Hi:
To modify the A77 to bypass the input board jumper pin 1 to pin 4 , and pin 7 to pin 10 with a 5k resistor, This eliminates the input board.
You can also replace R809 on the playback amp board with a 10 K trimpot to adjust the hi freq equalization on playback.
These and other mods are from an article " Greening the Revox A77 " by Charles Repka from Audio Amateur magazine 4/1981 issue.
You can also check out posts from " Audio Karma " "Diy Audio " and " Reel to Reel Groups "
Hope all this helps
Ed