[Elecraft] K2 -- loss of output power control

Thu Dec 11 13:47:01 EST 2008

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.

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

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'.

73,
Don W3FPR

Brian Lloyd wrote:
> 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?
>>>
>>>       
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