[Elecraft] [K2} K2-100 Test and Alignment Issues

[Don Wilhelm]

Bill,

For the J3+ lead, I think the PA transistors may be producing that 
voltage in response to light shining on them.  Shield the transistors 
from light and see if the voltage changes - if so, ignore it.

As for the C1 adjustment - do the following steps in order.  Preset R26 
and R27 to about mid-range.  If you want a closer setting, measure the 
resistance to ground from the center pin on the pot and set it close to 
43k - you can measure from U5 pin 3 and pin 5 for easier access than the 
actual potentiometer.

Balance the wattmeter by adjusting C1 - monitor the voltage at TP4 while 
sending 40 meter power through the wattmeter.  You should be able to 
obtain a very near zero voltage if all is well.  If you cannot obtain a 
very low voltage, there is a problem with one or more of the following:  
T4, D16, D17, R28, R29, C95 or C1.
Check the windings of T4 first to be certain the correct leads are in 
the right holes and have been well stripped and tinned (if there is any 
question, rewind T4 with new wire).  Then check the orientation of D16 
and D17 against the parts placement diagram at the back of the manual.  
Be certain all the parts I mentioned are well soldered and there are no 
solder bridges.
T4 does have a 3rd winding, and that is the red wire from the center of 
the SO-239 jack to the board for T4-6 and the bare wire from the center 
of the SO-239 to the board hole marked T4-5.  Be certain the bare wire 
is not touching the shell of the SO-239 and was trimmed so it is not 
grounding against the heatsink.

Once you have achieved the balance point for C1, do not change its position.
Adjust R26 during a TUNE to indicate the same power on the K2 display as 
you read on your external wattmeter.
Then adjust R27 to the same position as R26 unless you have a load which 
can produce a known SWR below 2.0:1, in which case you can use that load 
and adjust R27 until the K2 shows that known SWR.

You can initially set R26 and R27 with the low power from the base K2 
(do not connect the APP power cable).  Later you may want to refine the 
R26 setting using 80 or 100 watts.

Let us know how you make out.

73,
Don W3FPR

On 1/27/2014 2:02 PM, William Moore wrote:
> I am now into the test and alignment phases of my just constructed KPA-100
> amplifier to be integrated into K2 #3842. Right from the start of this
> phase, I have run headlong into a two issues I just can't figure out.
>
> I am constructing from the Revision G September 2010 manual. On the page 47,
> the voltage test chart, all measurements are in accordance with the ranges
> in the chart except for J3+ lead where instead of 0 volts, I am reading a
> stable and consistent 0.044 volts; certainly not a significant voltage but
> none the less, it is not zero as called for in the chart.
>
> The second issue is with reference to the C1 SWR Bridge Null Adjustment
> described on page 48. Adjusting C1 varies the power output reading on the K2
> LCD but does not vary the SWR readout which remains at 9.9 to 1. However,
> the SWR/watt meter on my Kenwood AT-200 antenna tuner indicates the K2 is
> running 15 watts with an SWR of 1 to 1 into the dummy load, a Heathkit
> Cantenna. Twisting the power pot has no effect on the actual power output as
> measured on the Kenwood AT-200. Removing the all connections between the K2
> and the KPA-100, the K2 functions normally with the power setting pot.
>
> I would really appreciate hearing from anyone who can offer any clues to
> what I am doing wrong in conducting these measurements and adjustments or
> perhaps identifying a possible construction mistake but gee, I checked so
> carefully.
>
>