Azgael, please check schematics for QMX+ Rev. 2 / 3 page 1.
Gate of Q101 together with LIN_REG_EN is connected via R101 to "+12V". With pressed vol-encoder and not running controller LIN_REG_EN is pulled to around the voltage at "+12V". After the controller gets VDD via IC101, starts to run and VDD is proper generated via Q109 the controller will pull LIN_REG_EN low to around 0 V.
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Checking schematics Rev. 4 for QMX you will find nearly the same things but additional R115 from LIN_REG_EN to GND, forming a voltage divider 0.5:1. This pulls LIN_REG_EN for the start only to 50% of "+12V".
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But if you would see also only 50% of "+12V" at LIN_REG_EN of your QMX+ maybe R115 is hidden somewhere in the "deep parts" of QMX+ schematics.
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73 Ludwig
