[Elecraft] K2 S/N 4023 saga

[James T. "Jim" Rogers]

Rich wrote:

2.  Checking Q1 and Q2 on the control board.  When I try and key, the
voltage 8R goes to zero, the voltage for 8T goes high for about 20
milliseconds, then drops.   I do hear a blip in the headphones. If I set the
keyer to HAND, and hold the key down, the DOT/PTT line to pin 29 of U6 on
the control board goes to zero and stays as long as I hold the key down.  U6
pin 27 goes high for about 20 milliseconds and repeats the cycle similar to
voltage 8T.

Rich, here is what we now know. The MCU is controlling 8T(MCU 27 goes high
and turns on 8T). That's a good thing. But why is the MCU after 20ms or so
deciding to shut the 8T line down. The MCU monitors the current draw at all
times thru pin 4, ISENSE. Normally I would expect under an overcurrent
condition for the LCD display to display a HI CUR message indicating an
overcurrent situation. This is a programmable function through the menu, but
I am going to assume that in your trouble-shooting you have RESET the system
which would have restored the default setting eliminating the chance that
this function was programmed too low.

R115, the big black resistor on the front right corner of the control board
is where current is sensed. My DVM would never catch a 20ms current
measurement but your scope will. Make sure the POWER control is set to
MINIMUM. Now there are two ways of doing this.

1) If your scope (If my memory serves me right, you have access to a scope)
supports differential inputs, place a probe from Ch A on one side of R115
and a probe from Ch B on the other side. Record the measurement. PRESS and
HOLD TUNE. You should see an increase in voltage (from what you said above
it is only going to show for 20ms so look fast). Record that measurement. Go
to step 3.
***OR***
2)The first step is to measure 12V on R115 which will be the end closest to
the ON/OFF switch. Record that reading. Now move the probe to the other side
of R115 and record that reading ever how slight the change. Now keep the
probe on that point and press and hold TUNE. Record the highest level your
scope indicates.

3)Take the second reading and subtract the first reading from it. This will
give you the adjusted voltage drop across R115 in our suspected over current
condition. Now calculate the current I = E/0.05. If you indicate more than
two amps, there is a problem.

Good Luck
Jim W4ATK
K2 #4028