From: https://yamahadx.yahoogroups.narkive.com/cCCZDQNe/dx7-and-tx816-master-tune-editing-via-sysex

Switch to Function Mode:
F0 43 10 08 27 7F F7
F0 43 10 08 27 00 F7

Select Switch 1 = Master Tune:
F0 43 10 08 00 7F F7
F0 43 10 08 00 00 F7

Change value down with repeating pressing of No switch:
F0 43 10 08 28 7F F7
F0 43 10 08 28 00 F7

or up with Yes switch:
F0 43 10 08 29 7F F7
F0 43 10 08 29 00 F7