Sarah,
Congratulations on getting your license and venturing to build your K2.
The AGC Threshold high limit is directly proportional to the 8 volt
regulator actual output - if you follow the circuit you will discover
that that voltage is really the result of a 3 legged resistive voltage
divider. With a full 8 volts out of the regulator, the typical highest
voltage is about 3.85 to 3.9 volts depending on the exact values of the
5% tolerance resistors. Since your 8 volt regulator is near the lower
edge of its tolerance range (actually a bit lower than it should be),
you can set R1 to its full limit and things should work fine.
I would recommend that you replace the regulator since a 5% tolerance
would produce an output between 7.60 and 8.40 volts.
73,
Don W3FPR
Sarah K. wrote:
Hi,
I'm Sarah, AF6FH! I just got my license a couple weeks ago, and I'm
now assembling K2# 0622, which will be my first HF radio. I started
assembly on Wednesday night, and so far, everything's been going just
fine.
But...
I'm having a little issue with setting the AGC Threshold.
The instructions (pg. 48) say to set the RF GAIN control fully
clockwise, measure the voltage on pin 5 of U2 on the control board,
and adjust trimmer R1 for a reading of 3.80 volts.
I did my voltage measurements with my own (cheap) DMM, not the
internal voltmeter.
My concern is that, by adjusting R1, the highest I could get the
voltage to go was 3.59v, so for the moment, that's where it's set.
I measured the voltage on the LM29307-8, and it has 12.23V at the
input, but only 7.53V at the output. The 78M05 has 4.93V at its
output.
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