Yes I have tried running the 50W PA with my QMX (make sure you pick the correct band!). The QMX PTT is indeed not putting 5V out and you do need to switch the PA on. I used a separate external power supply for this, just to see if the PA would work. And yes, it did work! Then I did something stupid: while the QMX was transmitting, I switched off the PTT 5V input to the PA. The PA stopped, that for sure, but so did my QMX, which stopped working properly all together. I replaced its finals (4x BS170) but without success. The finals input is all right (scope measurement), but the output is not, even with fresh BS170s. I need to investigate further...
