Ludwig,
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After looking at the schematic I think I can come up with a plausible explanation for my symptoms.
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When the left knob is pressed it provides PWR_ON, passing the input voltage, in my case 9VDC to the 3.3V converter.? If power PWR_HOLD does not come from the processor then it will not latch the power on.? The point that 9V will show up as 3.3V when the knob is pressed? is output of IC101 linear 3.3V regulator which appears to be there to bootstrap VDD to the processor until it can provide PWM to the 3.3V switched mode power supply.? So I am thinking that this was the point I was measuring when I saw 9V turn into 3.3V and go back to 9V when I let off the knob.? Clearly the 3.3V PWM supply was not turning on.? This rules out an intermittent at the input power connector.??
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That the unit was not coming up when I ran these tests suggests that there is a possible fault between the processor coming up and providing PWR_HOLD and/or PWM to the 3.3V SMPS (could be caused by an intermittent in VDD from the 3.3V regulator output to the processor).?
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The part I cannot explain yet is why hooking up the USB resolves this especially assuming it is a mechanical intermittent.? The schematic I am looking at does not show a VDD connection from the USB port, although it is possible that it is not shown.? It does show the USB port connected to pins 70 and 71 of the processor.? These are port pins so not directly connected to VDD unless a weird path through internal pullups, which seems not plausible.
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So the next time this happens I will be looking at VDD to the processor at L202, PWR_ON, PWM_3V3 to see if I can narrow it down.??
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Brien - KE7WB
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Brien - KE7WB