Commodore 8-bit
Interactive Flowchart Commodore VIC-20
ASSY 324003 (2-Prong) & ASSY 250403 (Cost-Reduced CR) — Early LM323K linear board & late 7-pin DIN CR board
Complete Diagnostic Flowchart: Commodore 64 Blank / Black Screen Fault Isolation (Commodore 64 (ASSY 326298 / 250407 / 250425 / 250466 / 250469))
STEP 1: Measure +5V DC Logic Rail at User Port Pin 2 or CPU Pin 6
[node-1]
Disconnect cartridges and serial cables. Set your digital multimeter to DC Volts. Place black probe on User Port Pin 12 (GND) or RF modulator chassis shield. Probe User Port Pin 2 (+5V DC) and MOS 6510 CPU (U7) Pin 6 (VCC).
Expected Reading: Steady +4.75V to +5.25V DC with <50 mVpp ripple. Above +5.25V indicates a failing epoxy brick regulator.
STEP 2: Check 9V AC Transformer Rail & VIC-II Pin 13 (+12V VDD)
[node-2]
Switch multimeter to AC Volts and probe across User Port Pins 10 and 11 (9V AC from PSU through fuse F1). Switch back to DC Volts and probe VIC-II (U19) Pin 13 (VDD): expect +12.0V on 6567/6569 longboards (from 7812 VR1 & C88/C90) or +5.0V on 8562/8565 shortboards.
Expected Reading: 8.5V–11.0V AC on User Port Pins 10–11, and +11.4V to +12.6V DC at VIC-II Pin 13 (ASSY 250407–250466).
STEP 3: Verify Φ2 System Clock at CPU Pin 39 & /RESET at Pin 40
[node-3]
Probe MOS 6510 CPU (U7) Pin 40 (/RESET): must hold LOW (<0.4V) for ~400 ms at power-on (via 556 timer U20 & C24), then snap HIGH (>3.8V). Check Φ2 clock at CPU Pin 39 (and Φ0 input at Pin 1 from VIC-II Pin 17): verify a clean 0.985 MHz (PAL) / 1.023 MHz (NTSC) TTL square wave.
Expected Reading: Pin 40 (/RESET) steady HIGH (+4.8V); Pin 39 (Φ2) pulsing at 0.985 MHz (PAL) swinging 0.15V to 4.65V.
STEP 4: Probe 82S100 PLA (U17) Chip-Select Strobes (/CASRAM Pin 18 & /KERNAL Pin 16)
[node-4]
Check thermal state of U17 (82S100 / 906114-01 PLA): if scorching hot (>60°C), suspect internal nichrome fuse degradation. Scope PLA Pin 18 (/CASRAM), Pin 16 (/KERNAL), and Pin 15 (/BASIC) during Φ2 high. Verify clean active-low strobes (<0.4V low, >3.8V high) without mid-rail (~1.8V–2.2V) runt contention steps.
Expected Reading: Crisp active-low pulses on /CASRAM (Pin 18) and /KERNAL (Pin 16). If Dead Test 781220 cartridge flashes border 1–8 times, PLA works and DRAM is faulty.
STEP 5: Isolate 4164 DRAM Bus Contention vs. KERNAL ROM / 6510 CPU
[node-5]
Insert a C64 Dead Test (781220) cartridge or scope DRAM data pins D0–D7 across U9–U12 & U21–U24 (4164 Pin 2 Din / Pin 14 Dout). Does the Dead Test flash the screen border 1–8 times, or do you see a ~1.8V mid-rail contention plateau on a data line D0–D7?
Expected Reading: Yes = Faulty 4164 DRAM (or 74LS257 multiplexer U13/U25). No = Check KERNAL ROM (U4), 6510 CPU (U7), or shorted 6581 SID (U18) pulling bus down.
[ISOLATED FAULT: RES-PSU] Fault Isolated: +5V DC Supply Rail Out of Spec / Failing Brick PSU
VERDICT
The mainboard is receiving an invalid or over-voltage +5V DC rail. Original epoxy-potted Commodore brick PSUs suffer internal 7805 thermal drift or short-circuit failure.
[ISOLATED FAULT: RES-9VAC-12V] Fault Isolated: Missing 9V AC or +12V VR1 (7812) / C88/C90 Rail Failure
VERDICT
Without 9V AC or the derived +12V DC rail on ASSY 250407–250466, the NMOS 6567/6569 VIC-II cannot generate video sync or Φ0 clock.
[ISOLATED FAULT: RES-CLOCK-RESET] Fault Isolated: /RESET Stuck Low or Missing Φ2 / 8701 Dot Clock
VERDICT
The 6510 CPU is either held permanently in reset (Pin 40 LOW) or lacks the 0.985 MHz Φ2 clock (Pin 39) generated by the 8701 (U31) and VIC-II (U19).
