Seeking info on the correct position of the switch on the IR tether receiver board 691-PCB1-039 with the AD20A01 Chip

It doesn't matter which position that switch is in for your locomotive.

If I remember correctly, it was only in use on one locomotive, the 6-18064 New York Central Mohawk from 1998, to serve as the RailSounds/SignalSounds switch.

Every other locomotive, through 2004-05 when the AD20 was phased out, had a separate switch mounted either on the frame, under the coal load or hatch, and therefore the switch mounted on the board no longer had any function.

I hope this helps.
TRW

That's not really true, if either switch is in the ON position, it will switch into Signal Sounds mode.  I agree it's not terribly useful with the external switch, but it sadly still affects things if it's set wrong.

On the 691-PCB1-038 motherboard, the switch on the AD20 will short pins 15 and 17 of the Railsounds audio card and drop the card into Signal Sounds mode.

Thanks PAPERTRW and GUNRUNNERJOHN!

I'll follow the traces on the board to make sure the switch is in the open position.

Thanks!

Looking at the board front, switch at the top, left is turned off, which enabled the full RailSounds.

Thanks, Gunrunner!