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omniagc [2024/02/10 17:44] n5na |
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| + | **Alternative AGC Pop Solution for Ten-Tec Omni D by Steve, KA0SM** | ||
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| + | I wanted to pass along an alternative solution to the AGC “pop” issue that I implemented on my Ten-Tec Omni D (series B). | ||
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| + | The solution involves installing signal limiting diodes on the audio output of the IF-AGC board. | ||
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| + | Looking at the schematic for the Omni D, the main issue appears to be that the audio out from the IF-AGC board (80448) can momentarily overdrive the input to the op amp on the audio amp sidetone board (80447) when a strong signal hits the receiver suddenly. The "surge" gets sent on from the IF-AGC board to the audio amplifier board before the AGC circuit can react. | ||
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| + | The surge on the audio amp input momentarily hits or exceeds the max input rating for the LM380 op amp (0.5V Peak to Peak), causing the op amp to be over driven, which creates the pop on the output to the speaker/headphones. | ||
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| + | The fix involves installing a pair of 1N914 diodes (in reverse directions) between the AF output of the IF-AGC board and ground to limit the peak voltage coming off the board to roughly 0.5V (the forward bias voltage of the diodes). The pair of diodes are installed between pins 1 (ground) and 3 (AF out) of the IF-AGC board so one has the stripe towards ground and the other has the stripe towards Audio out. | ||
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| + | The output from the IF-AGC board is nominally around 0.1 to 0.2V RMS, so installing diodes has no effect to the signal under normal conditions. The diodes only conduct when the signal exceeds their forward bias voltage. | ||
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| + | Steve\\ | ||
| + | KA0SM | ||
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