Even if it is not yet Easter my QMX stood up from the deaths yesterday:
Based on Hans' feedback to my dead QMX as was pretty sure, that the CPU would have been killed due to a voltage difference between
- the USB connection to my laptop (running on one potentially ungrounded power supply)?
- the 12V supply power comming from an ungrounded lab power supply
Yesterday I disassembled my QMX from the enclosure and pulled out both power supply (SMPS) PCBs.
I then added jumper wires for GROUND and VDD (3.3V) to the main board PCB#2 connector and supplied 3.3V (JP104 Pin 4 and Pin 2 GND) from a current limited lab power supply and slowly increased the voltage while watching the current.
At 3.3V I saw a current consumption of approx. 150mA --> Hooray, CPU not shorted, most probably OK!
I then checked the PWM_3V3 and PWM_5V signals on the main board, that are directly coming from the CPU --> 116.4kHz rectangle signals --> CPU is working.
As a next step I put in the SMPS PCB#1, to create the 5V and pass through the 12V, that I connected through the power supply connector with 3.3V still being supplied directly to the main board.
Again I had high current consumption on 12V --> problem must be on SMPS PCB#1!
I pulled out SMPS PCB#1 and put in SMPS PCB#2 for 3V3 supply. I fed 12V to the main board 12V connector of PCB#1 (JP101 pin 4) and saw 3.3V coming out of SMPS PCB#2 and a nice 116.4kHz signal on PWM_3V3:
I was now suspecting, that the 5.6V Z-diode D108 on SMPS PCB#1 had a short, desoldered it and soldered in a new one.
Unfortunately a 3W 5.6V Z-Diode, that I had ordered, was too big to fit into this place, so that I had to put in a glass 5.6V Z-diode.
Put in PCB#1, connected 12V to the power connector of the PCB, slowly raised to voltage and again hooray, QMX was working again!!
I updated to the firmware from 1_00_012 to 1_00_017 without any problems and the QMX seems to be working fine.
That D108 obviously took up its protection role and died with a short is for me a strong indication, that the SW based switching regulator still has problems...
I think my use case to provide power to the QMX from one power supply and have a laptop conencted with a different power supply with at least one ungrounded is not an common scenario, if you run FT8.
--
Vy 73, DB2OO -Joerg
