Sinclair ZX81 Troubleshooting Guide: Difference between revisions
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[[File:Sinclair ZX81 motherboard (3327788413).jpg|alt=Sinclair ZX81 Motherboard|thumb|Sinclair ZX81 Motherboard]] | |||
This guide provides detailed, component-level troubleshooting for the '''Sinclair ZX81''' home computer, covering both Issue 1 and Issue 2 motherboard revisions. It addresses common failure symptoms, diagnostic procedures, and repair techniques for: | This guide provides detailed, component-level troubleshooting for the '''Sinclair ZX81''' home computer, covering both Issue 1 and Issue 2 motherboard revisions. It addresses common failure symptoms, diagnostic procedures, and repair techniques for: | ||
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''Note: This guide does not cover external peripherals such as the ZX Printer or cassette recorders.'' | ''Note: This guide does not cover external peripherals such as the ZX Printer or cassette recorders.'' | ||
== | == Diagnostic Tools & Techniques == | ||
=== Visual Inspection === | === Visual Inspection === | ||
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* If display appears after removing a chip, the removed chip was likely faulty. | * If display appears after removing a chip, the removed chip was likely faulty. | ||
== | == Power Supply & Voltage Checks == | ||
{| class="wikitable" style="text-align:left" | {| class="wikitable" style="text-align:left" | ||
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Never operate the ZX81 with a faulty or unregulated power supply. | Never operate the ZX81 with a faulty or unregulated power supply. | ||
== | == “No Display” / “White Screen” Flowchart == | ||
# Confirm PSU output and 7805 regulator voltage. | # Confirm PSU output and 7805 regulator voltage. | ||
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# Remove external RAM pack and peripherals—test again. | # Remove external RAM pack and peripherals—test again. | ||
== | == ULA (Uncommitted Logic Array) Faults == | ||
* The ULA (IC1, 2C184E or 2C210E) generates video, clock, and system glue logic. | * The ULA (IC1, 2C184E or 2C210E) generates video, clock, and system glue logic. | ||
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'''Replacement:''' Use a compatible ULA (2C184E for Issue 1, 2C210E for Issue 2). Modern FPGA-based replacements are available. | '''Replacement:''' Use a compatible ULA (2C184E for Issue 1, 2C210E for Issue 2). Modern FPGA-based replacements are available. | ||
== | == Z80 CPU Faults == | ||
* Z80 rarely fails, but can be damaged by over-voltage or ESD. | * Z80 rarely fails, but can be damaged by over-voltage or ESD. | ||
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'''Replacement:''' Use any standard Z80A or Z80B (DIP-40). | '''Replacement:''' Use any standard Z80A or Z80B (DIP-40). | ||
== | == RAM Issues == | ||
* Internal RAM: 1K (2114 or 4118, DIP-18). | * Internal RAM: 1K (2114 or 4118, DIP-18). | ||
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* Secure RAM pack to reduce wobble (use support bracket or tape). | * Secure RAM pack to reduce wobble (use support bracket or tape). | ||
== | == ROM Faults == | ||
* 8K ROM (2364 or 2716 EPROM) contains BASIC and OS. | * 8K ROM (2364 or 2716 EPROM) contains BASIC and OS. | ||
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'''Replacement:''' Use compatible 8K ROM or EPROM (adapter may be required for 2716). | '''Replacement:''' Use compatible 8K ROM or EPROM (adapter may be required for 2716). | ||
== | == Keyboard Membrane & Input Issues == | ||
* Flat membrane keyboard is prone to cracks and trace breaks. | * Flat membrane keyboard is prone to cracks and trace breaks. | ||
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* Clean edge connector contacts. | * Clean edge connector contacts. | ||
== | == Video Output & RF Modulator Issues == | ||
* ZX81 outputs composite video to RF modulator (UHF channel 36). | * ZX81 outputs composite video to RF modulator (UHF channel 36). | ||
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* For direct composite mod: disconnect modulator input and wire composite out to RCA jack. | * For direct composite mod: disconnect modulator input and wire composite out to RCA jack. | ||
== | == Cassette Loading & Saving Issues == | ||
* ZX81 uses audio cassette for program storage. | * ZX81 uses audio cassette for program storage. | ||
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* Replace C8, C9, and associated audio capacitors. | * Replace C8, C9, and associated audio capacitors. | ||
== | == Final Notes == | ||
* '''Start with power and voltage checks.''' | * '''Start with power and voltage checks.''' | ||