The ZX Spectrum 16k, 48k and Plus models all have a DC/DC converter/inverter power system on the main board to generate +12V and a -5V supply rails for the 4116 DRAM chips. The +12V is also used by the colour encoder chip (LM1889).
Models using unmodified issue 1,unmodified issue 2, some issue 3 (unmodified) and some issue 4 (unmodified) boards used a version of this DC/DC converter/inverter circuit that would not work correctly if the nominal 9V input voltage was 11.9V or greater.
If a computer was returned for repair or service to Sinclair, or to a a good independent service engineer, the board would be modified to the latest version of this DC/DC converter/inverter circuit.
So unless you know that you have an issue 5 or issue 6A board, or that you have an earlier board that does include the (better) later version of the DC/DC converter/inverter circuit, I strongly recommend that you do not connect it to a PSU that provides an on load voltage of 11.9V or more.
The problem with the earlier version of the DC/DC converter/inverter circuit, is that with the voltage on the nominal 9V input at 12V or more, DC current flows via ‘the coil’ from the 9V rail to the +12V rail. The control circuit sees that the +12V rail is at or above +12V, so turns the switching transistor off. Then there is no AC drive to ‘the coil’. The result is that ‘the coil’ is no longer able to generate the -5V supply. The 4116 DRAM chips (or equivalents) that provide the ‘lower’ 16k RAM then don’t work correctly. Worse, the lack of this -5V bias supply can cause these DRAM chips to self destruct
If the lower RAM is not functional (which means
any of the eight 4116 DRAM chips not working), then the system will crash at power up.
The recommended on-load input voltage on the nominal 9V input is between 7.5V and 11.5V for regulated power units, or between 8.5V and 11.5V on-load for unregulated power units.
To reduce the heat output of the 7805 voltage regulator, it is better if the input voltage on the nominal 9V supply is at the lower end of these ranges
The current requirement varies depending on which issue the board is. But all require 800mA or less. So if no expansions or interfaces are used, a PSU rated at 800mA is fine. If expansions or interfaces are to be used then a more powerful PSU is needed. For simple interfaces like simple joystick interfaces, you need a 1A (1000mA) PSU. Anything else and a PSU rated at 1.4A or greater is recommended
Mark