Hi Colin @M1BUU
Not just me then, phew!
I’ve stayed with the supplied toroid cores. They measure low in inductance on my Peak LCR meter, but using my home brew meter, they measure correctly. The Peak isn’t giving consistent results either - something else to investigate!
Progress so far:
Reading the instruction manual, I checked the waveform at the output mosfet drains, and found that the amplitude increases as I reduce the frequency below 10MHz (still increasing as I get down to 8 MHz) With 13.8v supply, the peak amplitude is 48v at 10 MHz, and 61v at 8 MHz.
This suggests that it is not just the LPF which is out of kilter. So, I have removed a couple of turns off L4. It is still tuned low, I think, but somewhat better. I’ll leave it at that for the time being.
(L4 is what I would think of as the “tank” circuit for the output stage, and the manual suggests that the tuning of this is not critical, but involved some experiment at the design stage, to take account of the capacitance of the mosfets - so I thought a little more experiment might be in order)
I then removed a turn off each of the LPF toroids, L1, 2 and 3.
These changes have increased the output, but there is still room for improvement.
If I have time today, I’ll do another frequency run from 8 to 11 MHz, and compare the results with my earlier test. That will give me an idea of what to do next.
Thanks for your comments comparing the MTR output design - interesting.
None of my comments are meant as a criticism of the kit - the manual is really informative, and the circuit board is large enough to allow changes and measurements without too much trouble.
I’m having great fun, but must get out on a summit before too long…
Luc @ON7DQ - thanks for the feedback, interesting.