[TenTec] Questions about mic input for "Omni C"

[Dave Haupt]

Gents, I have a modest challenge in front of me.  I'm to supply an audio processor to be used with a rig that I won't get my hands on until hours before it has to all work.  So, I need to "get it right".

The rig, I'm told, is a "Ten-Tec Omni C".  Going through the archives here, I think that means it's an "Omni D, Series C", as there seems to have been no such animal as an "Omni C".

The closest thing to a manual I've found is that of an Omni D, model #546.  In that manual, the only stated requirement for the microphone is "high impedance".  Usually, this means relatively high output voltage...in other words, the 5 millivolts from a Heil HC-5 won't drive the rig well at all.

Looking at the schematic, the mic input Z is evidently set by a 1 Megohm resistor feeding the input of what's nothing more than a two-stage preamp made of a dual 741 op amp.  Overall stage gain is about 26dB, and the second stage will begin clipping with mic levels of about 150mV peak.  That's quite a bit lower than many high-Z mics, like the EV638.  Mind you, I've not got my hands on a rig to measure - this is all based on a quickie SPICE analysis I performed on the circuit involved.

Can someone who owns one of these rigs place an oscilloscope directly on the microphone lead and read out what the actual audio voltage is during normal operations?  That would be exceptionally useful.

The pinning of the quarter-inch connector seems straight-up traditional: tip is PTT, ring is MIC, sleeve is GND.

Also from the aforementioned manual, page 2-7:

"Since the crystal ladder filter system is common to both the receiver and transmitter circuits, any audio signal picked up by the microphone while receiving may be applied to the system and appear in the audio output.  Acoustic feedback may occur and cause a sustained howl.  To completely eliminate any microphone pickup, it is recommended that a SPDT switch be used for the PTT function in the microphone, and that the microphone signal lead by shorted to ground during receive."

Would those of you who own and operate this model agree with the statement?  This will make some of my external switching ever so slightly more complex.

Thanks and 73,

Dave W8NF