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 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).

The solution involves installing signal limiting diodes on the audio output of the IF-AGC board.

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.

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 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.

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.

Steve
KA0SM