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QMX 5V SMPS Smoking #smoke #troubleshooting #PSU #qmx


 

I built a midband QMX a couple months ago; it worked fine for a solid handful of QSOs and many hours of use in code practice mode.
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I was in the process of testing a newly built manual antenna tuner. I hit the key to transmit and the SWR protection kicked in, exactly as it should. I turned off the QMX intending to power cycle it to clear the SWR protection but when I tried to turn it back on, the screen didn't turn on, I heard a strange buzzing through the headphones, and I smelled smoke.
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I opened up the case and tried again to turn it on under closer inspection. I determined that Q108 was the component that was smoking (or at least the component that produces smoke first when power is applied and the left button is held to turn the rig on). Additionally, L101 seems to have a ring of residue or crust (see photo) which is not present on L102. I don't remember seeing it when I built the kit, but I can't be 100% sure that it wasn't there before.

Has anyone else experienced a similar failure condition?
Would there be any harm in disconnecting the 5v SMPS board and powering up the QMX without it to inspect the status of the processor via the serial interface?
What other troubleshooting steps should I try next?
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Thanks for your time and 73,
Hudson, KL4LJ


 

On Mon, Dec 2, 2024 at 10:42 AM, Hudson Spillers KL4LJ wrote:
disconnecting the 5v SMPS board and powering up the QMX without it
Hudson, no switch on without the 5V SMPS board (see schematics page 1 and QMX QMX_plus Startup section 2).
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It looks like an overload to Q108 and L101 (broken D108?) I'm afraid L101 has shorted windings now.
You may:
  1. Test the 5 V SMPS off line (see QMX QMX_Plus Power supply test Process , but you couldn't find a L101 short in this test)
  2. Test the VCC rail of the mainboard (see QMX QMX_plus internal power supply section 3)
  3. Order a new 5 V SMPS if the test in the step before was ok
73 Ludwig