Maximus wrote: ↑Fri Apr 21, 2017 10:55 pm
Do think that replacing the 150 ohm supply resistors for higher value ones to the bfr96 would help?
To prove it quickly and easily, just unsolder one end of one of the 150Ω resistors and pull it out of the board. This will increase the resistance to 150Ω (it's 75Ω at the moment with the two resistors in parallel). The problem that you might run in to is that there may be insufficient drive for the final transistor - Class C stages need a certain amount of power to force the transistor to switch. There's no harm in a quick test, though.
If you find that you don't get much output with 150Ω there, solder the resistor back in and test again at full power to make sure that you haven't done any damage.
Have you measured the current consumption of the board? If it's 1A or less, get a 1N4001 diode and connect it in series with your positive supply connection. If it's more than 1A, use a 1N5401 (though I can't imagine that it will be). This will drop the supply by about 0.65V. Measure the output power (at full power) again, and you should find that it has reduced a bit. Try two diodes in series for a 1.3V supply reduction, and test again. You may find that reducing the supply like this - to the whole board - reduces your output power whilst staying clean because the "power control" section of the board is not doing anything.
If you find that everything's clean and happy at reduced voltage (and it should be), you should add a LM317 regulator circuit to the positive supply input. You'll then be able to set the output power by setting the supply voltage. You won't be able to reduce supply much below 11V because of the 78L09 feeding the oscillator stage - this may necessitate further modifications if 11V still gives you too much power.....
Give it a try and let us know how you get on!
