TKR750/850 signals and audio from DB25 connector:

Pin 7  - GND - ground for PTT
pin 8  - TD  - Flat TX audio to 5 kHz (TD in the test mode sets sensitivity)
Pin 10 - RD  - RX audio - raw unfiltered (RD in the test mode sets output level)
Pin 12 - RXG - RX signal ground (For RXA and RXD)
Pin 16 - PTT - active low external PTT
Pin 19 - TXG - TX signal ground (For TA and TD)


Under "Test Mode" in KPG-91D programming software:
Squelch open = 65
Squelch tight = 75
RD = 256
TD = 256
NOTES:
About 2.5 V P-P is required at TD for 5 kHz deviation, about 1.5 V P-P for 3 kHz deviation.  
A URI or standard FOB has insufficient TX audio without additional amplification.  
A RIM, RA-35,  RA-40, or RA-42 works fine.

RA-35/40/42 Pinout
Pin 1 Right (not used)
Pin 2 Left (composite)
Pin 3 COS (not used)
Pin 4 CTCSS (not used)
Pin 5 PTT
Pin 6 Receive Audio
Pin 7 Local Control (not used)
Pin 8 Ground
Pin 9 +12 (not used)

TKR-750 -- RA-35/40/42
Pin 7      Pin 8
Pin 12     Pin 8
Pin 19     Pin 8
Pin 10     Pin 6
Pin 16     Pin 5
Pin 8      Pin 2

Set RA-35/40/42 JU2 to supply +5VDC to audio amps (JU2 to "A" position)
Set RX pot half way
RA-40/42 set TX pots to maximum (fully clockwise)
Jump middle pins on filter headers - if not already done
In usbradio.conf file:
rxboost = 0
txboost = 0
carrierfrom = dsp
ctcssfrom = dsp
rxdemod = flat
txprelim = yes
txlimonly = no
txmixa = composite
txmixb = no
invertptt = 0
duplex = 1


In radio tune menu - Set the following:
Run the noise and level calibrations for the receiver audio.
radio tune txtone = 750 Hz deviation as indicated on service monitor.
radio tune txvoice = 3kHz deviation as indicated on service monitor.