Commodore SX-64 Machine Hub
The first full-colour luggable computer (1984) — multi-board stack diagnostics, 5″ CRT monitor & internal PSU recapping, Alps 1541 drive servicing, and DB25 keyboard restoration.
Internal Switching PSU & 5″ CRT Anode High-Voltage Warning
Inside the cramped steel chassis of the Commodore SX-64 sit two high-energy subsystems directly beside the logic boards: an enclosed internal mains switching/linear power supply module at the bottom-rear and an unshielded 5-inch colour CRT monitor assembly (~14 kV anode voltage) on the left. Never reach around the CRT neck board or flyback transformer to unplug the CPU/FDD boards without first discharging the CRT anode cap and letting the primary PSU filter capacitors bleed down!
1 Internal PSU Rail Spec
The internal SX-64 PSU powers the C64 logic, the 1541 floppy drive, the audio amp, and the 5″ colour CRT monitor:
| Rail | Spec | Tolerance | Feeds |
|---|---|---|---|
| +5 V DC (Logic) | 5.00 V DC | 4.90–5.15 V | CPU board, I/O board, 1541 logic, RAM & ROMs |
| +12 V DC (Drive/SID) | 12.00 V DC | 11.5–12.5 V | 6581 SID, VIC-II, Alps 1541 stepper & spindle motor |
| +12 V DC (CRT) | 12.00 V DC (Regulated) | 11.8–12.2 V | 5″ CRT deflection & flyback B+ regulation |
| 9 V AC | 9.0 V AC (50/60 Hz) | 8.5–10.0 V AC | 6526 CIA Time-of-Day (TOD) 50/60 Hz reference |
2 PSU & CRT Recapping Guide
- Internal PSU Module: Drilling out the rivets (or removing the machine screws on later units) on the bottom-rear silver PSU box reveals aging 85 °C radial electrolytics and a RIFA/mains EMI filter capacitor. Replace the X2 mains cap immediately and recap the secondary +5 V/+12 V filter caps with 105 °C Panasonic FR / Nichicon PW low-ESR parts.
- 5″ CRT Monitor Chassis ( Matsushita / Sony ): Dry electrolytic capacitors in the vertical deflection stage cause vertical foldover (compressed top half of screen) or rolling sync. Replace the 10 μF–470 μF caps around the vertical IC and check for cracked solder joints on the flyback transformer pins and CRT neck socket.
3 Safe Disassembly Sequence
- Remove the top cover (undo rear bezel screws and slide the top aluminium skin backwards — watch the ground strap!).
- To access the vertical CPU/VIC/PLA board on the right wall, remove the upper 1541 FDD logic board first (mounted horizontally above the floppy drive).
- Label every wire harness (J1–J12) with masking tape before unplugging — several 4-pin and 6-pin Molex connectors on the I/O and FDD boards have identical pin spacing!
Executive Portable Hardware Architecture
Select a subsystem to view component layouts, mechanical service procedures, and adjustment guides for the SX-64.
Modular 4-PCB Architecture ASSY 250410 • 1983–1986
To fit a complete Commodore 64 and a 1541 disk drive around a 5-inch CRT tube and power supply, Commodore split the electronics across four separate interconnected daughterboards:
- 1. CPU / Video / RAM Board (Vertical Right Wall): Houses the 6510 CPU (U1), 906114-01 PLA (U4), 6567/6569 VIC-II (U2), eight 4164 DRAMs (U10–U17), 2114 Colour RAM (U7), and the three 24-pin ROM sockets: BASIC (U18), Kernal
251104-04(U20), and Character ROM (U19). - 2. I/O & Sound Board (Main Floor / Backplane): Houses the two 6526 CIAs (UA1 & UA3), the 6581 SID (UB1), joystick/user/serial port connectors, and the 62-pin upright edge connector that the vertical CPU board plugs into!
- 3. Cart Expansion Riser PCB (Top Rear): Bridges the I/O board up to the top-loading cartridge slot door.
- 4. 1541 FDD Controller Board (Horizontal Top Deck): Complete 1541 logic board (6502 CPU, two 6522 VIAs, 2 KB SRAM, 16 KB DOS 2.6 ROM, and gate array) mounted directly above the Alps drive mechanism.
