Sinclair ZX Spectrum Troubleshooting Guide

This guide provides detailed, component-level troubleshooting for the Sinclair ZX Spectrum home computer series (16K, 48K, 48K+, 128K, +2, +3). It covers common failure symptoms, diagnostic procedures, and fixes for:

Power supply faults
“No display” (blank screen) scenarios
RAM (4116/4164/41464) faults
ULA (Uncommitted Logic Array) failures
ROM and CPU (Z80A) faults
Keyboard and membrane issues
Video and audio output faults
Edge connector and peripheral problems

Differences between major board revisions (Issue 1–6, 128K, +2, +3) are noted where relevant.

Diagnostic Tools & Techniques

Visual Inspection

Remove the case; inspect for burnt, cracked, or corroded components, especially around the power input, edge connector, and RAM chips.
Check for cold solder joints—especially on the voltage regulator, RAM, and ULA.
Look for leaking or bulging capacitors.

Thermal Checks

After 1–2 minutes of power-on, gently touch (or use an IR thermometer) on the ULA, RAM, and voltage regulator.
Too-hot-to-touch RAM or ULA often indicates internal failure or short.
Use freeze spray or compressed air: if behaviour changes when cooling a chip, it is likely faulty.

Power & Signal Probing

+5 V DC at RAM, ULA, CPU, and ROM Vcc pins.
-5 V DC and +12 V DC (16K/48K only) at lower RAM (4116) chips.
RESET line: low at power-on, then high (5 V).
System clock: 3.5 MHz at Z80 pin 6.
Composite video at modulator input (before RF conversion).
ROM chip-enable and RAM multiplexing signals.

Chip Substitution

Swap socketed chips (ULA, ROM, CPU) one at a time with known-good parts.
Observe orientation and always power off before removal/insertion.
For 128K/+2/+3, test with known-good upper/lower RAM chips.

Piggy-back Testing

Press a good DRAM chip on top of each suspect RAM (pins aligned).
If symptoms change or machine boots, the underlying IC is bad.
Warning: Ineffective if the bad chip is shorted; never piggy-back ULA or custom ICs.

Minimal-Configuration Boot

The Spectrum will not boot without ULA, ROM, or CPU.
On 16K/48K, you can boot with only lower RAM (16K) fitted; upper RAM (48K) can be removed for testing.
Disconnect keyboard membrane to rule out shorts.

Power Supply & Voltage Checks

Always verify the PSU before suspecting mainboard faults.

Test Point	Expected Voltage	Purpose / Notes
Regulator input (7812/7805 tab)	+12 V DC	Main supply rail (16K/48K)
Regulator output (7805)	+5 V DC (±5%)	Logic rail for CPU, ULA, ROM
Lower RAM pin 1	-5 V DC	Only on 4116 RAM (16K/48K)
Lower RAM pin 8	+12 V DC	Only on 4116 RAM (16K/48K)
CPU Vcc (pin 11)	+5 V DC	Z80A supply
ULA Vcc (pin 40)	+5 V DC	ULA supply
RESET (edge connector pin 16)	Low → High	Must release to 5 V after power-up

Common PSU Faults

5 V missing/high → no boot or chip damage.
12 V missing → no video, no sound, or RAM errors.
-5 V missing → lower RAM (4116) failure, often black screen or stripes.
Intermittent drop-outs due to cracked power jack solder or faulty regulator.
Buzzing/hum bars = dried-out capacitors or PSU ripple.

Never run a Spectrum on an unstable PSU; modern regulated adapters are recommended.

“No Display” (Blank Screen) Flowchart

Check PSU rails (+5 V, +12 V, -5 V), RESET, and clock first.
Swap/feel ULA (most common failure).
Replace/logic-probe ROM and CPU (Z80A).
Test/replace lower RAM (4116) – black screen or stripes often indicate RAM.
Inspect upper RAM (48K models) – garbage screen or crashes.
Remove keyboard membrane; check for shorts.
Inspect modulator and composite video output.

RAM Failures

Lower RAM (16K/48K: 4116)

Each chip = 1 bit; all 8 must work for 16K operation.
Needs +5 V, +12 V, and -5 V.
Failure symptoms: black screen, coloured stripes, random resets, or garbage display.
One chip hotter than others often indicates failure.

Upper RAM (48K: 4532/4164)

Adds 32K for 48K total.
Failure symptoms: boots as 16K, random crashes, corrupted graphics, or “R” errors on startup.
Can be removed for 16K-only operation.

128K/+2/+3 RAM (41464/4464)

8 × 41464 (128K) or 8 × 4464 (256K).
Failure: random crashes, corrupted display, “RAMTOP” errors, or beeper noise.

Diagnosis / Fix

Swap suspected chip, or socket & replace.
If still faulty after RAM swap, check multiplexers (74LS157/74LS32).
Test for correct voltages at each RAM chip.

ULA (Uncommitted Logic Array) Faults

The ULA (IC1, Ferranti 5C102E/6C001E/7K010E) is the most failure-prone chip in 16K/48K models.
Controls video, keyboard, tape I/O, and RAM multiplexing.

Failure Signs

No display (blank/black screen)
Vertical or horizontal stripes
No sync or distorted video
No sound or tape I/O
Keyboard unresponsive

Diagnosis / Fix

Swap with known-good ULA (matching type)
Check for excessive heat
Inspect for dry joints on ULA socket

Z80A CPU Faults

Rare, but possible after over-voltage or static discharge.

Symptoms

No clock at pin 6, or address/data lines stuck
No response to RESET
Dead after ULA and RAM ruled out

Diagnosis / Fix

Swap with known-good Z80A
Check for correct clock and RESET signals

ROM Faults

16K/48K: 8K ROM (IC5, Sinclair 2364 or 27C64 EPROM)
128K/+2/+3: 32K or 64K ROM

Symptoms

Blank screen or border only
Corrupt startup screen or “1982 Sinclair Research” missing
Machine boots with test cartridge but not to BASIC

Diagnosis / Fix

Swap with known-good ROM or EPROM
Check chip-enable and address lines

Keyboard and Membrane Issues

The keyboard membrane is a common failure point (cracks, broken traces).
Symptoms: unresponsive keys, stuck keys, or no keyboard at all.
Test with continuity meter; replace membrane if needed.
On 48K+, check for broken key plungers or springs.
On 128K/+2/+3, check for loose ribbon cables and corroded connectors.

Video Output Faults

No Video (but power LED on)

Check composite video at modulator input (bypass RF for testing).
Suspect ULA, modulator, or video amplifier transistor (TR1/TR2).

Rolling or distorted image

Wrong ULA type for board revision (PAL/NTSC mismatch)
Bad clock crystal (14 MHz)
Faulty modulator or dry joints

Colour missing

Faulty ULA or modulator
Check chroma circuit (128K/+2/+3)

Jailbars or checkerboard

RAM or ULA fault; check for hot chips

Audio Output Faults

No sound: ULA failure, speaker disconnected, or dry joints.
Distorted sound: faulty ULA or speaker.
128K/+2/+3: check AY-3-8912 sound chip and associated circuitry.