Ed,
It certainly sounds like you have a bad solder connection there. It may
have worked just fine in the past and is just now developing enough
oxidation to be troublesome.
You may actually be looking in the wrong place if you are restricting
your search to U1. U1, U2, Q12 and their associated components are all
involved - including the RF Gain control on the front panel (and the
connectors to create the paths to the front panel too). In fact,
everything in the whole lower left corner of the schematic is
potentially suspect.
A brief description of the circuit - U1 mixes the 4913 kHz IF input with
the crystal and comes out with an output signal that is amplified by
U2b. The output of U2B is rectified by D1 and applied to Q12 which in
turn drives U2A. The output of U2A is sent to the microprocessor so it
can indicate the AGC level on the bargraph and also to the IF amplifier
to control the gain of that IC. Those points beyond D1 respond to DC
levels and those before D1 respond to the AC amplitude of the signal.
73,
Don W3FPR
Ed Lambert wrote:
My K2 s/n 1999 was completed in February 2002.
Last Sunday, during the EC Nets, I noticed that some strong stations were
actually getting audio signals into my ears that were close to the threshold
of pain. I also noticed they were not "deflecting" the "S" meter one bar.
After some experimenting, I discovered that my agc was intermittent.
Recalibrating it sometimes brings it back to life. Touching pin 7 of U1 on
the control board seems inconclusive for me. I cannot hear the signal on or
about 10.136 MHz, but I have been doing this in the operating position while
standing on my head on a banana peel with three tools in each hand. Maybe
when I get it on the bench I'll be able to hear the signal.
At this time I suspect a marginal solder joint somewhere in the signal path
from pin 8 of U12 on the RF board to pin 1 of 1 on the control board, or one
of the pins on U1 or U12, or a bad crystal X1 on the control board. I intend
to do some signal tracing in these areas to see if I can induce the problem
one way or the other. (Simply resoldering all of the joints is a last
resort; I'd prefer to learn something and identify the exact problem.)
I am curious about the S meter calibration process since it sometimes seems
to "shock" the system into operation. Can anyone tell me how that process
interacts with U1?
I am also curious if the collected wisdom of the reflector has seen a
similar problem and might be able to shorten my search before I start
digging into this.
Ed Lambert KD3Y
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