Board Revisions: Scalloped vs Smooth Heatsink Edge
| Production Run | Serial Prefix | Hardware Notes |
|---|---|---|
| Early Run (1983–84) | GA1 / GA2 (early) | Uses discrete 8701/PLL clock components, ceramic 6567R8/6569R3 VIC-II, and original 906114-01 PLA. Top cartridge flap hinges can be fragile. |
| Mid/Late Run (1984–85) | GA4 / GA5 / GA6 | Updated PCB revisions with MOS 8701 clock generator IC, revised PSU harness, and 251104-04 Kernal ROM. |
Why the SX-64 Runs Hot: The vertical CPU/Video board sits in a narrow air pocket between the 1541 drive and the right aluminium side panel. Always fit low-profile copper heatsinks to the PLA (U4), SID (UB1), and VIC-II (U2)!
Built-in 5″ Colour CRT Monitor Separate Luma / Chroma Input
Because the internal 5-inch colour CRT is driven via separated Luminance (Y) and Chrominance (C) lines straight from the VIC-II (effectively internal S-Video, bypassing any RF modulator!), the SX-64 produces a remarkably sharp, dot-crawl-free image despite its small screen size.
Front Control Knobs (Behind Flip-Down Door)
- VOLUME: Controls the internal loudspeaker amplifier (fed by the 6581 SID).
- BRIGHTNESS: Adjusts CRT black level / DC grid bias.
- CONTRAST: Adjusts luminance video drive amplitude.
- COLOR (Saturation): Adjusts PAL/NTSC chroma gain (pull or turn depending on sub-pot).
- TINT (Hue — NTSC) / V-HOLD: Adjusts phase/vertical sync lock.
CRT Monitor Troubleshooting & Adjustments
| Symptom | Cause & Bench Fix |
|---|---|
| External Monitor Works (on Rear DIN), Internal CRT Black | Proves the C64 logic and VIC-II are 100% working! Check the +12 V CRT supply fuse/line from the PSU, look for a glowing orange heater filament in the CRT neck, and inspect the flyback solder joints. |
| Black & White Picture Only on Internal CRT | Oxidised front COLOR pot behind the control door, dirty internal Chroma connector wire from the I/O board, or drifted chroma crystal trimmer pot on the CRT PCB. |
| Single Horizontal White Line Across Middle | Vertical deflection collapse! Turn brightness down immediately to avoid burning the phosphor. Replace the dried-out electrolytic caps around the vertical deflection IC and reflow its pins. |
| Blurry Text in 40 Columns | Adjust the FOCUS potentiometer on the flyback transformer / neck board using an insulated plastic alignment tool while viewing text. |
Internal 1541 Floppy Drive Alps DFB111M25A Mechanism
Almost all SX-64s use the rugged Japanese Alps Electric DFB111M25A push-button (toast-pop) mechanism (unlike Newtronics/Mitsumi twist-lever drives, the Alps rarely suffers from open read/write heads!). However, the mechanical eject latch, spindle belt, and Track 0 stop require periodic care.
Mechanical & Optical Checklist
- Read/Write Head Cleaning: Lift the upper pressure pad arm gently by a few millimetres (never bend the spring!) and swab the lower glass/ferrite head surface with 99% Isopropyl Alcohol.
- Stepper Rails & Worm/Band Guide: Clean old hardened white lithium grease off the two steel head guide rails with IPA and apply a single drop of light sewing-machine oil or PTFE lubricant.
- Spindle Speed (300 RPM): Use the 50/60 Hz strobe marks on the underside of the spindle flywheel (or a diagnostic disk speed test) and adjust the small trimpot on the motor servo PCB to lock at exactly 300.0 RPM (200 ms/rev).
Common SX-64 1541 Drive Faults
| Symptom | Cause & Fix |
|---|---|
| Red Drive LED Stays On Solid + Spindle Spins Forever | 1541 power-on self-test failed! On the horizontal FDD PCB, check the socketed 6502 CPU, both 6522 VIAs (UC2/UC3), the 16 KB DOS ROM (325302-01 / 901229-05), or reseat the power/reset connector. |
| Machine-Gun Head Banging on Track 0 | Normal 1541 bump on error, or radial head alignment drifted. Run the 1541 Alignment Tool or check the stepper phase driver transistors / 6522 VIA. |
| Disk Won't Eject When Pressing Front Button | The plastic eject button extension rod has slipped off the Alps push-rod, or the metal skate-plate inside the drive needs cleaning and dry-lubrication. |
| Device 8 vs Device 9 Switch Mod | Cut the jumper pad between the two halves of Jumper 1 (J1) on the 1541 PCB (connected to 6522 VIA pin 15 / PB5) and wire a miniature rear toggle switch to swap the internal drive between Device 8 and Device 9 when using an external SD2IEC/Pi1541! |
Detachable Keyboard & Membrane Restoration Mitsumi Hybrid Membrane
Unlike the desktop C64's conductive rubber-cup plunger keyboard, the SX-64's slim detachable keyboard uses a Mitsumi multi-layer flexible Mylar membrane actuated by small coiled springs and conductive elastomer pads under each keycap. After 40 years, oxidised silver tracks on the membrane cause high contact resistance (2 kΩ+) and dead keys.
