Editing Sinclair ZX81 Capacitor Replacement Guide (section)

== Checks after the work ==

The ZX81 has one internal rail. There is no −5 V and no +12 V — those belong to the [[Sinclair ZX Spectrum Capacitor Replacement Guide|ZX Spectrum]], whose 4116 DRAM needs them. The ZX81's RAM is static.<ref name="zx81sch" />

{| class="wikitable styled-table" style="width:95%; text-align:left;"
|+'''ZX81 checkpoints'''
! Point !! Expect !! Source
|-
| Power jack, 3.5 mm, '''tip positive''' || DC only, need not be regulated but should be well smoothed; 8–12 V on load, about 14 V open-circuit and roughly 11 V on load at 400 mA; not less than 600 mA capability, or 1.2 A if the ZX Printer shares the supply. Below about 7 V the regulator will not work correctly || <ref name="zx81man" /><ref name="tv2" />
|-
| REG (7805) output || +5 V, reaching every IC on the board || <ref name="zx81man" /><ref name="zx81sch" />
|-
| IC3 (Z80A) pin 11 Vcc / pin 29 ground || +5 V / ground || <ref name="zx81sch" />
|-
| IC4 (4118 RAM) pin 24 Vcc / pin 12 ground || +5 V / ground || <ref name="zx81sch" />
|-
| IC4a, IC4b (2114 RAM) pin 18 Vcc / pin 9 ground || +5 V / ground || <ref name="zx81sch" />
|-
| IC3 (Z80A) pin 26 || /RESET low briefly at switch-on, then high || <ref name="zx81sch" />
|-
| IC1 (ULA) pin 16, to the modulator || About 2 V peak from picture peak to the bottom of the sync pulses || <ref name="tv2" />
|}

'''The ZX81 has no reset switch.''' The only reset is the R15/C5 network acting at switch-on, which is why the remedy for a hung machine is to unplug it and wait a few seconds.<ref name="zx81sch" /><ref name="zx81man" />

A clear, steady black-and-white picture with no rolling or flicker, and the inverse-K cursor at the bottom left, is the sign of a good result.<ref name="tv2" />