Editing Sinclair ZX81 Troubleshooting Guide (section)

== No display or white screen ==

# Confirm the unregulated input and the 7805 output.
# Check /RESET at IC3 pin 26 actually releases. If the screen is clear with no inverse-K cursor, Sinclair's own first response is to disconnect the supply, wait a few seconds and try again — that gives '''C5''' time to discharge.<ref name="zx81man" />
# Check the 6.5 MHz at IC1 pin 35 and the 3.25 MHz at IC3 pin 6.
# Check the video at IC1 pin 16 into the modulator — about 2 V peak.
# Remove the RAM pack and every peripheral and retest.
# Swap the ULA (IC1), then the CPU (IC3), then the ROM (IC2), then the RAM.
# Inspect for broken traces, especially in the video section around TR1, C1, R1 and R17.