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.
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