Troubleshooting the Omni VI S-meter

(posted by K0CQ)

Chasing the schematics, I find the meter is driven by the S-meter circuits for SSB and FM through diodes for all positions of the meter switch. The difference is that its also driven by a op amp on the 9 MHz Mixer/IF board 81594 with a series cal pot and a diode. There's also a shunt capacitor. If the cal pot got turned way up it might take over, if the op amp failed or a board ground failed, or something failed in the ALC FWD detector circuit to push lots of voltage to the op amp (U1A) that would drive the meter full scale and beyond all the time. I didn't chase down the FWD power detector circuit, but I think its also used for ALC and I recall reports of Tentecs sometimes losing diodes in that circuit from nearby lightning while connected to an antenna.

(Omni V s-meter comments that may be applicable)

As far as I know the manual with circuit diagrams that you can download from Tentec is all the "service" manual there is. Looking at it I have found that there is an S-meter sensitivity adjustment R45 on the IF/AF board along with an S-meter offset R43. It looks like R43 sets the zero and R45 sets sensitivity from low to very high. I don't know the value, but there is no fixed resistor to limit hypersensitivity. Providing the AGC is enabled, you should be able to adjust R45 to any sensitivity you like, providing the pot hasn't gone bad with age. If the pot has an open resistive element it may only work set up way too high. The pot may have an identification resistance on it to allow finding a generic replacement or to allow a will guess for value, at about 1/2 the pot's total resistance to be put in as a fixed resistor.

The other thing that can cause an S-meter to be hypersensitive is leaky bypasses on the AGC line so that AGC control is poor, e.g. that it takes a large AGC voltage to reduce the RF and IF gain. Maybe the RF gain controlled stage is on another board and the cable between them is open. It might be two common Tentec service procedures are in order. One is to tighten all the board mounting screws to make sure the grounds are good. The other is to apply a quarter drop of DeoxIT to each cable connector pin and slide the connector off and on a few times to get a solid connection again.

If all that's needed is setting R45, you will have to judge by ear what signals are strong and weak, like we did before we could run AGC on a receiver on CW or SSB, and adjust the meter for a satisfying reading that matches your perception by ear.

If R45 proves to be open, Cramolin who makes DeoxIT makes a pot restorer product that might work.

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