On Sun, Apr 20, 2025 at 04:52 PM, Randy Paulson wrote:
Source on Q111 = 10.6vdc. With a scope I see a PWM'd square-ish wave from +11.9 to +10. Not returning to 0. My suspicion is that this is due the Q111 drain floating.?
No.? L102 and C106 filter the voltage, trying to average it, so it won't go down to zero.
But this is BAD, and needs investigation before replacing D109.
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The processor produces PWM (the PWM_3V3 signal) trying to get the 'averaged' voltage at the source of Q111 to 3.3V
- NOT 10.6V.
The processor measures this voltage, actually half this voltage, at the ADC_3V3 connection.?
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Since you see 10.6V ave, the processor's measurement of the ADC_3V3 voltage is likely not working.?? If the processor can't see the voltage on ADC_3V3, it will continue to increase the PWM percentage to try and get it there, giving a too-high voltage.? This causes D109 (when present) to conduct and keep the voltage near 3.3V, and possibly burning it out.? So check carefully to make sure R110 and R112 are in place, and measure the voltage at the ADC_3V3 junction, which should show VDD/2.? Also inspect to and make sure the processor pin where ADC_3V3 is received isn't shorted to something (solder flake, etc.).
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If the above failure is happening, and you bring up the diagnostics page in the terminal program, you would likely see the 3V in 'linear' mode, and the 3V 'duty cycle' equal to 'duty max', which means the processor is trying hard to get the 3.3V smps working but not seeing the result.
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[Alternately, something in the PWM circuit may be broken (e.g. Q109, Q107, Q113), but if that was so, I don't think you would see the pulsing at the source of Q111.]
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Good luck, Stan KC7XE