Step-by-Step Keyboard Restoration
- Never Pull Keycaps Straight Off the Front! The SX-64 keyboard opens from the back: undo the perimeter screws on the rear plastic cover, lift off the metal backing plate, and peel back the three-layer flexible membrane sheet without removing a single keycap!
- Clean the Membrane Contacts: Separate the top and bottom Mylar sheets carefully and wipe every circular contact pad with a soft microfibre cloth dampened with deionised water or mild IPA (do not scrub aggressively with harsh solvents or you will strip the silver ink!).
- Restore Weak Pads: If specific key pads measure above 500 Ω when pressed, lightly burnish the black conductive rubber puck inside the key stem on a sheet of plain white printer paper, or apply a tiny dot of CaiKot 44 silver conductive coating.
DB25 Keyboard Cable Warnings
- Original Shielded Flat Cable: The thick original Commodore SX-64 keyboard cable uses heavily shielded conductors and metal-hooded DB25 connectors with custom strain reliefs.
- Do NOT Use Cheap Unshielded Serial Printer Cables: Inexpensive 25-pin RS-232 switchbox cables often omit several pins (or lack shielding), causing RF interference on the CIA keyboard lines and missing keys. If building a replacement cable, use a fully wired 25-conductor shielded DB25 male-to-female cable under 1.5 m in length.
- Latching Ears: If the plastic side-latches on the keyboard casing are cracked, 3D-printed replacement latches (in PETG or ABS) prevent the heavy keyboard from falling open when carrying the machine by its handle!
Diagnostic Checklist & Multi-Board Faults
Isolating faults across the SX-64's interconnected CPU board, I/O backplane, and ribbon harnesses.
"Black Screen / No Boot" Multi-Board Decision Tree
Red power LED and green/red drive LEDs come on, CRT screen glows dark grey/black (or white raster), but no white screen with cyan border appears.
- Step 1 — Connect an External Monitor to the Rear 8-Pin DIN: Before opening the case, plug a TV/monitor into the rear A/V port. If the external monitor shows the SX-64 boot screen while the internal 5″ CRT is dark, your C64 boards are fine — only the internal CRT monitor or its +12 V feed needs service!
- Step 2 — Reseat the Vertical CPU Board & Inter-Board Harnesses:
- Thermal cycling and luggable transport vibration cause the vertical CPU/Video board (ASSY 250410) to creep slightly out of its 62-pin edge connector on the floor I/O board.
- Clean the gold/tin edge fingers on the bottom of the CPU board with a soft pink pencil eraser followed by 99% IPA, and reseat all white Molex ribbon jumpers between the CPU board, I/O board, and Cartridge riser.
- Step 3 — Check the PLA (U4 — 906114-01) on the CPU Board: Just like the breadbin C64, the original 28-pin 906114-01 / 82S100 PLA at U4 is the single highest-failure IC in the SX-64! Because it is trapped vertically in stagnant hot air, it fails frequently. Swap U4 with a modern cool-running replacement (PLAnkton, SuperPLA, or GAL/CPLD PLA) first.
- Step 4 — Test with a C64 Dead-Test Cartridge (Rev 781220): Flip open the top cartridge hatch and insert a Dead-Test cartridge. Because Dead-Test runs in `Ultimax` mode without needing the Kernal ROM or CIA chips, screen flashes (1 to 8 flashes) immediately identify which of the eight 4164 DRAMs (U10–U17) on the CPU board has shorted.
- Step 5 — Unplug the 6581 SID (UB1 on the I/O Board): A shorted 6581 SID can drag down the +12 V rail (which is also needed by the VIC-II and clock circuitry!). The SX-64 will boot normally with UB1 removed from its socket.
CPU Board 4164 DRAM Map & Ribbon Connector Checklist
Garbage characters on boot, `OUT OF MEMORY IN 0` error, or Dead-Test flash codes.
