Editing Sinclair ZX80 Troubleshooting Guide (section)

== 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.