Editing Sinclair ZX Spectrum Troubleshooting Guide (section)

== 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.