My 60-15m QMX has been working really well since I built it. This evening, however, when I pressed the encoder - I got nothing but a brief click in the headphones!
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After checking the obvious - battery, cables etc, I opened up the radio to investigate. There were no signs/smells of overheating parts or magic smoke. There was no short from either VCC or VDD to ground. 12V was present on the V_IN pin of the 5V supply board and pressing the encoder dropped the PWR_ON pin to 0V, as expected. I did notice that the 3.3V regulator on the 3.3V board was getting slightly warm if I repeatedly pressed the left encoder. Figuring that its output might be being dragged down by something, I pulled the 3.3V board out of the radio and briefly applied 9V to the input pins of the 3.3V regulator chip. D103 instantly got hot so there was something pulling a lot of current on the VDD line. From the schematic, the main suspect was D109. Removing that from the board cleared the fault.?
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As I don't have any 3.6V zeners handy, I popped the 3.3V board out of my Low-Band QMX and fitted it to the Mid-Band radio which brought it back to life.
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Having now had a search on this forum, it appears that there are quite a few mentions of D109 failures. Would it make sense to order a slightly higher rated replacement part or would it be wiser to stick to the 500mW surface mount component? Why did it fail in the first place? Any thoughts?
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Cheers,
Ronan
MM0IVR
