Stan - good point. A physical connection between +12V and VDD could blow up D103. A couple components that are just a bit too close is all it would take to set up a failure mode when the main board is flexed a bit.
My QMX+ is still waiting to be built. I did fix a QMX+ for my good friend Rich N3III a week ago. The problem with his radio was traced to a missing D103. Looked like it was ripped from the board, along with its pads. Fortunately that was easy to fix, and his QMX+ is working super-well now.
73, Steve, N3SB?
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-------- Original message --------
From: Stan Dye <standye@...>
Date: 9/19/24 10:44 PM (GMT-05:00)
Subject: Re: [QRPLabs] qmx+ SCHEMATIC QUESTION hELP UNDERSTANDING
A momentary short from the 12V bus to VDD could blow up D103.? Is there something mechanically close that could come together and short these two? Maybe pins contacting the chassis? I don't have QMX+, I got QMXmini, so I can't directly check.
Lee;
Wow - that 3.6 Volt Zener should not have seen an overstress condition sitting on the 3.3 Volt VDD bus - unless the switching regulator did something it shouldn't have.
Looking forward to hearing about the result you obtain when the D109 is replaced.
73; Steve, N3SB?
-------- Original message --------
Date: 9/19/24 8:54 PM (GMT-05:00)
Subject: Re: [QRPLabs] qmx+ SCHEMATIC QUESTION hELP UNDERSTANDING
Steve,
?Thanks for the info.? I checked the ground to vcc and showed 330 ohms.? found D109 Zener Diode exploded.on one end. I did get a voltage reading of 2.1 volts. on it though.
?I removed D109? and the ground is gone.
?Mouser Parts on order.?
?I will update this when I get the part.
?Thanks for the help.
?Lee
AA7HC
