KPA100 unable to get a low null for SWR cal

[Elecraft mailing list]

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
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [hidden email]

[Don Wilhelm-4]

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!
>  
>

______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [hidden email]

[Elecraft mailing list]

Thank you to Don W3FPR who solved my problem!
 
I could not get the swr null lower than 200 mV.
 
Provided trim cap C1 was brown and did not have the minimum  capacitance
required.  I used a Murata green trimmer cap which now allows me to go  
down to
7.5 mV and the SWR shown on all bands into a dummy load it 1.0:1.
 
The replacement cap from Elecraft is part number E540012 (5 pF to 15 pF  
white body) which should be used at C1.
 
Michael n2zdb
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [hidden email]