I got lucky again at the local flee market when i managed to get a Spectrum ZX powerbrick.
Some basic information:
Input: 240V AC, 50Hz
Output: 9V DC, 1.4A (without peripherals 700mA would do but not recommended)
Center Polarity: -ve (inner diameter is 2.5mm and it’s center negative)
However when connected the power, adapter was dead. So it was time to call in the superhero in form of my friend, who also happens to be an electronics engineer. He managed to tear the thing apart, its easier than with the commodore adapters which are sealed with epoxy, Sinclair only used three flat screws.
However when my friend unsoldered the transformer it had a surprise underneath.
After the thermal fuse was replaced the powerbrick started working again. As a fun fact, the original adapter has a throughput of 15V when not connected and 13,45 V when connected to a load. The Spectrum is declared to work at 9V, but it’s capable to work even with higher than specified voltages. It is truly designed like a Russian tank as most of the electronics of the era. However it is recommended to use a modern swiching adapter, which gives a more constant and stable output, which directly translates in less heating of the passive elements and longer lifetime.
Reference if you want to know more:
A more technical video (Video is not mine).