TR-7800 Tone pad conversion for 4th column function
Users who desire ABCD control functions may be accommodate by an alteration to the Control unit and to the
REV switch. This switch will then perform dual functions.
Procedure
Remove top and bottom covers, 4 screws each.
To free the front panel remove 4 screws securing the front panel bracket to the side frame.
Unplug 5 multiconnectors from the Control unit along the front and left edges of the board.
Remove 6 screws from the Control unit and swing the board over.
Remove and save R51 22 Kohm located between IC's Q19 and Q28. (Q28 pin 5).
Resolder this 22 Kohm resistor to Q28 pin 5, and solder another 22 Kohm resistor to Q28 pin 9. Using a short piece of
hookup wire, extend these connections t the REV switch as shown.
Connect a line from the center switch contact to the original feed point of R51, which is the B3 line.
To reassemble, reverse steps 4 through 1.
To operate the ABCD column, hold the REV switch IN and key the 3, 6, 9, or # keys. Normal 3rd column tones are
supplied when the REV switch is not depressed. The receive REV function is not altered by this change.
Note: This switch is a DPDT and many have both sets of contacts bridged. Free one set of contacts for this new function.
