Prior to modification I measured the following:

J1 pin 2 - audio out 5.5 volts peak to peak, centered @ 0.0 VDC
J1 pin 28 - squlelch .75 volts peak to peak at threshold, 4 volts p-p full tight

For an HLN4838B common circuits card transmit audio was feed to U502B pin 6
via a 11K series resistor and a .1 ufd cap.  I removed R17 on the iMic (output
to ground), but I don't think that was necessary.  Without the .1 the 150 ufd 
output cap on the iMic was is reverse biased by the Syntor's opamp DC bias.
The output cap had enough leakage when reverse biased to mess up the bias on 
U502 enough to rail it's output. I thought the output resistor was causing the
problem, but removing it didn't fix it.

For an HLN4797 common circuits card the feed point is pin 1 of U502A.
The feed point may be different on other common circuits cards.  The point is
the inverting input of the opamp section that feeds the splatter filter.        

PTT is from GPIO 5 on the iMic via the Xcat control cable pin 3 (UF7), I pulled
pin 3 on the Xcat to prevent it from interfering.  PTT connects to U1.6 on the
personality board.

iMic UAC3556B           Xcat cable
28 GPIO 0               17 COS
23 GPIO 5               3   PTT (Xcat pin pulled)
20 GPIO 8               18  Doug Hall clock
19 GPIO 9               4   Doug Hall data


Rx Audio is from J1 pin 2 via the following:

            +5
             ^
             |
             \
             /
             \  4.7 k
             /
             \
             /
             |
             +-----< J1 pin 2 (detected audio)
             |                 
             \
             /
             \  3.3k
             /
             \
             /
             |
             +--------> Audio in to iMic
             |  pin 7
             \
MCP41010     /  6
digital      \<-------< J1 pin 28 (squelch)
squelch      /
 pot         \
(10k)        /
             | 4,5
             |
          [ground]



MCP4101         Xcat Ribbon cable
1 CS            11      UF3
2 SCK           9       UF4
3 SI            5       UF6
4 VSS           19,20   Ground
5 PA0           19,20   Ground
6 PW0           -
7 PB0           -
8 VDD           1,2     +5


The iMic should be powered by the radio, not the USB bus to solve a power
sequencing problem.  If the iMic is powered by the USB bus it must be connected
before radio is powered up or it will not be recognized.  Not a good thing
for recovering from power failures since the radio will power up before the
computer has had time to boot and look for USB devices.

Modification:  Lift UAC3556B pin 32 (Vbus) from its pad and connect to ground
via a 100k resistor.  Connect the USB Vbus line to pin 32.  Connect the VBUS
pad on the iMic to +5 from the Syntor.