Sinclair ZX80 Troubleshooting Guide: Difference between revisions
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This guide provides detailed, component-level troubleshooting for the '''Sinclair ZX80''' home computer. It covers all major PCB revisions and notes differences between early and late production models where relevant. | This guide provides detailed, component-level troubleshooting for the '''Sinclair ZX80''' home computer. It covers all major PCB revisions and notes differences between early and late production models where relevant. | ||
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''Note: This guide does not cover ZX81 or ZX80-to-ZX81 upgrade kits.'' | ''Note: This guide does not cover ZX81 or ZX80-to-ZX81 upgrade kits.'' | ||
== | == Diagnostic Tools & Techniques == | ||
=== Visual Inspection === | === Visual Inspection === | ||
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Remove one at a time; if the ZX80 suddenly boots, the removed IC was dragging the bus down. | Remove one at a time; if the ZX80 suddenly boots, the removed IC was dragging the bus down. | ||
== | == Power Supply & Voltage Checks == | ||
Always verify the PSU '''before''' blaming main-board silicon. | Always verify the PSU '''before''' blaming main-board silicon. | ||
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Never run a ZX80 on an unstable PSU; modern regulated supplies are highly recommended (≥500 mA @ 5 V). | Never run a ZX80 on an unstable PSU; modern regulated supplies are highly recommended (≥500 mA @ 5 V). | ||
== | == “No Video” or “Garbage Video” Flowchart == | ||
# Check PSU rails, RESET, and system clock first. | # Check PSU rails, RESET, and system clock first. | ||
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# Remove keyboard membrane if stuck in reset. | # Remove keyboard membrane if stuck in reset. | ||
== | == RAM Failures == | ||
* '''2114 (1K x 4-bit) RAM'''—two chips for 1 KB ZX80, four for 2 KB. | * '''2114 (1K x 4-bit) RAM'''—two chips for 1 KB ZX80, four for 2 KB. | ||
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* If still faulty after RAM swap, check address and data bus logic (74LS157, 74LS00, etc.). | * If still faulty after RAM swap, check address and data bus logic (74LS157, 74LS00, etc.). | ||
== | == Video Output & ULA Faults == | ||
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* Replace with same ULA type (early and late variants differ). | * Replace with same ULA type (early and late variants differ). | ||
== | == ROM Faults == | ||
* 4K mask ROM (IC3) – contains BASIC interpreter and OS. | * 4K mask ROM (IC3) – contains BASIC interpreter and OS. | ||
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* Ensure correct ROM type for board revision. | * Ensure correct ROM type for board revision. | ||
== | == Z80 CPU Faults == | ||
Rare, but possible (over-voltage, ESD). | Rare, but possible (over-voltage, ESD). | ||
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* Replace with Z80A or compatible. | * Replace with Z80A or compatible. | ||
== | == Keyboard Matrix Issues == | ||
* Membrane cable tears or corrosion at connector. | * Membrane cable tears or corrosion at connector. | ||
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Test continuity from each keyboard pad to ULA; repair or replace membrane as needed. | Test continuity from each keyboard pad to ULA; repair or replace membrane as needed. | ||
== | == Other Logic ICs == | ||
* 74LS00, 74LS157, 74LS32, 74LS04—glue logic for address decoding, video timing, and bus control. | * 74LS00, 74LS157, 74LS32, 74LS04—glue logic for address decoding, video timing, and bus control. | ||
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* Piggy-back or swap with known-good chips to confirm. | * Piggy-back or swap with known-good chips to confirm. | ||
== | == Edge Connector & Expansion Issues == | ||
* No response from RAM pack or printer = dirty or bent edge connector. | * No response from RAM pack or printer = dirty or bent edge connector. | ||