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| - | **Alternative AGC Pop Solution for Ten-Tec Omni D by Steve, KA0SM** | + | **Alternative / Additional AGC Pop Solution for Ten-Tec Omni D by Steve, KA0SM** |
| - | I wanted to pass along an alternative solution to the AGC “pop” issue that I implemented on my Ten-Tec Omni D (series B). | + | I wanted to pass along an alternative/additional solution to the AGC “pop” issue that I implemented on my Ten-Tec Omni D (series B). |
| - | The solution involves installing signal limiting diodes on the audio output of the IF-AGC board. | + | The solution involves installing signal limiting diodes on the audio input of the audio amplifier sidetone board (80447). |
| - | 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. | + | As N4LQ describes, the source of the AGC “pop” is the audio out from the IF-AGC board (80448) can momentarily peak at saturation of the op amp producing a square wave and overdriving the input to the audio amp sidetone board (80447) when a strong signal hits the receiver suddenly. The "surge" gets sent on from the IF-AGC board, through the AF Gain control, and to the audio amplifier board before the AGC circuit can react. |
| - | 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. | + | The output from the op amp (RC4558P) on the IF-AGC board can be a high as 7 volts in this situation. Even after passing through the AF Gain control and the resistors on the input side of the op amp on the audio sidetone board, the surge can momentarily hit or exceed the max input rating for the LM380 op amp (which is +0.5V), causing the op amp to be over driven into saturation, which creates the loud pop in the speaker, headphones, and phone jack. |
| - | 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. | + | My fix involves installing a pair of diodes (1N4148 or equivalent) in reverse directions between the AF input on the audio sidetone board and ground to limit the peak voltage coming into the board to roughly +0.5V (the forward bias voltage of the diodes). The pair of diodes are installed between pins 1 (ground) and 6 (AF in) of the audio sidetone board so one has the stripe towards ground and the other has the stripe towards Audio in. |
| - | 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. | + | The audio signal coming into the audio sidetone board from the AF Gain control is nominally around +0.1 to 0.2V RMS, so installing the diodes has no effect to the signal under normal conditions. The diodes only conduct when the signal exceeds their forward bias voltage. Without N4LQ’s fix, you may still experience a small pop as the audio amp is hit with a squared off wave, but the effect is significantly reduced. |
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| + | I would also like to add to N4LQ's Omni A/B/C/D AGC fix write-up that the R39 100K ohm resistor in the OMNI C that he refers to is labeled as R32 on the earlier OMNI D series A and B schematics. The difference in numbering comes from the fact that the Omni C added the hang AGC feature which increased the number of resistors on that board. | ||
| Steve\\ | Steve\\ | ||
| KA0SM | KA0SM | ||