[ISOLATED FAULT: RES-PLA] Fault Isolated: Failed 82S100 / 906114-01 PLA (U17)
VERDICT
The 82S100 PLA is failing to decode memory strobes (/CASRAM Pin 18, /KERNAL Pin 16, /BASIC Pin 15) or asserting multiple chip-selects simultaneously.
[ISOLATED FAULT: RES-DRAM] Fault Isolated: Bad 4164 DRAM (U9–U12 / U21–U24) or 74LS257 Mux (U13/U25)
VERDICT
One or more 4164 (64K×1) DRAM chips—or the MOS-branded 74LS257 address multiplexers (U13/U25)—are corrupting lower zero-page/stack memory.
[ISOLATED FAULT: RES-ROM-CPU] Fault Isolated: KERNAL ROM (U4), MOS 6510 CPU (U7) or Shorted SID (U18)
VERDICT
With rails, clock, PLA, and DRAM verified, the black screen is caused by a corrupted KERNAL ROM ($E000–$FFFF), a halted 6510 CPU, or a shorted 6581 SID locking the data bus.
Summary
Early 2-prong VIC-20 boards rectify 9V AC internally using a massive 4700 uF axial capacitor (C28) and a hot-running TO-3 LM323K 3A regulator. Later Cost-Reduced (ASSY 250403) boards use a C64-style 7-pin DIN PSU and far fewer electrolytics.
Interactive Bench Oscilloscope — Verify Pre- & Post-Recap Waveforms
Open Full Signal Visualizer →Click any test point below to compare a healthy recapped rail or clock against high-ESR electrolytic capacitor ripple and droop:
Safety & Warnings
Check C64-style brick PSU on Cost-Reduced (CR) boards
The 7-pin DIN VIC-20 CR uses the exact same epoxy brick PSU as the C64 - test the +5V rail before powering on!
Is This Your Board?
- Early ASSY 324003 / 1001006 boards have a 2-prong 9V AC power socket and a huge finned heatsink on the right edge.
- Cost-Reduced ASSY 250403 boards have a 7-pin circular DIN power socket and two 6116 SRAM chips instead of ten 2114 chips.
Verified
Check
Verified rows are agreed by two or more independent sources. Check rows are revision-dependent or single-sourced — confirm the value against your own board before ordering.
Cost-Reduced Mainboard (ASSY 250403)
| Ref | Value | Voltage | Type | Function | Notes | Confidence |
|---|---|---|---|---|---|---|
C1 |
10 uF | 25 V | Radial | 6560/6561 VIC video filter | Verified | |
C11 |
10 uF | 25 V | Radial | 555 /RESET timer | Verified | |
C16 |
2200 uF | 16 V | Axial | 9V AC / Datasette motor filter | Verified | |
C17 |
100 uF | 16 V | Radial | +5V rail filter | Verified | |
C35 |
470 uF | 16 V | Axial | +5V bulk decoupling | Verified | |
C40 |
10 uF | 25 V | Radial | Audio decoupling | Verified |
Cost-Reduced Mainboard (ASSY 250403) — shopping list
- 3 x 10 uF 25 V Radial
- 1 x 2200 uF 16 V Axial
- 1 x 100 uF 16 V Radial
- 1 x 470 uF 16 V Axial
Early 2-Prong Mainboard (ASSY 324003)
| Ref | Value | Voltage | Type | Function | Notes | Confidence |
|---|---|---|---|---|---|---|
C5 |
10 uF | 25 V | Radial | VIC video bias | Verified | |
C26 |
10 uF | 25 V | Radial | NE555 reset timing | Verified | |
C28 |
4700 uF | 25 V | Axial | Main 9V AC bridge reservoir | Giant axial can next to the bridge rectifier and LM323K heatsink. | Verified |
C29 |
100 uF | 16 V | Radial | LM323K +5V output filter | Verified | |
C30 |
10 uF | 25 V | Radial | Cassette motor regulator | Verified |
Early 2-Prong Mainboard (ASSY 324003) — shopping list
- 3 x 10 uF 25 V Radial
- 1 x 4700 uF 25 V Axial
- 1 x 100 uF 16 V Radial
Whole-Machine Shopping List
- 6 x 10 uF 25 V Radial
- 1 x 2200 uF 16 V Axial
- 2 x 100 uF 16 V Radial
- 1 x 470 uF 16 V Axial
- 1 x 4700 uF 25 V Axial
Recap Tips
- On early 2-prong boards, replacing the hot LM323K TO-3 regulator with a modern 3A buck module drops internal case temperatures by over 20 C.
Related Symptoms
- Hum bar rolling up the screen Dried-out 4700 uF axial reservoir C28 on early 2-prong boards.
Sources & References
- Console5 TechWiki - Commodore VIC-20 Early and CR board capacitor lists.