[Elecraft] KPA100 unable to get a low null for SWR cal

[Don Wilhelm]

Michael,

With all the holiday stuff, I think your question did not yet receive a 
response.

Yes, you should be able to obtain a very good null when you adjust the 
capacitor.

Pre-set the trimmer pots R26 and R27 to measure 43k from the wiper to 
ground - you can easily measure that resistance from pins 3 and 5 of U5.

If you transmit into an open 'load' you will have the same voltage on 
the forward and reflected - that part is normal.

Is this a newly built KPA100?  If so, what is the color of the C1 body?  
If it is blue, no problem, also if it is white, again no problem, but if 
it is brown, that capacitor will not work in the C1 location - email 
parts at elecraft dot com and request a PN E540012 capacitor to use at 
the C1 location.
If this is an older KPA100 with either a blue or ceramic capacitor at 
C1, there is a possibility that C1 has failed.  Order PN E540012 to use 
as a replacement (the blue trimmer caps are no longer available).

73,
Don W3FPR


On 12/28/2014 7:45 PM, Michael via Elecraft wrote:
> Usually I can get a SWR null around 10 - 15 mV with the power level at  2W.
>   
> For some reason I can only adjust it down to around 200 mV into a 50 Ohm
> dummy load. I have tried changing the diodes and U5 but they did not make a
> difference. Also rewound T4 using another toroid.
>   
> Also noticed that when I disconnect the dummy load and transmit at the 2W
> adj level both FOR and REFL voltages are about the same (TP4 = 1.1Vdc, IC5
> pin  7=1.1Vdc)!  Shouldn't the VREFL be much higher then the VFWD?
>   
> Both pots are adj to 27.8 kOhms for power calibration at 2W using a
> LP-100A.
>   
> When transmitting 2W into 50 Ohm load (K2 display shows "2.1W   1.4:1")
> LP-100A shows approx. 2W  SWR- 1.05:1
>   
> VFWD = 0.822 Vdc  (IC5 pin 7)
> VREFL= 0.173 Vdc  (IC5 TP4 )
>   
> Why is the K2 showing a much higher SWR then it should be showing?   Could
> there be a problem with U6?
>   
> I did not build this K2/100 and have found mistakes along the way then
> trying to get it working properly.... I can't seem to find any in this area yet
>   though!
>   
>