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Hi Dave, all

We DID find an issue but it's a small one and should be easy to resolve.?

Who knew - but apparently the BSS84 P-channel MOSFET from different manufacturers are significantly different... In particular, some have built-in ESD (Electrostatic discharge) protection with a TVS diode across the gate-source, and others don't. Q202 on page 2 of the schematic has its gate connected directly to the left rotary encoder button. At the time you're soldering on the 2x4 pin header for the controls board, nothing at all is connected to the gate. I didn't have any problems with my soldering station but some of my colleagues, with their lesser soldering irons, did show a tendency to kill Q202.?

On several assembled QMX here there was a clear tendency for the Q202 transistor to be killed by ESD during assembly. The symptoms then are that the QMX is on whenever power is plugged in, it doesn't wait for you to press the ON button; nor can you subsequently turn it OFF with the button while power is still applied. It can also be confirmed by a resistance reading of under 100K between Gate and Source (it should be extremely high resistance).

It's still not completely clear how it happened but the current best theory is that the Rev 3 boards were made with a different manufacturer's BSS84 to the Rev 1 and Rev 2 boards. It's also possible to test the voltage drop across the internal body diode, Rev 1 and Rev 2 always measure 0.58V on my meter but Rev 3, between 0.61 and 0.63V, indicates clearly that there is some difference.?

I had a few P-channel MOSFETs on hand having internal ESD protection circuits, but only 18. After replacement the?ESD sensitivity was removed,?in testing none were destroyed. We sent out those 18 QMX Rev 3.?

While the instructions quite clearly do state in bold red font (assembly manual page 5) that ESD precautions?should be observed, we all know that many constructors aren't going to observe ESD precautions or will do so inadequately. So though I could hide behind my bold red disclaimer, I don't really want to simply ship out the Rev 3 boards like that and cause an increased rate of construction failure.?

Therefore I suspended shipments; I located some BSS84AK made by Nexperia (formerly NXP, and before that,?Philips), in stock at a Turkish distributor, and ordered 350 on Tuesday. According to the Nexperia BSS84 datasheet they have integrated ESD protection diodes rated for 1,000V ESD.? See? . With any luck they should be here tomorrow. So the plan is to replace Q202 on all 1,000 QMX Rev 3 boards with this Nexperia BSS84AK, assuming it passes initial testing. I'll order the other 650 later.?

73 Hans G0UPL