My HDMI switch/splitter stopped working. I tried it with a different power supply/AC adapter and it worked. The original power supply output at 5V 1.4A whereas the 'new' power supply outputs at 12V 100mA. Is this safe?
God no. you could take a phone charger and clip the end off of it and wire the old charger connector to the phone charger as long as the phone charger is at least 1.4 amps.
No, it's not because the voltage output is way too high. The amperage is not a problem because if a device requires less amperage than the rating on the power supply is not a problem. If you can buy one of those multi purpose power supply with variable voltage and amperage above 1.4.