First, a reminder to all:? Please DON'T CONNECT ANY EXTERNAL VOLTAGE TO THE QMX/QMX+ MIC/PADDLE INPUT.
For electret mics, QMX/QMX+ already has an internal 3.3V bias voltage hooked up to the mic input (ring terminal) of the connector.
And indeed, only electret mics, or other high-impedance mics should be used.? There is also an internal DC-blocking capacitor on the microphone signal.? The 'tip' and 'ring' terminals of this connector also hook directly to processor input pins (for the paddle dit/dah) in addition to the processor microphone input - so external voltages improperly applied may cause damage.
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Now, on to your problem (QMX won't turn on, per the message title):
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Looking at the schematic, I don't see how connecting 3.3V to any of the terminals of the mic input jack could cause damage such that the QMX would fail to turn on.? Unless there was some other factor, like perhaps an ESD event.? Or if the voltage applied for some reason was 6V or 9V or 12V, it indeed could have caused damage to either the processor or to the 3.3V power supply - this could happen, for example, if you were using a external supply for the 3.3V bias, and turned it from off to on while connected - some supplies can spike significantly above their set voltages when turned on, or spike well below ground potential when turned off.
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Since your QMX won't turn on, somehow either the power supply or processor was damaged - hopefully only the power supply, since it is much easier to fix/replace.? The protection diode in the 3.3V supply could have failed if there was over-voltage.? Or over-voltage could have fried some input pins on the processor, preventing it from starting up.
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A good measurement to make is the current draw while you have the PWR_ON button pressed (but don't press it too long unless you have a current-limited power supply set to about 250mA).? This can help tell what kind of fault is present.
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Then I would remove the bottom cover, and (with no power attached), and measure the resistance between GND and the three voltage busses (12V, Vcc, and Vdd).? You can find the contacts for those busses on the soldered SMPS connector pins.? (Someone just today or yesterday posted a message with a nice pdf identifying all of the voltage pins on the bottom of the QMX card).? I? suspect the resistance between Vdd and Gnd will be a low number of ohms (rather than a few K-ohms).? Such a low reading would indicate a failed component or short.? Next step would be to disassemble the top cover to get to the SMPS cards and remove them.? Then re-measure the Vdd/Gnd resistance on the main board - if it is still low, the processor is likely damaged.? If it is now high (hopefully), then the failure is on the SMPS card, likely D109.? You can use the nice procedure written in the wiki of this list to independently test the SMPS cards, or just order new ones from Hans and replace them.
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Good luck in quickly finding this, and ask if you have any addition questions.
Stan KC7XE
