John,
Trying the 3.3 volt (maybe 3.6 volt?)? with or without the resistor could not hurt.? The diode would add capacitance that might make the I2C decode an issue, so I would do the Zener first, then add the resistor.? Adding the 7805 regulators ahead of the LM1117 is a good idea.
Using diodes to drop the input voltage on QRP transceivers works well.? The current required by ORO transceivers, like the Sbitx, may make it a problem.? 10 amps with a .6 volt drop require 6 watts to be dissipated.? A buck or buck/boost converter might be better if the noise is not an issue.
I am waiting to buy a new oscilloscope with I2C decoding.? This may be a good project to test the capability.
We should also look at the long 5 volts run from the 5-volt buck converter to the RPi board.? We may need RF bypassing on it, moving the converter, or replacing the trace with a shielded wire that can handle the current.? I like Gordon's idea of modifying how the RPi gets its power.? There are protections on the USB-C connector that are not on the GPIO connector.? The RPi schematic has a 5-volt Zener on the input from the USB-C connector.? That would be connected to the 5-volt line on the GPIO.? I would think it would keep the voltage at 5 volts regardless of the supply.? It would be interesting to do the forensics on one of the dead RPis to see if that diode is shorted or open.
I also do not like that the power warning had to be turned off when powering from the GPIO connector.? Low voltage may be the issue, with the pulses indicated by the audio when WiFi is turned on.? We are back to Gordon's suggestion to power through the USB-C connector with the voltage warnings on to see if there is an issue.
The above are questions, not suggestions.
73
Evan
AC9TU
