I recently wrote 'All LED'S I use with 5V-Supplies are connected via 220-Ohm-resistors'
and later 'after omitting the led's it does not go so hot as before..',
so actually there is No LED connected with Arduino Nano !
my power-supplies consist on plugin-transformers and self-built
regulators - wired by well-known circuit-diagrams in the related datasheets with condensators like recommended.
Good idea seem to check on oscillations anywhere, I'll switch on my
old tube-driven Oscillograph..