KX1 Q6 problem

[Colin WHITMORE]

'Crafters,

I having a bit of a problem.  I've just recently completed a build of a KX1
and everything has gone together smoothly, as usual.  The problem is with
the power transitor, Q6.  If I tighten the flathead screw, Q6 must be
shorting against something, as the display goes blank and the unit no longer
work. (if you listen closely you can even hear the crackle of the short
begining)  If I back out the screw the KX1 comes back alive!

Before anyone responds please note, EVERYTHING IS THERE AND IN PROPER
PLACEMENT!  I only say this to be clear that the spacers, thermal pad,
plastic washer, etc. are all present and accounted for, and that all are in
proper position.

>From the PC board going up:

Screw with split washer on each side of PCB - No obvious issues here.

Short hex stand-off - Again, no obvious issues here.

Plastic shouldered washer - It's properly seated, that is the shoulder is
towards Q6.  Also it sits centered within the hole of the Q6 case.

Power transistor (Q6) - Is parallel and square to the PCB.  No funny cants,
twists, or other unusual orientations noted.  The leads are properly curved
and do not contact anything within the case or on the PCB.

Thermal pad - Opening in pad is perfectly centered within the opening on the
top cover.  The pad sits parallel to the nearest edge.  The orientation is
exactly underneath the power transistor.  In fact there is a slight amount
(~1/16-1/8") of thermal pad visable on all 4 sides when the PCB is fully
installed in the top cover.

Final flathead screw - Also no obvious issues here (however, see below).  
When the screw is not present, the view of the whole assembly (pad, Q6 and
hex stand-off) is centered exactly within the screw opening.

My thought is that the conical shape of the flathead screw may be coming
into contact with the opening in the case of the transistor and grounding
Q6.  Any thoughts?

Cheers,
Colin - AC0S


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[W0rw]

As Elecraft has so carefully warned about the shorting of the PA  Q6
(See Fig. 24 in the assembly manual).
i have found after about 5 additional disassembly and reassembly cycles, that
there were microscopic metallic particles being scrapped off of the Q6
counter sunk mounting hole.
One of those particles caused a +12V short to ground.
Prior to reassembling the unit better use a microscope make sure the
hole is free of these particles.
The mounting screw can bite the edge of the chassis and that can make
these little particles if you take it apart frequently.
If you have not done any disassembly/reassembly,  the area is
probably OK. Always check the +12 V line to ground after reassembling the
unit.
Paul  w0rw
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[Paul - VE1DY]

HI Colin...

You're not the first to have this happen.

In the archives you will find that some people took a 1/4" or 3/8"
drill bit and beveled (by hand) the top of Q6, to give more clearance
between the screw and the collector.  This works.

On some rigs, that screw goes down a little further.

Good luck.  The kx1 is a super little rig.

73, Paul - ve1dy


http://www3.ns.sympatico.ca/ppike


--- Colin WHITMORE <[hidden email]> wrote:

> 'Crafters,
>
> I having a bit of a problem.  I've just recently completed a build of
> a KX1
> and everything has gone together smoothly, as usual.  The problem is
> with
> the power transitor, Q6.  If I tighten the flathead screw, Q6 must be
>
> shorting against something, as the display goes blank and the unit no
> longer
> work.

               
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