that varies nicely with
> output power. At the moment it is around 0.5V then jumps to 3.15V when
> power out hits 15W. This happens in a jump, from what I can tell it's
> because the MCU VRFDET line is still too low. When I measure VRFDET after
> R66 I only get 0.8V max. Way too low.
>
> I have removed and tested R66, R69, C77, D9 and all are correct. The path
> of VRFDET is fine, the 820 Ohm resistor on the control board is fine.
>
> R69 measures 99.9k out of circuit, but only 89k when in circuit. Taking the
> output of D9 directly to VRFDET input on the MCU shows that VRFDET tops out
> at about 2.3V in that case, again for 15W out, though I get a bit more
> variability of output.
>
> I have compared it with a recently acquired (and known good) K2. The only
> difference I can see is that in circuit R69 shows 102k.
>
> My plan of attack has been to get the base K2 back into a working state,
> then work on any issues we find in the KPA100 module, of which there are
> certainly some.
>
>