Commodore 8-bit
Interactive Flowchart Commodore 128
ASSY 310379 (C128 Flat) & ASSY 250477 (C128DCR) — Dual-CPU 8502 + Z80A 40/80-column mainboard
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
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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)
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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
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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)
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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
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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
The Commodore 128 generates its +12V DC rail (for the 6581 SID and 8564/8566 VIC-IIe) on-board from the 9V AC transformer line via C106 (2200 uF) and C107 (1000 uF). Drying axial filter cans cause 100Hz hum and VDC/VIC-IIe sync instability.
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
Never swap C128 and Amiga 5-pin Square DIN PSUs
Both use the identical 5-pin square DIN plug, but C128 Pins 4 & 5 carry 9V AC while Amiga Pins 3, 4 & 5 carry +12V DC, GND & -12V DC!
Renew thermal paste on the RF shield fingers
When lifting the top RF shield to reach the capacitors, clean and apply fresh non-conductive thermal compound to the spring tabs pressing on U7, U21, U22 and U5.
Is This Your Board?
- ASSY 310379 is the wedge C128 board inside a full RF shield cage that doubles as the heatsink for the VIC-IIe, VDC, SID and 8722 MMU.
- ASSY 250477 is the C128DCR metal-case integrated board with 64 KB VDC RAM and an 8580 SID.
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.
Mainboard (ASSY 310379)
| Ref | Value | Voltage | Type | Function | Notes | Confidence |
|---|---|---|---|---|---|---|
C12 |
10 uF | 25 V | Radial | VIC-IIe PLL loop filter | Verified | |
C20 |
10 uF | 25 V | Radial | 8563 VDC clock filter | Verified | |
C40 |
4.7 uF | 25 V | Radial | SID audio coupling | Verified | |
C43 |
10 uF | 25 V | Radial | Audio out coupling | Verified | |
C59 |
1 uF | 50 V | Radial | 556 /RESET timing | Verified | |
C61 |
10 uF | 25 V | Radial | Reset pulse timing | Verified | |
C92 |
100 uF | 16 V | Radial | Cassette motor regulator | Verified | |
C100 |
100 uF | 16 V | Radial | +5V logic bulk | Verified | |
C104 |
10 uF | 25 V | Radial | 7812 output decoupling | Verified | |
C106 |
2200 uF | 25 V | Axial | 9V AC rectifier main reservoir | Verified | |
C107 |
1000 uF | 25 V | Axial | 7812 regulator input filter | Verified | |
C109 |
470 uF | 16 V | Radial | +5V rail local reservoir | Verified |
Mainboard (ASSY 310379) — shopping list
- 5 x 10 uF 25 V Radial
- 1 x 4.7 uF 25 V Radial
- 1 x 1 uF 50 V Radial
- 2 x 100 uF 16 V Radial
- 1 x 2200 uF 25 V Axial
- 1 x 1000 uF 25 V Axial
- 1 x 470 uF 16 V Radial
Recap Tips
- Check the two large axial cans C106 (2200 uF 25V) and C107 (1000 uF 25V) near the bridge rectifier CR13 first if the +12V rail droops.
Related Symptoms
- Black screen in both 40 and 80 column modes Remember the Z80A (U10) boots first on the C128 before handing bus control to the 8502 (U6).
Sources & References
- Console5 TechWiki - Commodore 128 ASSY 310379 and 250477 capacitor schedules.