Editing Commodore 64 Troubleshooting Guide (section)

== Power-Supply & Voltage Checks ==
Always verify PSU '''before''' blaming main-board silicon.

{| class="wikitable" style="text-align:left"
! Test Point!!Expected Voltage!!Purpose / Notes
|-
|Expansion Port pin 2 (+5 V) ↔ pin 1 (GND)||+5 V DC (±5 %)||Main logic rail
|-
|User Port pins 10 & 11||≈9 V AC (rms)||Drives SID/VIC analogue rails & 50/60 Hz TOD clock
|-
|SID Vdd (p 28 6581 / p 25 or 28 8580)||12 V (*6581*) or 9 V (*8580*)||Analogue supply
|-
|VIC-II Vdd (p 13 6567/9)||12 V (old) / 5 V (new HMOS)||Video-core supply
|-
|RESET (Expansion Port pin C)||Low → High||Must release to 5 V after power-up
|}

=== Common PSU Faults ===
* '''5 V missing/high''' → no boot or chip death (>6 V destroys RAM/PLA quickly).  
* '''9 VAC missing''' → black screen, no colour/sound; check brick fuse & board fuse.  
* '''Intermittent drop-outs''' via dirty power switch or cracked power-jack solder.  
* '''Hum-bars / buzzing audio''' = dried-out PSU capacitors.

Never run a C64 on an unstable brick; modern replacements or crowbar protectors are highly recommended (≥1.5 A @ 5 V, ≥1 A @ 9 VAC).