[Elecraft] K2 KPA100 SWR Bridge Readings Way Too High

[Don Wilhelm]

Dick,

Make certain you have the diodes D16 and D17 mounted the right way around.

Replace the coax jumper to the dummy load - measure your dummy load with 
your ohmmeter to be certain it is a good 50 ohm dummy load (yes, dummy 
loads can fail).

Then balance the wattmeter.  Easiest way is to power the K2 from only 
the coaxial 2.1mm power jack (no power to the KPA100) with power set to 
10 watts and adjust C1 for a voltage as close to zero as possible read 
at TP4 (should be in the low mV range).  If you cannot see a very low 
voltage at TP4. then measure U5 pin 3 and adjust C1 for the lowest voltage.
Next with power off preset R26 to 43k ohms read at U5 pin 5 and R27 to 
43k read at U5 pin 3.

You can do the initial tests of the wattmeter still in low power mode 
(no power to the KPA100).  Set for 10 watts and do a TUNE - do not stay 
in TUNE for more than 30 seconds at a time, allow a 1 minute cooling off 
period between each TUNE.
Check the DC voltage at U5 pin 3.  It should be close to zero.  The 
voltage at U5 pin 6 should also be close to zero.  Also check the 
voltage at U1 pin 3 which should also be near zero, and the K2 display 
should show an SWR of 1.

Connect an external wattmeter between the dummy load and the KPA100 - 
make certain you use known good coax jumpers.

Now do a TUNE and set R26 so the display power is the same as your 
external wattmeter.

Next, with the same TUNE conditions, measure the DC voltage at U5 pin 5.
You should see the same DC voltage at U5 pin 7, and at U1 pin 2.

Those two voltages at U1 will determine the SWR calculated by the KPA100 
firmware and passed along to the base K2 MCU.

If all is well on the DC voltages to this point and the K2 still shows 
an SWR greater than 1.0 or 1.1, you have a KPA100 firmware problem.

If the firmware is showing the correct SWR, then set R26 so the K2 
display shows the same power as on your external wattmeter.  You will 
want to refine that power (R26) adjustment at 100 watts later but the 10 
watt level will be sufficient for the initial setting.

If you are not able to set the power display without deviating 
substantially from the 43k initial setting, then check the DC voltages 
during a TUNE at U6.  The DC voltage at U6 pin 3 should be the same as 
that measured at U5 pin 7, and the DC voltage at U6 pin 6 should be 
about twice that at pin 3.

Take it one step at a time and in the order that I indicated.  If you 
skip around, you will become confused because one thing depends on the 
step before it.

If all those conditions are correct, the KPA100 wattmeter is working 
correctly.  Power the KPA100 and try it at 100 watts (hold TUNE/DISPLAY 
together).  Quickly adjust R26 to display the actual power.  If you have 
to move R26 very far, then you have a problem with the scaling. 
Transistors Q10 and Q11 may not be working properly.

73,
Don W3FPR

On 10/2/2017 10:58 PM, Dick Lamb wrote:
> The SWR bridge on my K2/KPA100 had been working perfectly for years, but I
> apparently blew something while making comparison measurements with a
> friend's K2 I was trying to fix. Now the K2 panel LCD SWR indicator reads
> around 5:1 for a near-perfect dummy load.  Adjusting C1 makes almost no
> difference.  I can force the SWR to read 1:1 by setting R26 toward one end
> of its travel and R27 toward the opposite end of its travel (instead of
> both at midpoint), but then every load reads as 1:1 SWR.  Replacing D16 and
> D17 didn't help (I tried it twice).  Except for the SWR reading, everything
> seems OK.  Perhaps I should try replacing U5 and U6?  Other ideas?   Dick,