Editing Commodore 64 Troubleshooting Guide (section)

== Diagnostic Tools & Techniques ==

=== Visual Inspection ===
* Remove the lid; examine for burnt/cracked parts, corrosion, loose wires or '''cold solder joints''' – especially around the power jack and both joystick ports.  
* Re-flow or re-solder any suspect joints to cure intermittent power or I/O issues.

=== Thermal Checks ===
* Gently touch (or IR-probe) chips after ~60 s of power-on.  
* A '''too-hot-to-touch''' PLA or SID often indicates an internal short.  
* Use freeze-spray/compressed air: if behaviour changes while cooling/heating, that IC is likely faulty.

=== Power & Signal Probing ===
* '''+5 V DC''' at multiple IC Vcc pins (e.g. 6510).  
* '''9 VAC''' between PSU DIN pins (and user-port pins 10 ↔ 11).  
* '''+12 V DC''' (older boards) / '''≈9 V DC''' (250469) at SID & VIC-II analogue Vdd.  
* '''RESET''': low for ≈½ s at power-on, then high (5 V).  
* '''System clock''': logic-level 1 MHz at 6510 φ2 pin.  
* '''PLA chip-enable lines''', '''CAS / RAS''' pulses to RAM, etc.

=== Chip Substitution ===
Swap socketed chips '''one at a time''' with known-good parts:

* PLA (U17) – highest failure rate on bread-bin boards.  
* VIC-II, SID, 6510, CIAs.  
* Observe orientation (notch/pin 1) and always power-off before removal/insertion.

=== Piggy-back Testing ===
* Press a good logic or DRAM 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 MOS customs.

=== Minimal-Configuration Boot ===
You can power-up '''without''' the following chips:

* '''SID (U18)''' – no sound, otherwise normal BASIC screen.  
* '''CIA U1 / U2''' – boots but '''no blinking cursor''' (keyboard/IEC inactive).  
* '''CD4066 switches (U16/U28)''' – paddles disabled, possible subtle colour shift.

Remove one at a time; if C64 suddenly boots, the removed IC was dragging the bus down.

=== Cartridge Diagnostics ===
* '''Any autostart game''' (e.g. *Jupiter Lander*) bypasses BASIC/KERNAL; if it runs, suspect those ROMs.  
* '''Dead-Test Cartridge''':  
  * Runs with '''all ROMs, SID and both CIAs removed''' – ideal for core-logic/RAM checks.  
  * Coloured-border flash count = faulty RAM bit/chip.  
  * If nothing at all after stripping to bare minimum ⇒ PLA, CPU, VIC-II or clock logic.