KAT2 Alert #2: Important Assembly/Alignment Changes
28 Feb 2000
Please make the changes listed below to your assembly manual. Technical details on the R6 change appear at the end.
1. PAGE 4, Second assembly step: Delete R6 from the list of resistors to be installed in this step. Change the last sentence in the step to: "Be sure to install the 470-‡ resistor at R5 (yellow-violet-brown)."
2. PAGE 6: Change the illustration to show the lock washer going between the standoff and the control board. (This corrects the board spacing problem mentioned earlier, which is due to variance in relay and connector height.)
3. PAGE 11, 5th step from the bottom: Add a new step after this one:
[ ] Using an ohmmeter, verify continuity of each connection between J8 (the 10-pin housing) and J3 (the KAT2 control board end of the cable).
4. PAGE 16: Print out and attach these new steps to the bottom of the page:
[ ] Set up your DMM to read DC volts (20 or 30 volt scale).
[ ] On the bottom of the KAT2 control board, locate a pad labeled "B". Just to the left of this pad you'll see the round pad for pin 1 of U4 (LM358). (You'll know you have the right round pad if it's part of a group of 8 pads, 7 of which are square.) Touch your DMM's (+) probe to this pad. Connect your DMM's (-) probe to any ground point on the K2.
[ ] In receive mode, the voltage at this test point must be 0 volts. If not, trace backwards through the circuit to find where the voltage is coming from.
[ ] Go into TUNE mode briefly. At 5 watts, the voltage should be between 3 and 4 volts. If it is much lower or higher than this, you probably have a wiring error in the SWR bridge. Remove the KAT2 control board from the L-C board and check the wiring of T1. Make sure T1's leads are properly stripped.
[ ] Once you have the correct voltage indicated at U4 pin 1 in TUNE mode, install the 47-ohm resistor at R6 (yellow-violet black). The two pads for R6 can be found from the bottom of the KAT2 control board. First, locate a pad at one end of the board labeled "6V". The pads for R6 are above and below the "6V" label. R6 can be installed on either side of the board.
Technical Note on R6
The installation of R6 (47 ohms) is being moved to a later point in the assembly to prevent problems during initial testing of the ATU. With R6 removed, the K2's original RF detector circuit (RF-D9, etc.) will control power output. R6 will not be installed until the ATU's own SWR bridge is proven to be working correctly. This precaution is important, since any wiring error to the ATU's bridge could result in 0 volts output at U4 pin 1, making the K2 drive to full power output.
Question: Why is the KAT2's bridge output fed back to the K2?
As with most transmitters, the K2's PA output impedance is almost never *exactly* 50 ohms, even if the tuner says the SWR is 1:1. The actual output impedance could be 35 ohms, 65 ohms, etc., depending on the band, supply voltage, POWER level, and operating mode. Most tuners (including the KAT2) will happily match the antenna to this load without complaining, since their bridge circuits only look for minimum SWR--they don't impose a 50-ohm constraint on the source and load. (An exception is true absorptive-type bridge circuits, which use resistive elements to enforce a 50-ohm bilateral match.)
Since the K2 itself provides only a simple RF detector (D9 on the RF board), such a non-50-ohm match will result in incorrect power readings. For example, the K2 display could say 5 watts, but actual power output could be anywhere from 3 to 7 watts. However, the link-coupled bridge
circuit used in the KAT2 provides a DC output voltage that is reasonably accurate over a fairly wide PA impedance range. (The bridge's forward and reverse output voltages are a function of both the RF voltage *and* RF current, i.e. power.) So, by connecting the KAT2's forward voltage output back to the K2 control board by installing R6, we obtain much more accurate POWER settings.
The value of R6 was carefully selected. 47 ohms provides enough isolation to prevent U4 on the KAT2 from oscillating, but it is small enough to allow the KAT2's bridge output to "capture" the VRFDET line on the K2 control board. R6 also serves as a convenient place to break the circuit during initial test and alignment (as explained above).