Yes, do. Although I have one whose output seems OK but doesn't power the system ok when connected. They definitely were built to be cheap rather than to an engineering standard!

Not slating them, times were different and every penny counted. I've got 4 spectrums here... 3 are 48k but one is original 16k....which is the most interesting one but it's the one that doesn't work!

A modern Meanwell power supply is simply going to be safer as well as far more efficient (and smaller!) Just make sure you get the polarity right.

This stackexchange question is quite good on how and why the 9VDC supply at the jack gets converted to 5V, 12V and -5V internally: https://retrocomputing.stackexchange.com/questions/2242/how-...