Re: QCX 50w amp receive problem

Timothy Fidler

If Q5 has shorted as you put it then there will be next to no voltage on the drain because of the two 10 k dropper resistors on the way to that drain.

Many people operate the  PA from a separate PSU from the 12 V psu powering the Qcx mini . If that is so there is no guarantee the grounds are at the same ground 
or even that the  PA 's PSU os grounded at all.  If that ground is -2.4 V low with respect to the ground of the  MIni then that  Q5 mosfet would turn on... it all depends on contact resistances and so on because the two psus are nominally inter grounded via the key enable line plug  black wire.  (at the instant before it is plugged in of course they are not necessarily so ).  If you have a PSU for the PA which is a switcher you will often find that infact the ground connection betwen the two is the AC Neutral line and the earthing connection in your house wiring SB.  

THe US may not use the MEN system so  in that case I have not a clue how your house wiring neutral links and so on work. 

Measuring the  0 Volt  line of the Qcx and the PA when powered up and NOT connected with respect to a common known good earth like a copper piping system might show you if there are issues to look at.  In the worst case a zero volt strap between the respective PSUS will ensure continuity of the zero volt line (without the Amp enable cable being in place)  

Do yourself a favour and put  470R resistor in the  active line of that cable. I suggested this to someone else who had a completely different issue.  He ignored me an pulled the power on his PA while the Qcx was powered up and destroyed the Qcx.  I asked him why he ignored that advice  (he had an early revision Qcx mini with the suspect regulator).  His advice was  that he thought that was only for hot plug and hot unplug issues.  I asked him why puling the power on the PA with the Qcx connected was not a hot unplug......



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