Well... I just finished my second QMX, the first one was toasted by increasing the supply voltage from 6V to 12V wiith a ramp-up time of 3milliseconds.
The full story is here:?
/g/QRPLabs/message/106237That one was? running firmware 1.07. Image. D109 was destroyed (short) so I removed it.
After that my QMX turned on but power? consumption was200mA @12V input.
Image rejection was nonexistant and bandpass filters totally wrong, so I concluded IC402 and/or IC403 were damaged.
I decided to stop there and ordered a new kit
Now the second QMX, finished last night:
I loaded firmware 1.09 without problems.
Everything was fine and I was just about to write something on the forum of a successful build.
I wanted to put in some figures of RF power output and current consumption.
Power output was fine 4W on all bands @12V supply. Image rejection and filter sweeps were also fine. Supply current about 100mA in receive and 600mA in transmit
But while I was measureing supply current I heard a "crack" and the QMX turned itself off.
I turned it on again by pressing the left rotary switch.
Now I smelled burnt electronics.
I opened the case and out came the holy smoke...
C107 was now black and with a large crack in it.
I took out the power board and removed C107 and measured the resistance between Vcc and GND.
733 Ohms in both directions with the DMM, so i removed D108. Now I had 75 kOhm in one diection and about 7 kOhm in the other.
I also checked Q108, no short between terminals, so I guess it survived.
So I took the 5V power board from my first QMX and put it into the second.
And voila, everything back to normal.
So what happened?
During the current measurement It is possible that I interrupted the power supply line shortly when I disconnected the DMM without powering down the QMX before.
But htis case was addressed by Hans in his article about SMPS behaviour during input voltage step...
There is a saying in swedish "en g?ng ?r ingen g?ng, tv? g?nger ?r en gong-gong"
Free translation: One time is OK, two times is an alarm clock.
I can see that several QMXs have blown up C107.
I think this is no coincidence. There is something going on here
I recall that all who reported blown C107 were running 12V supplies with high current capability
I suspect a HW/SW interaction:
During a power interruption glitch, is it possible that the processor can leave the the PWM signal high, thus creating a voltage increase that kills C107 and D108?
I would suggest that the PWM-signals should be AC-coupled so that if stuck high, the SMPS would turn itself off.
73 de SM5EIE /Gunnar
?
So I ordered?
