K2: Flakey PA Output

[Clark Macaulay KE4RQ]

Was operating the ARRL CW contest and output on my K2 QRP rig dropped to zero.  Bummer. Anyway, I took it to the bench and started to check it out when it began to work.  Put it back together and took it to the operation position and it failed. I'm thinking a cold solder joint or something.  

Tonight I stared signal tracing and found the output of Q6 to be zero under key down after a few seconds.  I as about to order a new Q6 and it started working again.  I've been poking around, pushing this component and that one, hoping to find an intermittent but with no success.

Could Q6 be failing internally--that is the little beastie is going bad not because of a solder joint (which I have reheated) but because of something inside?  Is there another component in the driver circuitry or PA that could be causing this?  

I really wanted to use the K2 this weekend for the QRPARCI contest but may have to rein in my big rig.  

Your comments and suggestions are greatly appreciated.

73 de Clark KE4RQ

[Don Wilhelm-4]

Clark,

Intermittent operation does usually mean a marginal solder connection -
particularly those that might have a heat characteristic to them.
Q6 is quite husky, so I would not suspect the device itself.  Check R50
for good soldering, also look at the schematic Sheet 4 of the RF Board
to identify the other components associated with Q6.  How is the tinning
of the red leads on T2?  The Green leads of T1?
Q10 and R46, R47, R48 are also suspects.

73,
Don W3FPR

Clark Macaulay KE4RQ wrote:

> Was operating the ARRL CW contest and output on my K2 QRP rig dropped to
> zero.  Bummer. Anyway, I took it to the bench and started to check it out
> when it began to work.  Put it back together and took it to the operation
> position and it failed. I'm thinking a cold solder joint or something.  
>
> Tonight I stared signal tracing and found the output of Q6 to be zero under
> key down after a few seconds.  I as about to order a new Q6 and it started
> working again.  I've been poking around, pushing this component and that
> one, hoping to find an intermittent but with no success.
>
> Could Q6 be failing internally--that is the little beastie is going bad not
> because of a solder joint (which I have reheated) but because of something
> inside?  Is there another component in the driver circuitry or PA that could
> be causing this?  
>
> I really wanted to use the K2 this weekend for the QRPARCI contest but may
> have to rein in my big rig.  
>  
>

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[Clark Macaulay KE4RQ]

I guess I should have been a little more explicit in my trouble shooting. The problem of power dropping to zero was on all bands which seemed to eliminate the filter board.  The problem had happened once a year or so ago when I posted a question relating to "Lo Po" display (I have the KAT2) but then seemed to go away.  Hmmm.  

The DC voltages checked out fine.  Using the rf probe, I found rf (with values a whole lot higher than those listed in the manual) up to the base of Q6 using the TUNE function and/or a straight key for key down. There was at times some output on the collector of Q6 but would quickly drop to zero.  That's why I posted my question late at night (my eyes and brain were turning to mush) about the failure rate of Q6.  

The next day I reheated the solder joints for all components around Q6 and output returned. R50 value measures okay.  A closer visual on the toroids indicates I may have to remove them and retreat the leads.

Thanks to Don and Gary for their suggestions.  I think I'm on the right track to fix this intermittant.  If I get it solved by Dayton, I just might purchase another option for my K2 (thinking about the transverter option) while "she" is on the operating table.

73, Clark KE4RQ
K2 #