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omniagc [2007/04/06 20:15] n5na |
omniagc [2024/02/10 17:44] n5na |
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| 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: | 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: | + | 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 appears, the first dit or static pulse drives this amplifier 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. | *Remove the top cover. Locate the IF-AGC board. | ||
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| 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 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. | + | Now I can actually wear headphones without getting blasted 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 [[546caudio|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. | A reminder about the [[546caudio|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. | ||
