[Elecraft] K2 -- loss of output power control

[Brian Lloyd]

On Thu, Dec 11, 2008 at 6:59 AM, Don Wilhelm <w3fpr at embarqmail.com> wrote:
> Brian,
>
> You have some problem with the wattmeter in the KPA100 - T4, D16, D17, R26,
> R27, U5, U6 or on the VRFDET signal pathway back to the K2 control board.
>
> You seem to have a somewhat normal voltage variation at U5 pin 7, which (for
> the moment) says everything is working through U5, but check the U6 pin 6
> output under the same conditions and also check for the same voltage at K2
> control board P4 pin 10.

I did. I think I said that in the original message (looking ...), yes,
I did check and the output of U6 pin 6 is the signal VRFDET and tracks
VFWD (output of U5b). It is properly present on header for the aux-I/O
cable to the control board and I have verified the same voltages on
pin 10 of the Aux-I/O connector on the control board.

I did disconnect the Aux-I/O cable from the control board and I tested
the base K2. Power output control of the K2 without the KPA100
connected is working is working properly.

> Since this happened only by removing and re-installing the KPA100, be
> suspicious of a solder joint that is not well soldered - those may work for
> a while and then when touched can fail.

I did carefully remove the PA board from the heatsink and check all
connections visually and specifically all of those between U5, U6, and
the Aux-I/O connector. I even rang out the connections. That is all
good.

I have also verified the entire path from U6 pin 6 (KPA100) all the
way to pin 2 on the microcontroller, U6, on the control board. Voltage
is present there and it does follow power output. So VRFDET is
reaching the control board.

Just for grins, I looked at the values for VRFDET coming form the K2
RF board RF output detector. They are *much* higher than what I am
getting from VRFDET coming from the KPA100. At full output I have
something like 3.5V on VRFDET from the K2 RF board output but only
about 0.3V on VRFDET coming from the KPA100. I am thinking maybe D16?
What kind of voltage should I see at the cathode of D16? I see about
1.7V at full power output from the PA.

73 de Brian, WB6RQN


>
> 73,
> Don W3FPR
>
>
> Brian Lloyd wrote:
>>
>> Our K2 was working perfectly until I opened it up, tightened the power
>> transistor mounting screws, realigned the receiver, and put it back
>> together. Now power output no longer follows the power setting. When set to
>> a power output less than 10W, I hear the bypass relay but I get the High Cur
>> display (momentarily) and the display then shows a power output of 0.3W with
>> an SWR of 3.0. Output power as measured with a W1 is 20W. If I advance the
>> power control, as soon as I pass the 10W point the PA comes on and the power
>> jumps to 150W output. Turning the power level control fully CCW (0.1W
>> setting) produces 6W of output.
>>
>> Looking at pin 7 of U5 on the KPA100 I can see good analog voltage that
>> varies with power. I can see the value of VFWD change when the range changes
>> (SCALE changes) also. I can also see varying VFWD at pin 2 of U1 so this
>> does not appear to be an analog problem.
>>
>> FWIW, voltages at pin 2 of U1 for various power outputs are:
>>
>>  6W -- 0.15V
>>
>>  20W -- 0.30V
>>
>> 150W -- 0.29V
>>
>> At pin 7 of U5 (U5b):
>>
>>  6W -- 0.15V
>>
>>  20W -- 0.30V
>>
>> 150W -- 0.77V
>>
>> On the t-r menu item I have "8r hold" set.
>>
>> I do have output on VRFDET but the value only varies from about 0.3V-0.7V
>> so that appears to be working also.
>>
>> I am planning to fly to Dominica on Friday and operate from there but I
>> can't take the K2 unless I solve this problem.
>>
>> Suggestions?
>>
>