SX-64 CPU Board (250410) 4164 DRAM Bit Map
| Data Bit | CPU Board IC | Dead-Test Flashes | Data Bit | CPU Board IC | Dead-Test Flashes |
|---|---|---|---|---|---|
| Bit 0 (D0) | U10 | 8 Flashes | Bit 4 (D4) | U14 | 4 Flashes |
| Bit 1 (D1) | U11 | 7 Flashes | Bit 5 (D5) | U15 | 3 Flashes |
| Bit 2 (D2) | U12 | 6 Flashes | Bit 6 (D6) | U16 | 2 Flashes |
| Bit 3 (D3) | U13 | 5 Flashes | Bit 7 (D7) | U17 | 1 Flash |
Critical Inter-Board Harness Checks
- 62-Pin CPU-to-I/O Edge Connector: Carries all address/data/control lines between the 6510/PLA/VIC-II and the 6526 CIAs/SID/Ports. Inspect the solder side of the floor I/O board for hairline ring-cracks on this long connector.
- Cartridge Riser Ribbon Cables: Two flat ribbon cables link the top cartridge slot PCB down to the CPU and I/O boards. If cartridges fail to boot or freeze when wiggled, reflow the header pins on both ends of the riser ribbons.
- Internal IEC Jumper Cable: Connects the 1541 FDD board internally to the I/O board's serial bus. If the drive spins on boot-up reset but gives `DEVICE NOT PRESENT` on `LOAD"$",8`, check CIA#2 (UA3) and the internal IEC harness.
SX-64 Kernal ROM (251104-04) vs. Standard C64
Why the SX-64 boots to a White/Cyan screen, how SHIFT + RUN/STOP changed, and why there is no Datasette port.
1. High-Contrast White Screen / Cyan Border
Standard desktop C64s boot with light-blue text on a dark-blue background (hard to read on a 5-inch portable CRT in bright office lighting). Commodore modified the SX-64 Kernal ROM (251104-04 at U20) to initialise with a White background (`POKE 53281,1`), Cyan border (`POKE 53280,3`), and Blue text (`POKE 646,6`) for crisp legibility on the built-in 5″ tube.
2. SHIFT + RUN/STOP Loads from Disk (Device 8)!
On a standard C64, pressing SHIFT + RUN/STOP prints `LOAD` and waits for the cassette tape (`PRESS PLAY ON TAPE`). On the SX-64, the custom Kernal automatically executes LOAD":*",8,1 followed by RUN — immediately booting the first program on the internal 1541 floppy drive with a single keystroke!
3. Missing Cassette Port & Tape Routines Disabled
The SX-64 physically omits the 6-pin edge connector for the Datasette recorder, omits the +6 V TIP29 cassette motor pass-transistor circuit, and patches out the Kernal tape I/O routines (attempting `LOAD"",1` returns `?ILLEGAL DEVICE NUMBER ERROR`). Because the 6510 CPU port bits (Pins 24–29) are still present on the CPU, some games that use cassette-sense bit 4 (`$01`) as a software flag may require standard C64 Kernal (`901227-03`) switched in via a dual-Kernal ROM adapter.
DB25 Keyboard, A/V & Port Pinouts
25-pin D-Sub detachable keyboard pinout, 8-pin DIN External Monitor A/V, User Port, and IEC Serial Bus.
Detachable Keyboard Connector DB25 Female on Chassis
Maps CIA#1 (UA1) Port A (`PA0–PA7`, Columns `COL0–COL7`) and Port B (`PB0–PB7`, Rows `ROW0–ROW7`) plus the `/RESTORE` key and Shift-Lock LED across the 25-pin D-Sub cable:
| DB25 Pin | SX-64 Signal | CIA#1 / Function | DB25 Pin | SX-64 Signal | CIA#1 / Function |
|---|---|---|---|---|---|
| 1 | GND (Shield) | Chassis Ground | 14 | ROW 2 (PB2) | CIA#1 Pin 12 |
| 2 | NC | Unconnected | 15 | ROW 1 (PB1) | CIA#1 Pin 11 |
| 3 | /RESTORE | To NMI circuit (active-low to GND) | 16 | ROW 0 (PB0) | CIA#1 Pin 10 |
| 4 | +5 V DC | Shift-Lock LED supply | 17 | COL 0 (PA0) | CIA#1 Pin 2 |
| 5 | ROW 3 (PB3) | CIA#1 Pin 13 | 18 | COL 1 (PA1) | CIA#1 Pin 3 |
| 6 | ROW 6 (PB6) | CIA#1 Pin 16 | 19 | COL 2 (PA2) | CIA#1 Pin 4 |
| 7 | ROW 5 (PB5) | CIA#1 Pin 15 | 20 | COL 3 (PA3) | CIA#1 Pin 5 |
| 8 | ROW 4 (PB4) | CIA#1 Pin 14 | 21 | COL 4 (PA4) | CIA#1 Pin 6 |
| 9 | ROW 7 (PB7) | CIA#1 Pin 17 | 22 | COL 5 (PA5) | CIA#1 Pin 7 |
| 10–13 | NC | Unconnected | 23 | COL 6 (PA6) | CIA#1 Pin 8 |
| 25 | GND (Key Return) | Restore / LED Ground | 24 | COL 7 (PA7) | CIA#1 Pin 9 |
Bench Testing Without the Keyboard: You can type commands on a bare SX-64 chassis by momentarily bridging the appropriate `COL` pin (Pins 17–24) to a `ROW` pin (Pins 5–9, 14–16) on the front DB25 socket via a 470 Ω resistor!
