G3TXQ

Amateur Radio (G3TXQ) - TenTec Omni VI agc modification

When I got my Ten-Tec Omni VI I was a little disappointed with the quality of the received audio - particularly the agc characteristics. I guess I had become so used to the "hang" agc characteristic of my Corsair II, which prevents background noise rising during brief pauses in speech. Looking at the agc circuit of the Omni VI I realised that it wouldn't take much work to turn it into a "hang" system.

My modification leaves nearly all of the existing Omni agc circuitry intact. I have simply added an audio detector which modifies the delay characteristics of the original system: if audio is present, the agc response is very slow; once audio disappears, after a short delay (the hang period) the agc responds very quickly.


Board modification Close-up of modification

There is little originality in the circuitry - it was taken from the hang agc system described in "Solid State design for the Radio Amateur". I built the circuit "ugly style" on a small piece of copper-clad board which is mounted in a corner of the existing IF/AF (81602) board. I chose to fix the new board in place using nuts and bolts, but "double- sided sticky pads" would work just as well and would avoid having to drill holes in the Omni board.

Just 2 changes are made to the Omni board, both of which can be made without removing the board. Firstly, one end of R22 (330K) needs to be cut to open-circuit the resistor; if this is done carefully it will be possible to reverse the modification, if ever required, by re-soldering the resistor lead. Secondly, a 12.5v supply connection needs to be made to the new board; I tack soldered a lead onto the 12.5v end of R52 (3R3). Don't do this with the power on! All the other connections required are made using the existing flying leads and board connectors.

Full details can be found on the schematic. The existing front panel switch functions - AGC ON/OFF and AGC FAST/SLOW - are retained.

I have been very pleased with the results. If anything the performance is better than that of the Corsair because the design retains the Omni's IF-derived agc system and therefore responds more immediately to the onset of a large signal.