Thanks Stan. Let me clarify a couple points.
Source on Q111 = 10.6vdc. With a scope I see a PWM'd square wave from +11.9 to +10. Not returning to 0. My suspicion is that this is due to the Q111 drain floating.?
Correction of my statement: At the
drain of Q109 is the PWM'd square wave that alternates between 11.9 and
10v. On the other side of the filter at the source of Q111 we have clean 10.6 DC.
Where-as in the 5v circuit, the output of Q108 is alternating between 11.9 and 0 volts. And is being filtered to clean 5v DC at VCC.
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No.? L102 and C106 filter the voltage, trying to average it, so it won't go down to zero.
Correct. And the filter is working as I would expect. Filtering an alternating 11.9 to 10.0 28% PWM'd square wave to 10.6v of clean DC because it is oscillating between 11.9 and 10v not 11.9 and 0. Obviously not the voltage we want.
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The processor measures this voltage, actually half this voltage, at the ADC_3V3 connection.?
Ok, so that explains why the diagnostic screen shows 6.6v where I'm measuring 3.3 at ADC 3V3. ADC 5V measures correctly at 2.5v.
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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.).
R110 and R112 are in place and measure 27k each, in circuit.
ADC 3V3 measures 6.6v.
I used my Super loupe to inspect all of the processors legs and they are clean as a whistle.
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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.
They say a picture is worth a thousand words, so please see the attached diags screen and my notes on the schematic incl waveforms at G and D of Q108 and Q109.
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Thanks again Stan!
