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

[Michael via Elecraft]

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!
 
Anyone have an idea what could be the problem?
 
Thanks,
 
Michael
n2zdb