[Elecraft] [K2] TX Issue in Part III checks

[Don Wilhelm]

  Brian,

The DC voltages on Q24 are based on a properly working K2 at the 5 watt 
power level.
What  you now have is a K2 that will not develop power to the RF 
Detector, so the power control circuits will react to force the RF 
voltages to their maximum - the control element for that is the VALC 
voltage to Q24, and that will go to its maximum since there is zero 
power detected.

I note that you measure zero volts at the drain of Q24, but VALC at the 
source (CB emitter of Q8) is at 7.4 volts.  Something is broken (not 
soldered) between those two points because the only component between 
the two is R98 which should not be drawing any current and therefore 
should not have any voltage drop (given your situation with no power 
output).

Look for unsoldered connections (don't forget the connectors) or solder 
bridges in that signal path.
Look critically at you soldering - if the solder did not flow out to an 
almost invisible edge on the solder pad, it is suspect, and the likely 
cause is that the solder pad did not receive enough heat.  Re-flow the 
solder on all suspect connections using a hot (750 deg F) iron, and add 
a bit of flux if necessary.

73,
Don W3FPR

On 1/8/2011 10:44 PM, Brian Handy wrote:
> Hi All,
>
> I've been working on my K2 and I'm on the home stretch.  I had to dabble a bit
> to find a cold solder joint in the Part II checks, but eventually got a
> home-brewed test oscillator to reach through on 40m!  Quite happy with things so
> far, and learning a lot.
>
> I still have a lot to learn about the transmitter side though, that doesn't
> work.  When I first hit 'tune' in the Part III checks, I noticed high current on
> the power supply, high current on the front panel, and changing the power level
> didn't help, it was always high draw.
>
> It didn't take much of this to zorch the PA finals and short things straight to
> ground.  Nothing worked until I pulled T3, and while I had the soldering out I
> took out the PAs and started looking around and reading the mail archives.
>
> I noted one suggestion from Don to someone else and checked the voltages on base
> on the finals - both are around 0.65V, which I think is OK.
>
> I realize things won't be quite the same without the PAs there and proper
> feedback happening, but ... I then started going through all the voltages on the
> RF board with TUNE on.  Everything looked pretty much fine except for Q24 ~
>
> Leg - Expected - Measured:
> G    - 0V - 5.6V
> S - 1.2 - 2.5V
> D - 13V - 0V
>
> That doesn't look right at all to me.  Exploring further, I started chasing VALC
> down the rabbit hole.
>
> Pin 2 of U8 (V PWR) seems to vary between 3.8-5V as I turn the PWR knob in
> TUNE.  I decided that was OK.
>
> Pin 1 of U10A doesn't move around a lot, only up and down about 0.1V.  Finally
> the emitter of Q8 (V ALC) seems stuck at 7.4V.
>
> I was firmly confused at this point, so went back and started checking Control
> Board voltages per the page of expected readings in the back of the manual with
> CAL FCTR turned on.  Everything seems turned off when I do that and I'm seeing
> pretty much 0V everywhere.
>
> I'm a bit saturated at the moment, but thought I would ask if this sounds like
> I'm on the right track, or if I have to get to poking elsewhere ...
>
>
>
> Cheers and thanks in advance,
>
> Brian Handy
> AE7JW
> Tokyo, Japan
>
>
>
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