[Elecraft] K2 ALC Problem

[Craig Sande]

I completed the construction of K2 #3589 and all works well except for =
the ALC.

No matter how I set the power requested, the power output is high.  =
There was initial concern that I may have damaged MCU pin 2 =
inadvertantly with ESD, however the problem persisted after installation =
of a new MCU.  Therefore, I went through RF and DC voltage measurements, =
as well as checked components, solder joints, etc.  Based upon the =
following findings, I believe the problem is with pin 2 of U8 and wonder =
if there is anything else I am overlooking.

During transmit, D9 cathode goes from 0 to 4.1VDC (i.e. the RF output =
detector is functional), U6 pin 2 goes from 0 to 4.1VDC (the analog =
input of the RF output detector made it to the MCU), but U8 pin 2 =
remains 0.0VDC (this voltage drives the ALC circuit).  There is no short =
from U8 pin 2 to ground to explain the low voltage.  U10A, Q8, and RP2 =
of the ALC circuit appear to be correctly soldered.  There is no open =
circuit between U8 pin 2 and the anode of D3 in the ALC circuit.  U10A =
pin 1 remains at 0.6VDC during RX/TX transition (because there is no =
VPWR input).  The side tone function of U8 works.  I'm not sure how to =
test the other functions of U8 other than measuring voltages.  All of =
the voltages are within the range in the table except pin 2 which is =
0VDC and pin 10 which is at 5VDC.  Also, the BFO Buffer/Attenuator =
circuit has no RF voltage at test point 2, which I assume is due to the =
fact that their is no V ALC.

To test my theory that U8 is defective, I carefully desoldered U8 pin 2 =
(i.e. VPWR output) and pulled the lead out of the circuit board.  The =
voltage on this pin remains at 0VDC both on TX and RX.  I have no way to =
assess the digital input to U8, but at least other functions appear to =
work.  Therefore, I think that the pin 2 output circuit of U8 is bad.

Any other ideas, or am I on the correct path?

Craig, AE7I

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