Editing Sinclair ZX81 Troubleshooting Guide (section)

== The board at a glance ==

{| class="wikitable styled-table" style="width:95%; text-align:left;"
|+'''What you are working on'''<ref name="zx81man" /><ref name="zx81sch" />
! Ref !! Part !! Notes
|-
| '''IC1''' || Sinclair Logic IC (ULA), 40-pin || Video, keyboard scan, tape I/O, chip selects and the CPU clock
|-
| '''IC2''' || '''2364''', 24-pin || 8 KB mask ROM. '''Not a 2716''' — that is a 2 KB part and cannot hold this ROM
|-
| '''IC3''' || Z80A or D780C-1, 40-pin || CPU
|-
| '''IC4''' || MK4118, '''24-pin''' || 1 KB × 8 static RAM
|-
| ''IC4a, IC4b'' || µPD2114LC, '''18-pin''' || The alternative fit — two 1K × 4 static RAMs in place of IC4
|-
| REG || 7805 || 5 V regulator on an aluminium heatsink
|-
| TR1, TR2 || ZTX313 || Video output stage; master oscillator
|-
| '''X1''' || CDA 6.5MC || '''6.5 MHz''' ceramic filter at IC1 pin 35
|-
| MOD || UM1233 || UHF modulator, channel 36
|}

'''Board issues.''' At least three PCB versions exist and the component reference numbers are the same on all of them; the notable Issue 3 change is that resistor packs RP1 and RP3 are replaced by individual resistors R35–R42 and R43–R47.<ref name="tv2" /><ref name="zx81man" />