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Hello. I'm assembling a 50 watt power amplifier for use with a QCX+ that I assembled last month, for the 80 metre band. I have very high harmonics at the output of T3 - as a result, I get only about 9.3 watts out when running at 13.8 V. Output from the LPF looks OK on the 'scope. I found the excellent videos by NA5Y which shows the output of T3 should be much closer to a sinusoid than what I get. I also found the discussion with KO1KR in August of 2020, but unfortunately I can't see his scope photos so I don't know if he had the same problem. I've since skimmed through the many discussions on this forum but haven't found the same issue exactly, though I did check out some of the suggestions given here. I found the amplifier took 67 mA with R5 fully CCW - I adjusted R5 till the amplifier drew about 70 mA with the QCX+ key-down in practice mode. Here are my scope traces: 0092 is the gate drive, yellow is Q1 gate, purple is Q2 gate - all is well here. 0090 is the drain voltage, Q1 in yellow and Q2 in purple - these look like there is oscillation ? 0091 is the voltage at the output of T3. This has a lot of third harmonic and doesn't look like the waveform NA5Y showed at this point. (QCX+ was set for 3.520 MHZ , amplifier connected to a dummy load.) I thought that perhaps my bench power supply is doing something odd when the amp is drawing power, but I probed the 12 volt line right at the barrel connector and the 'scope showed almost no disturbance of the +12 v supply. In frustration, I observed the output of T3 with the 'scope set for a long capture - it almost looks as if the distortion grows with time, for the first few tens of microsceconds the scope showed a small but sine-ish waveform, which over the span of a few hundred microseconds got wigglier and wigglier till it developed into the mess seen in my scope capture 0091. I've checked DC continuity to the windings of all the transformers and coils. I did find a bad joint on the primary of T1 but I cleaned it up and can now get power out of the amplifier. I haven't yet checked bias on all the 1N4007 diodes but the input looks quite sinusoidal at R3 closest to the input. The few watts the amplifier does produce look OK at its output. The QCX+ makes about 4.3 wats into a dummy load (43 V peak to peak) at 13.8 V DC. I'm assembling these units for a friend, and I had really hoped to have them ready for Field Day this year. The instructions are excellent and I have followed them closely. What can I look at next? Bill VE4STW

what can I do to a bit more power on the higher bands

I would like to share my QMX+ mishap. The Failure I was working in the shack and needed to unplug the buck converter power supply going to my QMX+. Without thinking or checking if the QMX+ was power on, I grabbed the two ends of the Anderson power pole and wiggled the connectors apart. Just as I disconnected the connectors, I heard a distinct loud "POP". You all know that sinking feeling that "you done messed up". You see, I am very careful with my QMX rigs and know that these little gems are very sensitive to the input power and always follow a strict power up/down process.. oh, not this time. So, when I heard that noise and started to smell the magic smoke, I knew that my cherished QMX+ was dead. The Damage When I opened up the rig, I knew that the buck converters were probably toast. I did a visual on the two boards and confirmed that C107 was blown. In fact, there the capacitor was no longer yellow in color, but charred black. There was a nice little burn hole at the top. I did some further inspection and found, what initially looked like, was a hole in the PCB. My heart sank further until I looked at it with a magnifier and found that it was just remnants of the capacitor that needed to be scrapped and cleaned off the PCB board. What To Do Next? I assumed that the internal buck converters are toast. I replaced the 5V, and still the QMX+ failed to start. I then replaced the 3.3V. I was able to power on the QMX+, but there were issues. I found that the current being drawn was at .200ma. I could power the QMX+ off, but failed to power back on until I pressed the power button until all voltage was off the power supply. I checked out the receiver and it appeared to be functional except deaf. Looking at the diagnostic screen and found that the 5V was not at 5V, but at 4.7V. I started looking around the board and feeling the IC chips for heat. It didn't take long for me to burn my finger. IC403 was running way too hot. Chip Removal I got my flux and hot air rework ready, taped off some of the areas to avoid issues and started heating up the board. I found that my inexpensive rework station was just that inexpensive, and never was able to remove the IC. I heated the area for quite some time and was afraid that I might damage the board. Plan B... I cooled down the board and thought for a few minutes and remembered that I got some low temp de-solder metal bar for just this issue. I was able to melt this on each side of the chip and then heat one side and slowly lift the edge. It took no time to lift one side. Did the same for the other and now the chip has been removed. I cleaned off the flux from the board and used "solder wick/braid" to remove excess solder, and re-flux the area. I then re-soldered the new chip in place. Looks like it never happened. Both internal buck converters died, IC403 had an internal short. What was the root cause? I knew from a past experience that there was a transient that took out the 5V converter, but never like this time. I started looking at the buck converter and found something interesting. The Drok buck converter has a feature to "power on" when input voltage is applied. This feature was turned on. I removed the load and applied the voltage to the Drok and wiggled the connector on and off to trigger a spike. That's when I noticed the output voltage spike up to 25V. No wonder why the failure. I turned off the "Power On" feature and repeated the same steps. No further output or spikes were observed. So, if you are using the Drok buck converter that has this feature, beware. Make sure that the power on power feature is off and always power off the Drok before unplugging the QMX. I do still like the Drok converters and the current and voltage adjustments. They can be a bit noisy, but with a couple of ferrite beads and some distance from the QMX, it works great. Suggestions Do not use the Power ON feature of the Drok buck converter. Plug power source to Drok first, making sure that the output is turned off. Connect the QMX to the converter. Once connected, power on the Drok and verify output vol

I discovered a feature of my toolkitrc M6 charger that could be a buck converter. I can limit voltage and output amps starting from an energy source, for example 3cell lipo battery. What do you think?

Hi All, After putting together another QMX+ I found the time and inspiration to look into my heating issue of T2, T3 transformers on my 50w QRP-Labs Amp. As my T2 ferrite ring was cracked upon initial reception and mended using super glue, first on my list was to rewind T2 onto a new core. That made no difference at all. Then I replaced the pair of IRF510's just in case, that made no difference. Looking at the gates of the IRF510's with O-scope there is a nice sinus wave, and on the drains there is a much higher not-so-pretty signal when in transmit: (probe at 10x) Any ideas on what to check next? For instance, what impedance is the expected on the input side of T3? I was thinking of removing T3 and connecting a power resistor instead to see how the signal looks there then. (to get a hint of if the issue is on secondary side of T3 or LFP's. Perhaps insert signal from signal generator on secondary side of T3 and check signal as it Regards Jonas - SM4VEY Den tis 22 okt. 2024 kl 22:15 skrev Jonas Sanamon via groups.io <sanamon@...>: