Sinclair ZX80 Troubleshooting Guide

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.

Common failure symptoms, diagnostic procedures, and fixes are outlined for:

Power supply faults
“No video” or “garbage video” scenarios
Memory (RAM) issues
ROM faults
ULA (Uncommitted Logic Array) failures
Z80 CPU faults
Keyboard matrix problems
Video sync and display issues

Diagnostic techniques (chip substitution, piggy-backing, logic probing, thermal checks), voltage test points (+5 V), and critical signals (RESET, clock, chip-enable lines) are all explained.

Note: This guide does not cover ZX81 or ZX80-to-ZX81 upgrade kits.

Diagnostic Tools & Techniques

Visual Inspection

Remove the top cover; examine for burnt or cracked components, corrosion, or cold solder joints—especially around the power jack, ULA, and edge connector.
Re-flow or re-solder any suspect joints to cure intermittent power or I/O issues.

Thermal Checks

Gently touch (or use an IR thermometer) on chips after ~60 seconds of power-on.
A too-hot-to-touch ULA or RAM IC often indicates an internal short.
Use freeze spray: if behaviour changes while cooling/heating, that IC is likely faulty.

Power & Signal Probing

+5 V DC at multiple IC Vcc pins (e.g. Z80 pin 11, RAM, ULA).
RESET: low for a moment at power-on, then high (5 V).
System clock: logic-level 3.25 MHz at Z80 pin 6.
ROM chip-enable, RAM chip-enable, and address/data bus activity.

Chip Substitution

Swap socketed chips one at a time with known-good parts:

ULA (IC1) – highest failure rate.
Z80 CPU, ROM, RAM.
Observe orientation (notch/pin 1) and always power off before removal/insertion.

Piggy-back Testing

Press a good logic or RAM IC on top of each suspect chip (pins aligned).
If symptoms change or machine boots, the underlying IC is bad.
Warning: Won’t help if the bad IC is shorted; never piggy-back the ULA.

Minimal-Configuration Boot

You can power up without the following:

Keyboard membrane—system will boot, but no input.
RAM—system will display a blank or unstable screen, but should still show sync.

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.

Test Point	Expected Voltage	Purpose / Notes
Edge connector pin 7 (+5 V) ↔ pin 8 (GND)	+5 V DC (±5%)	Main logic rail
ULA pin 24 (Vcc) ↔ pin 12 (GND)	+5 V DC	ULA supply
Z80 pin 11 (Vcc) ↔ pin 29 (GND)	+5 V DC	CPU supply
RAM pin 8 (Vcc) ↔ pin 16 (GND)	+5 V DC	RAM supply

Common PSU Faults

5 V missing/high → no boot or chip death (>6 V destroys RAM/ULA quickly).
Intermittent drop-outs via dirty power switch or cracked power-jack solder.
Ripple/hum bars = dried-out PSU capacitors.

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.
Swap/feel ULA (dead clock = blank display).
Replace/logic-probe ROM (statistically #2 cause).
Swap CPU (Z80) only after ULA & ROM ruled out.
Test/replace RAM (blank screen or random characters).
Inspect video output transistor (TR1) and modulator.
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.
Expansion RAM packs connect via edge connector.

Symptoms

Blank screen (lower RAM failure).
Garbage characters, freezes, or random resets.
One RAM chip hotter than neighbours.

Diagnosis / Fix

Swap suspected chip, or socket & replace.
If still faulty after RAM swap, check address and data bus logic (74LS157, 74LS00, etc.).

Video Output & ULA Faults

Symptom	Likely Cause	Notes
No video at all	Dead ULA, bad modulator, or no +5 V	Confirm ULA pin 24 = 5 V
Rolling, unstable, or no sync	ULA or clock circuit	Check crystal, C4, C5, and associated logic
Black/white bars only	ROM failure or ULA	Try known-good ROM first
Weak/ghosted video	Faulty TR1 transistor or modulator	Replace as needed

ULA Failure Signs

No video output, or random patterns regardless of ROM/RAM/CPU swaps.
ULA runs very hot, or draws excessive current.
Replace with same ULA type (early and late variants differ).

ROM Faults

4K mask ROM (IC3) – contains BASIC interpreter and OS.
Early ZX80s use 2364 ROM; later ones may use 2716 EPROM.

Symptoms

Blank screen with border, or random characters.
No response to keyboard.
System boots with cartridge/ROM swap but not with original.

Repair

Replace with known-good ROM or compatible EPROM adapter (e.g. 2732 with adapter).
Ensure correct ROM type for board revision.

Z80 CPU Faults

Rare, but possible (over-voltage, ESD).

Address lines static despite proper clock ⇒ dead CPU.
No data bus activity with working ULA/ROM ⇒ suspect CPU.
Replace with Z80A or compatible.

Keyboard Matrix Issues

Membrane cable tears or corrosion at connector.
Key rows or columns dead = membrane or PCB trace open.
No keys work = membrane not seated, or ULA pin 14/15/16/17/18/19 failure.

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.
Failure can cause random boot, video, or RAM issues.
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.
Random resets with RAM pack = “RAM pack wobble”; secure with tape or support bracket.
Check for shorts or broken tracks near the edge connector.

Final Notes

Start with power checks.
ULA is statistically the most common failure, followed by ROM and RAM.
Always fit sockets on replacement parts and monitor PSU health.
Avoid static discharge when handling the ULA or RAM chips.