Editing Commodore 64 Troubleshooting Guide (section)

== Component reference and test points (deep dive) ==

=== Chip map ===

The main custom and memory ICs (Commodore service manual, boards 250407/250425/250469):

<templatestyles src="Template:StyledTable/styles.css" />
[[File:Commodore 64 (photo).jpg|thumb|right|300px|Commodore 64. Source: Wikimedia Commons.]]
{| class="wikitable styled-table" style="width:100%; text-align:left;"
! Ref !! Device !! Function
|-
| U1, U2 || 6526 CIA || Keyboard/joystick/paddle + IRQ + serial (U1); user port + serial + NMI (U2)
|-
| U3 || 2364 (901226) || BASIC ROM
|-
| U4 || 2364 (901227) || KERNAL ROM
|-
| U5 || 2364 (901225) || Character ROM
|-
| U6 || 2114 || Colour RAM
|-
| U7 || 6510 || CPU (with the parallel port P0–P5 that banks the memory map)
|-
| (8× 4164) || DRAM || 64 KB main RAM
|-
| U17 || 906114 PLA || Address decode / ROM & I/O select
|-
| U18 || 6581 (or 8580) SID || Sound (audio out pin 27, audio in pin 26)
|-
| U19 || 6567 (NTSC) / 6569 (PAL) VIC-II || Video and the system clock
|}
<ref name="c64svc">[https://portcommodore.com/rcarlsen/cbm/c64/C64_SERVICE_MANUAL.pdf Commodore C64 Service Manual] (Ray Carlsen / PortCommodore); [https://oldsilicon.com/technologies/commodore-64-250407-parts-locator/ C64 250407 Visual Parts Locator], OldSilicon; and [https://www.c64copyprotection.com/commodore-64-chip-set/ Commodore 64 Chip Set]. Source for the IC map, the PLA output-select pins (F1 pin 17 BASIC, F2 pin 16 KERNAL, F3 pin 15 character ROM, F5 pin 12 I/O) and the SID audio pins (27 out, 26 in).</ref>

=== Clock signals (where to probe) ===

* '''Master crystal''': 14.31818 MHz (NTSC) or 17.734472 MHz (PAL) at the VIC-II, which divides it to make the dot and system clocks.
* '''Dot clock''': 8.18 MHz (NTSC) / 7.88 MHz (PAL).
* '''System clock φ2''': about 1.023 MHz (NTSC) / 0.985 MHz (PAL), generated by the VIC-II (U19) and fed to the 6510 and both CIAs. '''No φ2 means the VIC-II or the crystal is dead and nothing runs.'''<ref name="c64clk">C64 clock rates per the [https://www.c64-wiki.com/wiki/Clock C64-Wiki] and the service manual.</ref>

=== PLA (U17) output selects — a scope short-cut ===

The 906114 PLA does the address decoding; probing its outputs shows at a glance whether the CPU is fetching ROM. Each output goes '''low''' when its area is selected:<ref name="c64svc">[https://portcommodore.com/rcarlsen/cbm/c64/C64_SERVICE_MANUAL.pdf Commodore C64 Service Manual] (Ray Carlsen / PortCommodore); [https://oldsilicon.com/technologies/commodore-64-250407-parts-locator/ C64 250407 Visual Parts Locator], OldSilicon; and [https://www.c64copyprotection.com/commodore-64-chip-set/ Commodore 64 Chip Set]. Source for the IC map, the PLA output-select pins (F1 pin 17 BASIC, F2 pin 16 KERNAL, F3 pin 15 character ROM, F5 pin 12 I/O) and the SID audio pins (27 out, 26 in).</ref>

* pin 17 (F1) low = BASIC ROM selected
* pin 16 (F2) low = KERNAL ROM selected
* pin 15 (F3) low = Character ROM selected
* pin 12 (F5) low = any I/O device selected

A PLA showing no activity on these pins (or lines stuck high/low) is the classic no-boot / black-screen cause.

=== Symptom → suspect chip ===

<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table" style="width:100%; text-align:left;"
! Symptom !! First suspect(s)
|-
| Black screen, no border || PLA (U17) → VIC-II (U19) → CPU (U7)
|-
| Garbage / checkerboard border || A 4164 DRAM (the border colour/pattern hints the failed bit)
|-
| Border + cursor, no KERNAL banner || KERNAL ROM (U4) or the ROM/PLA select
|-
| Screen OK but no blinking cursor || CIA U1
|-
| No sound || SID (U18)
|-
| Keyboard / joystick dead || CIA (U1 / U2)
|-
| Runs then crashes when warm || Overheating PLA or SID (feel/freeze-spray)
|}

=== Diagnostic cartridges ===

* '''Dead Test''' cartridge runs with all ROMs, the SID and both CIAs removed — the border-flash count identifies the faulty RAM bit.
* '''586220 Diagnostic''' cartridge, used with the keyboard/user-port/serial/joystick '''loopback harness''', tests every port and reports pass/fail — the definitive full-machine test.<ref name="c64svc">[https://portcommodore.com/rcarlsen/cbm/c64/C64_SERVICE_MANUAL.pdf Commodore C64 Service Manual] (Ray Carlsen / PortCommodore); [https://oldsilicon.com/technologies/commodore-64-250407-parts-locator/ C64 250407 Visual Parts Locator], OldSilicon; and [https://www.c64copyprotection.com/commodore-64-chip-set/ Commodore 64 Chip Set]. Source for the IC map, the PLA output-select pins (F1 pin 17 BASIC, F2 pin 16 KERNAL, F3 pin 15 character ROM, F5 pin 12 I/O) and the SID audio pins (27 out, 26 in).</ref>