KAT2 Symptom: High RF output that is not controllable
27 Feb 2000 AB7MY
KAT2 Tech Note:
If the SWR sensor (T1) is not wired and installed correctly, there will be no detected Forward Power from the KAT2. The result will be that the K2 does not sense any RF output. This is because the very low output impedance of the LM358 op-amp at U4A pin 1, will pull the RFDET line in the K2 toward ground through the 47 ohm resistor at R6 without any sensed Forward RF from T1.
The K2 will then "think" it needs to increase the drive to "see" the requested RF output set by the Power control on the K2's Front Panel. If you barely increase the Power control from the minimum setting when this trouble occurs, the RF output of the K2 will go to maximum and possibly cause damage to the PA devices, or some other components. Don't let this happen to you!
If you do have such a problem, then:
1. Put a voltmeter on the cathode end of D1 on the KAT2 Control PCB, or the "hot" terminal of trimmer pot R1 when you transmit to see if there is any rectified RF voltage present. No voltage here at even low RF levels indicates a problem with the SWR / Power sensor circuit. You should see zero to about 3-4 volts DC present, depending on the RF output level.
2. Leave the RF input connector in place from the K2 to the KAT2 (J7), but remove the Control cable connector (J8) from the KAT2 to the K2's Control PCB. <whew!> You will then see normal power control provided by D9 on the K2's RF PCB, since it is the source for all power measurements in the K2's power control loop without the KAT2 installed.
Of course, always provide a good 50 ohm dummy load when doing any of the preliminary tests of the K2 or KAT2, and use caution when you first test the new KAT2 option. Be sure you have carefully inspected the completed L/C and Control PCB's of the KAT2 "sandwich". Watch out for loose particles of solder or lead debris, and be sure to trim all leads closely as mentioned in the manual.
If you install any of the red or green windings of T1 on the KAT2 Control PCB incorrectly, your Forward or Reversed power detection will not work. If there are any solder shorts around T1, or if D1 or D2 is reversed, or the LM358 is in backwards, then you will have this problem too. This same thing applies to the Reverse power or SWR detection, but having no sensed Forward power (when there may actually be a *lot* of power present), is much more dangerous to the transmitter circuits. The toroid is, of course, the most likely place for errors.
Since it is not possible to use an ohmmeter to check T1 when it is installed (all the leads will show a short between them), you may have to remove T1 or separate its red and green center tap windings to verify it is correct. Be careful to not allow solder to flow up onto the leads that are to be insulated from each other, thus causing T1 to not function properly when you transmit. Wind T1 carefully and install it carefully. Be sure you know the correct wires are inserted into the 1,2,3,4 pads provided on the KAT2 Control PCB. Check page 5 of the KAT2 manual. Some time spent in this portion of the KAT2 construction can save a lot of work later, since this is a fairly critical circuit for the RF output control of the K2.
Toroid tinning Notes:
I'm still seeing lots of poorly-tinned-toroid-leads (or PTTL for short), as I repair some K2's that are here for service. Please practice tinning the leads of your toroids if you are not comfortable with this process. The best way I have found (and the quickest, too) is to use a soldering iron of 650-700 degrees F, and a "blob" of solder on the tip of the iron. After a few seconds, you can move the tip back and forth on the enamel wire and add some fresh solder to help remove the melted wire coating and oxides. Flip the toroid and wire(s) over, and repeat the operation on the other side of the wire(s). Be sure you can see brightly-tinned wire right up to the edge of the toroid core. This is much more effective and faster than using a flame to burn off the enamel, and then scraping and sanding the wire clean. Try it and I'm sure you will agree. Try to make your toroids look like the ones in the manual, or those shown in the photos on the Elecraft Builder's Resource pages. This is one way you can add some personal skills to the kit construction, and produce a fine looking (and performing) product!
"PTTL" still holds it's spot at the top of the Ten Most Frequent Reasons for Kit Failure!!! :-)