I had my QMX up and running with expected power consumption, good power output and fine image rejection and RF filter response.
When I prepared for portable use, I took out my QCX-mini power supply:
It has two switched mode power supplies, one for 12V and the other for 6V
I can switch between the supplies in order to change between 5W outut to less than 1W output
This worked fine with the QMX also, but when I switched from 6V to 12V, the QMX just died.
On afterthought I realise the the buck-converters in the QMX were running from 6V and with relatively high dutycycles
When switching to 12V the dutycycle was much too high, resulting in a voltage spike on both th Vdd and Vcc lines.
I found that D109, which protects the Vdd had a short. I removed D109 and QMX came to life...
Transmission was fine with same power output as before.
Reception seemed to be fine also
But power consumption was 150mA instead of 100mA on 12V input.
I noted the the linear 3.3V regulator was still active and getting warm (not hot). Despite? the LIN_REG_EN being low.
Gate-source voltage of Q102 was about -1.5V, so Q101 has a leak turning on Q102.
I connected gate and source of Q102 to turn it off, but it was still on, so Q102 has a short source to drain.
I ran the PUTTY diagnostics, it shows nominal operation of all buck converters and nominal Vdd and Vcc
But the RF-filter sweeps were terrible as compared before.
And image rejection was not existant, it was fine before. I verified this by tuning the receiver 25kHz up and found the same signal with the same strength...
I removed T401 and put new windings. No change in image rejection.
I concluded that IC402 or IC403 or both were damaged. These ICs are powered by Vcc which does not have a protection Zener.
So I gave up on repairing, thinking that more damaged circuits would continue to pop up.
I have ordered an new kit, I hope that I will not have to wait too long for it...
The existing QMX is operational as it is, but with performance issues and a little high power consumption
I also have QCX-minis for 80 and 40 meters that help me survive the waiting time
So be careful with sudden power supply changes to higher voltage.
If you want to change supply voltage, either make it slow or switch off the QMX during the voltage change
Ideal would of course be if Hans can implement power setting in the menu.
73 de SM5EIE / Gunnar
