I do have a Raspi 3B that slowly drops USB voltage when running and I can measure that at the USB ports.
The Raspi is powered by an industrial/lab PSU with exactly 5V. All time when the Raspi is running this PSU provides 5V.
The PSU is connected using a cable (25cm, 0,75mm^2) that is soldered to the GPIO pins of the Raspi (so no fuse problem).
The Raspi has nothing connected except a HDMI monitor. But it has nearly no influence what devices are connected.
The Raspi is turned off for about 60 minutes and now turned on. The voltage at the USB ports is then between 4,90V and 4,92V.
(While booting the USB power is turned off and on again by raspbian (seems to be normal)).
After booting the running system's USB voltage is at about 4,86V-4,88V. Within the next hours the voltage is pretty constant, it goes down to 4,85V but not less. After 5-8 hours something strange happens and the voltage starts to drop down to 4,35V and the maximum is not more than 4,70V.
Now I turn off the Raspi and expect it to start with 4,9V again. But that does not happen. It starts with about 4,75V-4,78V and after boot it goes down to less than 4,5V. I have to turn it off for about 60 minutes to start again with 4,9V. Sometimes when I first boot I get a kernel panic telling me something about being unable to initialise CPU Core 2 or whatever. This is exactly at the moment when the USB ports are turned off and on while booting. I turn it off and on again and it starts fine with 4,9V.
Any ideas anyone? Maybe the Raspi is simply broken. But is there any explanation for this strange voltage behaviour?
I would like to understand what may have caused that because I tried several hours (days) now. I can reproduce but cannot understand.
Thanks a lot!
Sorry, I know under-voltage is boring...