[Elecraft] K2 PA runaway current

Fri Feb 8 17:30:18 EST 2008

     The tuner has been ruled out as a cause to the problem. The K2 will show the same behavior into a dummy load either with out without the internal tuner installed. The radio itself did work for about a week with full (15W) output. My best guess is the initial problem happened as a result of excessive vibrations causing the ceramic coating to wear down enough to let RF pass between relay K12 and C4 then to ground. 

     All inductors are ok visually and they show continuity (no open conditions) with their respective areas of the circuit. I'm starting to question the resistors such as R50 and R58(?) as being damaged in some way. Usually a resistor will open with excessive current, possibly these are shorted internally... They seemed fine with my DMM. 

     To clarify, If I set the power to 2W (even down to 0.2W) I will see the correct value indicated on my external tuner (using the tune button). On the K2 display (and external tuner), after about 5 seconds or so, the wattage or current (depending on the readout selected) will creep up building "speed" until  the "cal Cur" set-point is reached. At this point, the radio will remain in a TX switched state without RF power output until I force it back into RX with the "tune" button. 

73, Damon

 --- On Fri 02/08, Robie Elms < ruler55 at gmail.com > wrote:
From: Robie Elms [mailto: ruler55 at gmail.com]
To: kc5cqw at excite.com
Date: Fri, 8 Feb 2008 10:28:39 -0600
Subject: Re: [Elecraft] K2 PA runaway current

Damon, Since you say the current builds I believe you have a spurious oscillation. I would remove the KAT2 and configure the rig so that you can connect the output directly to a 50 ohm dummy load. This will eliminate the possibility of a tuner problem. If you still experience the same type of problem I would look closely at T3 and T2 to make shure they have the proper number of turns and are connected correctly. I would also look at RFC's 8, 9 and 10 to verfiy they are the correct value. If removing the tuner eliminates the problem, then I would first verfiy winding and installation of T1 in the tuner. You will get more input from a number of individuals much more experienced with troubleshooting the K2 than I am on this e-mail list. However, perhaps my suggestions will get you moving forward again. Robie - AJ4F On Feb 8, 2008 9:52 AM, Damon Wascom <kc5cqw at excite.com> wrote: > > Hello elecraft 
list, > > I need some help with troubleshooting my K2. I destroyed my finals (twice) due to a shorted cap (C4) in the KAT2. For troubleshooting purposes, the finals have been temporarily replaced with 2SC2078's ($0.68 vs. $6-11 for the 2SC1969's). My problem is when I TX at any band and any power setting the current starts to build until I get a Hi Cur warning. > > At this point i've: re-worked ALL solder joints on the RF board, performed signal tracing per the manual, inspected every component visually, checked the resistors and semiconductors in the TR switch and PA with my DMM (power off and voltages during TX) and pulled out my remaining hair. > > The same problem happens with the 2SC1969's in place so the substitution of the finals has been ruled out. I just don't want to fry any more (I can't tell my XYL oops, I did it again...) > She is already upset with me about this. > > Here is my K2 configuration: > K2 #: 06351 (15W), 
KNB2, KSB2, KAT2, KAF2, KIO2, K160RX, K60XV and KBT2. > In other words, "Fully Loaded". > > I have been conducting tests with my MFJ-969's dummy load. My belief is that the problem is in the PA but that's just a guess. > > Thanks, Damon KC5CQW > > > _______________________________________________ > Join Excite! - http://www.excite.com > The most personalized portal on the Web! > > > _______________________________________________ > Elecraft mailing list > Post to: Elecraft at mailman.qth.net > You must be a subscriber to post to the list. > Subscriber Info (Addr. Change, sub, unsub etc.): > http://mailman.qth.net/mailman/listinfo/elecraft > > Help: http://mailman.qth.net/subscribers.htm > Elecraft web page: http://www.elecraft.com >

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