Editing Sinclair ZX Spectrum Troubleshooting Guide

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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.

{| class="wikitable" style="text-align:left"
! 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.

[[Category:Sinclair Computers]]