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QMX not powering on

 

I have just built a QMX, but it doesn't power on and appear as a USB device.
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I have built it for 9V, and have applied a voltage of 8V from a current limited supply. Prior to depressing the left encoder, current draw is 0A. On pressing and holding the encode, current goes up to 0.3A (or even 0.47A if I increase the current limit) but immediately drops to 0A on releasing the encoder.
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If I hold down the encode, current draw remains at a few few hundred mA, and IC101 on one of the power supply boards gets hot to the touch.
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The board has been very thoroughly checked visually and using a DMM prior to first power on. All the connections on the power board are fine,?
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Any suggestions on how to troubleshoot would be hugely appreciated.
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Many thanks
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Matthew
M0MZB?
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Something is pulling down on the 3.3V supply so the power-up sequence cannot complete.? That is why IC101 is getting hot - its output is shorted (or nearly shorted) to ground by something.? Or maybe it was defective in the first place, but that's not very likely.
Note that the general power-on process is this:
1 - pressing the power-on button turns on Q103/Q105, which puts the voltage from the input jack on the +12V rail
2 - this +12V goes to the input of IC101 which creates the initial 3.3V VDD voltage via D103, which is also connected at the top of D109.
3 - this VDD voltage powers on the CPU, which then sets up the switching power supplies, and turns on VCC via Q108 and switched VDD via Q111.? The CPU then turns OFF the IC101 3.3V (i.e. changing from the linear 3.3V supply to the switched one).? This all happens in about 1/4 second if all is working well.
So when you press the button and only 12V comes on, usually it means the initial linear 3.3V supply (IC101) isn't getting voltage to VDD to power up the processor.?
This can happen for multiple reasons.? If you can measure 3.3V at the output of IC101, but not at VDD, it often means that D109 (the protection zener diode) is blown and shorting the VDD pin to ground.? It could also be soldering issues, connector issues, or other components further downstream.
So the first trouble-shooting step is to check D109, and everything on the 3.3V power supply board.
And if you haven't already done so, look at the Wiki on this group, it has lots of pertinent info:
Startup process and signals you expect to see: /g/QRPLabs/wiki/36878
Power supply test process: /g/QRPLabs/wiki/37128
And this video is worth a review, it has lots of common issues to look out for:
Good luck.
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Matthew,
My 12 volt QMX draws plus or minus 200 mA at first, then drops to 95 mA once the switching power supplies are working.
Is your current limit set high enough to allow the initial inrush current? I had my power supply set too low at first, around 140 mA,
and the voltage would drop dramatically as the current compliance kicked in. I thought there was a big short at first.
Since the radio wasn't actually running there was no USB connection, either.
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For transmit I need to have the supply current set to 1.1 A or a bit higher.
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Best of luck,
David N6AN
 

Thanks for the pointers. I initi6had the current limit set to around 100mA and then increased it to 500mA, but still no joy.
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Stans explanation of what this component is doing is very useful, I will take a look at his suggestions, I now have something to investigate.
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I had same simptoms recently.
Pull out smps, measure vdd to gnd resistance if around 2ohms thats bad, you have a short
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Measure ESR of tantals on vdd line of PCM1804 if any of the two shows over 60 ohms welcome to the club, you have shorted PCM IC,
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Unsolder it and order new ones
 

Remove the GPS module if you installed it.? My power jumper under it was too big and it grounded out on the GPS antenna connector.? I had the exact same symptoms.? It came to life when I removed it.? I have fixed my jumper.
 

Thankyou so much for your advice. It was indeed a short pulling oc101vto ground. I had a blob of solder across the capacitor C103, as shown below.?
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With the short removed it nowvworks fine.
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Glad I used a current limited power supply for first power uo!
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