5/29/00
I've been working for 2 days on the Omni C trying to modify the agc to
reduce the popping effect of strong signals. The cw contest provided plenty
of super strong signals to test with! Here's what I learned:
The audio amp section of IC-1 on the IF-AGC board is biased to provide a
huge amount of gain. So much in fact that without sufficient agc voltage,
even weak signals drive it to distortion. When a strong signal suddenly
apprears, the first dit or static pulse drives this amplifer way into
distortion producing a square wave. The square wave hits the AF stage like a
ton of bricks and nearly blows out the speaker. Also, it's rectified by the
agc diode and applied to each IF amplifier making matters even worse. So
here's what I did:
Remove the top cover. Locate the IF-AGC board.
Tack solder a 5.6k ohm resistor across R39 on the IF-AGC board. R39 is the
100k resistor directly in front of IC-1.
Now readjust the S meter pot R-20. You'll need to turn it clockwise to
increase it's sensitivity. Calibrate it if you wish but don't trust it! You
will find the meter's action is a bit different now.
Now I can actually wear headphones without getting blased and the overall
distortion and noise level is so low it almost sounds like a direct
conversion receiver. You can still expect some minor blasting from local
stations but
overall this seems to be an improvement. At least if you're not happy with
it, just
remove the resistor.
A reminder about the audio mod I posted several months ago. This really
peaks up the af
response for cw and only one resistor is required. Simply put a 1meg
resistor between pin 2
of the AF power amp (input) and the negative lead of C11.
Give it a try and let me know what you think.
Steve
N4LQ
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