Service manual

ARS Analog Series User and Service Manual P/N 821518 Rev-B

General Circuit Description

Analog Switch and Button Backplane PCBs

Since a single-sided 24V supply is used, all op-amps are “biased” with a 1/2 supply reference. The CMOS

devices are powered from a separate zener-regulated 18V source on each analog switch and backplane board.

The analog switch elements are in series with an opamp summing junction, so they can handle very high input

signal levels without any "breakthrough" effect. The 4053's shunting contacts to AC ground improve off isolation.

The inverting input of any op-amp is sensitive to any stray capacitance to circuit common; though the summing amp

design includes stabilizing elements, care should taken to avoid capacitive loading of the summing busses. They

are jumpered between switchboards in some configurations.

The power supply board connects directly to the rightmost buffer/latch board ("backplane" board). Looping

through that board, it is jumpered to the remaining one or three other buffer/latch boards, and is then jumpered over

to the audio switch boards. The 4 position power harness between buffer/latch boards also carries the keypad

interlock bus.

The momentary contact closure to circuit common from the front panel button switch is translated by the backplane

board (assy 719125) to a latching high level (approximately 15-18 VDC) to drive the control inputs on the CMOS

analog switch board. The interlock function is implemented thus: Each button switch actuation generates a reset

pulse to shut off any circuit(s) which were previously ON. The reset pulse to any particular channel is overridden

while that channel's switch is closed, so two or more inputs may be switched on simultaneously. .

A tally/brightness control line for linkage to MSM units may be paralleled from the analog switch control inputs.

This circuit point may also be used for other remote control applications (either input or output) so long as loading,

input voltage limiting, and backplane driver isolation precautions are observed. External connections to these

control lines are via one or more D-sub connectors at the rear panel. Momentary actuation remote control circuits

may instead be connected to the backplane inputs, but there is no extra header for the physical connection. There is

also an "enable" bus connected to pin 6 of each 4053. It is available on H3 and H4, pin 12. (for bank switching or

other special functions)

If it becomes necessary to dissassemble a stacked audio switchboard assembly, insure that the upper board is

positioned directly over the lower one when reassembling. Should it inadvertently be displaced toward either

the front or rear, the power connections may become shorted.

Section 3: Tecnical Information