[Elecraft] Elecraft K3 problem

[Mike K8CN]

Hi Per-Tore,

I don't know if there's an easy way to check the PIN diodes in the T/R
switch, especially since the SMD components of the switch are mostly on the
underside of the RF board.  Here's where I would start:

1) Find spots on the RF board (top side) where you can measure the control
voltages 7R and 7T.  Verify that these change in opposition to each other,
i.e. if 7R is high, 7T should be low and vice-versa, when changing from RX
mode to TX mode and back.

2) Presuming that both voltages 7R and 7T behave properly in switching from
RX to TX mode and back, I would measure the voltage on the drain of Q7, the
MOSFET switch, when 7R is high.  Assertion of 7R should pull the Q7 drain
voltage low, which causes sufficient current to pass through both PIN diodes
D26 and D27 to enable the RX signal to pass through, presuming there is not
a problem with PIN diode D25.

3) It is possible that the Q7 drain voltage could be pulled low if only one
of PIN diodes D26 or D27 functions properly and the other has failed open,
so test (2) is not conclusive.

4) If you have a scope with a 10X probe, I would check for presence of RF on
the anode of PIN diode D43 when in TX mode - any frequency on the VFO within
the scope bandwidth should suffice.  D43 must be forward-biased by the 7T
voltage in order for RF drive to be coupled to the low power (10 Watt) PA
assembly (Z2 on the block diagram).  The fact that you have not been able to
detect any transmitted power in TX mode whether the 100 watt PA is bypassed
or not may indicate that the low power PA is not receiving drive from the
mixer.  The 7T voltage is supplied to the anode of D43 via the low power PA
input connector, but it's unlikely that the simple path internal to the low
power PA board is at fault, i.e. 7T is not reaching D43.

I haven't had my K3 opened up for many years, nor have I experienced the
problem you described, so I can't easily point out where to access the
circuit points mentioned above.

There is another set of PIN diode switches around the mixer, but I would
start with those closest to the low power PA before going further back up
the signal processing chain.

73,
Mike, K8CN



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