Hi Stan, thank you so much for your analysis!
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I used a fixed 12V PD trigger, so it's probably not the 20V issue. I'll take a closer look later.
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I'm not sure your analysis is correct. ?How are you determining when the SMPS kicks in? ?The 5V supply is only an SMPS supply, and it is what powers the display LCD and backlight. ?So if the LCD ever comes on, it is because the 5V SMPS is already going. ?But it could still be damaged in some way
You're absolutely right! I realized this as well. It looks like when the controller is still on the linear regulator, the 5V SMPS and LCD control works fine (at least partially).
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So my best shot-in-the-dark guess at an action plan is to remove IC503, and see if the short is gone, preferably by measuring the resistance between Vcc and gnd, which should be a few k-ohms. ?Then powering up with a current-limited supply to see if it comes up, and things start working again in receive mode.
I'm removing IC503 now. Without power, the resistance between Vdd/GND is 2.6k so I guess the short is either on the Vcc rail, or only happens when the 3.3V SMPS is running.
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Thanks again!
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73 de N6HAN Zhenxing Han
