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K2 PA runaway current

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KC5CQW

K2 PA runaway current

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

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Don Wilhelm-4

Re: K2 PA runaway current

Damon,

First, remove the KAT2 and operate the K2 into a good 50 ohm dummy load,
and use some valid method of monitoring the actual power output.

If the same thing occurs, than look in the K2, but if it behaves
properly, look at the KAT2.

If you have a K2 problem, How much is the power output? You say the
current builds up - over what time period? What happens if you set the
requested power to about 2 watts? Do you still get the HI-Cur and how
much power is actually being produced?

If it is a KAT2 problem, what happens if you operate at 2 watts with the
ATU in CALP and a dummy load connected to the ANT1 output? Have you
nulled and calibrated the KAT2 wattmeter for correct power output
indication? If the K2 does not see the power output level being sent
back from the KAT2, it will increase power to the maximum in an attempt
to obtain the requested power output level so the KAT2 wattmeter must be
calibrated for proper operation.

73,
Don W3FPR

Damon Wascom 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
>
>

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KC5CQW

Re: K2 PA runaway current

In reply to this post by KC5CQW

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 < [hidden email] > wrote:
From: Robie Elms [mailto: [hidden email]]
To: [hidden email]
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 <[hidden email]> 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: [hidden email] > 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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KC5CQW

RE: K2 PA runaway current

In reply to this post by KC5CQW

At this point I'm entertaining the idea of a full PA rebuild. Considering the fact that it keeps getting worse and today, I faintly heard a sizzle somewhere... possibly one of the electrolytic caps.

I've now clocked more time troubleshooting the PA then building the whole K2 kit!!!
This makes me wish I would have just bought a K3 instead...

Damon KC5CQW

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