API Audio 1608 Recording Console - Manual (Page 49)

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9.7 840B Ancillary Connections
The rear panel connections for the 840B Central Facilities module are as follows:
9.7.1 Talkback Remote Port and Oscillator Output
NOTE: The screened pinout on the rear panel of very early
1608’s is actually WRONG (serial numbers below 020). The
CORRECT pinout is as above.
9.7.2 Stereo Headphone Output
Stereo Headphone Jack: (under armrest on the Right side of the frame)
 Headphone amplifier output
 1/4" tip-ring-sleeve jack
TALKBACK: Talkback Remote Port
 Pin 1 = +5 volts (fused)
 Pin 2 = T/B To Echo
 Pin 3 = T/B to All
 Pin 4 = T/B Slate
 Pin 5 = Ground
 5-Pin Female XLR connector
OSCILLATOR: Oscillator Output
 Balanced, Line-level
 Male XLR connector