Post
by 1024MAK » Sun Feb 10, 2019 2:41 pm
I think some clarification is needed.
In a ZX80 or a ZX81, all the logic on the main/motherboard runs from the +5V rail. Unless the computer fails to work, there is no need to change any of the capacitors on the computers main / motherboard. And even then capacitor failure that results in a non-working machine is very rare.
In a ZX Spectrum 16K, 48K or ZX Spectrum+, there is an on board DC/DC converter/inverter circuit. This converts the (nominal) +9V input to a +12V supply and a -5V. These are needed for the eight 16K bit 4116 (or equivalent) DRAM chips. As Paul says, these are not happy if some of the supply voltages are missing while others are present. So it is a recommendation to replace the electrolytic capacitors associated with the DC/DC converter/inverter. But as these computers run hot, especially the rubber key versions, while in there, you may as well replace all the electrolytic capacitors.
With the ZX Spectrum 128 and the ZX Spectrum +2 (grey), there is still a DC/DC converter/inverter circuit, but none of the main logic chips/circuits are powered from it (only the serial port, aux/keypad port and some of the video circuits are powered from it). All the main logic chips/circuitry run from the +5V supply. So normally there is no need to replace the electrolytic capacitors unless there is a fault.
However, in some machines (all of the above mentioned ZX Spectrum computers) if some of the electrolytic capacitors are near to / are life expired, it may affect the quality of the video picture. If this is the case, replacing the electrolytic capacitors may improve the quality of the video image. But this a bit subjective, as not all computers are equal due to different issue boards and slightly different parts being used during the many years of manufacturing.
The later ZX Spectrum +2A, +2B, +3 and +3B models, these use an external PSU that provides a +12V and a +5V supply. So there is no DC/DC converter/inverter circuit. With these, normally there is no need to replace the electrolytic capacitors in the computer unless there is a fault.
I hope that helps.
Mark