Hier für alle Nachbauer die Nachricht von Hans Summers, es geht um den unbedingt notwendigen Einbau eines zusätzlichen 10uF Kondensators in den QCX Mini (QSP de DL9SCO):
With great sorrow I have to report that a problem has arisen with the QCX-mini; I have investigated and found a simple modification which prevents the problem. This should be viewed as an essential change for ALL QCX-mini constructors.
Four different constructors have reported their QCX-mini destroyed itself in a little puff of smoke. There was some commentary that this occurred on plugging in the power connector and in one case switching on/off rapidly.
SMD capacitor C38 is found to be not up to the job. Presumably of a type which is inadequate for the task. On a high speed oscilloscope, when the power is applied a large spike of up to 1.5x the supply voltage is observed, with a duration of the order of 0.1us. Where a supply voltage of 12 or 13.8V is used, the spike can exceed the maximum voltage rating of the AMS1117-5.0 voltage regulator IC and cause it to become unstable and self-destruct.
The failure mode of the AMS1117-5.0 is to connect its input to its output. This applies +12V to the +5V rail of the QCX-mini circuit, to which is connected the FST3253 1:4 MUX, the Si5351A Synth, the 74ACT08 PA driver, the LCD module, and the ATmega328 microcontroller. OUCH! The Si5351A synthesizer IC fails internally fusing its supply line to ground. That results in a high current flow through D1 and D2, the voltage dropper diodes which power the Si5351A via the +5V rail. One of these diodes fails first under the stress of the very high current flow, and this diode is what generates the "Psssst" sound, the smoke, and the bad lingering smell.
The problem can be solved by soldering a 10uF electrolytic or tantalum capacitor from the voltage regulator input to ground (in other words, in parallel with the pathetically inadequate C38 SMD 10uF capacitor).
In testing with the 10uF electrolytic capacitor installed, I set my bench PSU to 14V and rapidly inserted and removed the 2.1mm power connector to the QCX-mini board. Without fully inserting it, only pushing it just about enough to make contact. My oscilloscope was connected to the voltage regulator input and output pins. No voltage spikes were seen and although I continued this attack for 10 seconds or more, no failure occurred.
I then removed the 10uF electrolytic capacitor and repeated the test. This time after several seconds, the failure occured: manifested by the statutory "Pssst" sound, smoke which in this instance arose from diode D1, and the bad lingering smell of burned diode. After the event a small crater was visible on the top of diode D1 and another on the FST3253 MUX. Result: dead QCX-mini requiring extensive repairs.
I then soldered a 10uF capacitor to another QCX-mini (my last, gulp) and again tried to kill it with repeated intermittent connection to my 20A bench power supply set to 14V and again, there was NO failure.
The modification is described in the 1.04 version of the manual http://qrp-labs.com/images/qcxmini/manual_1_04.pdf in section 3.42 at pages 67 and 68.
The modification requires a small electrolytic or tantalum capacitor, value at least 10uF and rated at least 16V. The capacitor needs to be small enough to lie down in the space between the voltage regulator, op-amp IC10, and the power connector; it needs to be thin enough to not obstruct the gain potentiometer when the Controls PCB is plugged in. Practically speaking this means the capacitor should be no more than 5.5mm diameter and 9mm long. If you are familiar with the 10uF capacitor used in the QCX+ kit, this capacitor fits perfectly, it has diameter 4mm and length 7mm.
For those people who have already received their QCX-mini kit, or those whose kit is in the post... if you could find a suitable capacitor in your junkbox or local supplier, that would be much appreciated here. But if not, please email me and I will send you a suitable capacitor. For all future orders to be shipped out, we will include a suitable 10uF capacitor with your kit.
73 Hans G0UPL