[TenTec] OMNI VI Plus OPT 3 VOX problems

[A R]

Recently acquired a nice OMNI VI Plus-OPT 3, and have disposed of all issues except for an inop VOX function. In fact, the VOX misbehaves. When the VOX pushbutton is pressed, I get the VOX lamp lit, but even without a mic connected, the radio goes into TRANSMIT mode regardless of where the VOX gain, delay, or anti-vox settings are,  as long as the AF GAIN is advanced anywhere above full counter-clockwise. If the AF GAIN is full counter-clockwise, the radio stays in receive mode (with VOX engaged), as I can see received signal strength on the S-meter, but of course can't hear the received sig with the AF gain turn all the way down. If the AF gain is advanced at all from full counter-clockwise, the RF output pulses to full output on a 1-2 second period. And...the radio does NOT recognize input from the mic with any VOX or MIC gain setting (i.e.,no apparent modulation). 
Looking at the schematic, I suspect that U3 LM348 on the XMIT AUDIO board may be the culprit, since it processes both the MIC (XMIT) and SPEAKER (RCV) audio. But theres a lot of stuff downstream of U3 (including the DSP processor on the logic board), as well as a few electrolytics wrapped around U3. Haven't started t-shooting with the scope...yet. 
Anyone heard of or experienced any such VOX symptoms, or have any ideas what may cause this units VOX problem? Any known or notorious "bad actors" in this VOX circuitry. Any and all inputs are welcomed.
TNX es 73,Allen--W7GIF