OMNI A/B/C/D AGC Mod

From the Ten-Tec Reflector May 29, 2000

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 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.
  • 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 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 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

Alternative / Additional AGC Pop Solution for Ten-Tec Omni D by Steve, KA0SM

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 input of the audio amplifier sidetone board (80447).

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 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.

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 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.

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
KA0SM


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