This modification applies to QMX PCB Rev 1, 2 and 3. All factory-assembled radios shipped from 10-Feb-2024 onward already have this modification. Kit boards do NOT have this modification since it can only be installed after connector jack X201 is installed. 

Many thanks to Yuji JJ1GPM who noticed this error on the QMX PCB. 

The symptom is that the grounded PTT output does not work. The reason is that the output signal is mistakenly connected to pin 4 of the 3.5mm stereo jack which is the PTT output port. Pin 4 is the switch connection which is disconnected from the actual jack "tip" connection when a plug is inserted into the socket. 

To fix the problem a jumper wire should be soldered from pin 2 to pin 4 as illustrated below. This must be done AFTER the connector X201 is installed.

If this modification is not done: then the PTT grounded output will not work. However, there is no possibility of damage to a QMX and all other functions including the positve-going +5V PTT output (on the "ring" connection) should work correctly.