Hello and good afternoon to you.
To get into service mode for the GY-HM650U, as per the instructions of the service manual:
5.1.1 Modes required in servicing
While holding down the specified button (FOCUS ONE PUSH AUTO), press the [MENU] button to display the first layer menu of the service menu hierarchy. The items in the first layer vary according to the specified button being held when the [MENU] button is pressed. (Characters are displayed on LCD monitor screen
or View finder.)
5.1.2 Service menu Display
(1) While holding down the [FOCUS ONE PUSH AUTO] button, and then press the [USER1] + [USER2] + [MENU] buttons.
(2) And in the state which has pressed [FOCUS ONE PUSH AUTO] button, a press on [USER3]+[MENU] button will display Service menu. (Pressing a [FOCUS ONE PUSH AUTO] button is continued to the last.)
If you would like a copy of the service manual to see the graphical illustrations, please click here. The service menus info can be found in Section 5.