Rear External Monitor A/V 8-Pin DIN (262°)
Note that the SX-64 has NO RF TV modulator — it outputs clean Composite Video, separated S-Video (Luma/Chroma), and SID Audio on the rear 8-pin horseshoe DIN socket:
| DIN Pin | Signal | Notes |
|---|---|---|
| 1 | Luminance / Sync (Y) | S-Video Y output (75 Ω) |
| 2 | GND | Signal ground |
| 3 | Audio Out (SID) | Line-level mono audio from 6581 SID |
| 4 | Composite Video Out | Combined CVBS for RCA/SCART monitors |
| 5 | Audio In | External input to SID filter |
| 6 | Chrominance (C) | S-Video C output (add 300 Ω series resistor for modern LCDs) |
| 7, 8 | NC | Unconnected |
Rear User Port (24-Pin Edge) & IEC Serial (6-Pin DIN)
The rear 24-pin User Port (0.156″ pitch) and 6-pin DIN IEC serial bus are electrically identical to the desktop C64, EXCEPT Cassette Motor (+6V) is absent and the internal 1541 drive is permanently daisy-chained onto the IEC bus internally as Device 8.
Chip & EPROM Reference
Direct links to interactive IC pinout pages for every major chip on the SX-64 CPU, I/O, and 1541 FDD boards.
MOS 6510 CPU
CPU Board U1 — 1 MHz processor with 6-bit internal I/O port (`$00/$01`) controlling LORAM, HIRAM, and CHAREN banking.
View Pinout →906114-01 / 82S100 PLA
CPU Board U4 — Programmable Logic Array. #1 point of failure in the SX-64; replace with cool-running PLAnkton/GAL.
View Pinout →MOS 6567 / 6569 VIC-II
CPU Board U2 — Video Interface Chip driving both the internal 5″ colour CRT and rear 8-pin DIN monitor port.
View Pinout →MOS 6581 SID
I/O Board UB1 — 3-voice Sound Interface Device driving the internal front-panel speaker amp and rear A/V audio out.
View Pinout →MOS 6526 CIA (×2)
I/O Board UA1 & UA3 — Complex Interface Adapters handling the DB25 keyboard, joysticks, User Port, and IEC bus.
View Pinout →4164 DRAM (×8)
CPU Board U10–U17 — Single-rail +5 V 64K×1 dynamic RAM bank.
View Pinout →MOS 6502 (1541 FDD)
FDD Board — Dedicated 1 MHz CPU running Commodore DOS 2.6 on the internal floppy controller board.
View Pinout →MOS 6522 VIA (×2)
FDD Board UC2 & UC3 — Versatile Interface Adapters controlling the 1541 IEC serial handshake and Alps stepper/head logic.
View Pinout →SX-64 ROM Part Numbers & Dual-Kernal EPROM Upgrade Guide
All three ROM sockets on the SX-64 CPU board (U18, U19, U20) use the 24-pin 2364 mask ROM pinout (8 KB each), while the 1541 FDD board uses a 24-pin 8 KB (or 28-pin 16 KB) DOS ROM:
| Location | Commodore Part # | Size & Package | EPROM Substitute & Dual-Kernal Notes |
|---|---|---|---|
| U20 (SX-64 Kernal ROM) | 251104-04 (Rev 4) (or early 251104-01) |
8 KB (24-pin 2364) | MCM68764 / 68766 (direct 24-pin drop-in) or 27C64 / 27C128 / 27C256 via a 2364-to-27xxx adapter. Popular upgrade: fit a dual/triple-Kernal switcher with SX-64 Stock Kernal + JiffyDOS SX-64 + Standard C64 Kernal (901227-03)! |
| U18 (BASIC 2.0 ROM) | 901226-01 | 8 KB (24-pin 2364) | Identical to standard desktop C64 BASIC 2.0 ROM. |
| U19 (Character ROM) | 901225-01 | 4 KB (24-pin 2332) | Identical to standard desktop C64 Character generator ROM. |
| 1541 FDD DOS ROM | 901229-05 (or SX-specific JiffyDOS 1541) | 8 KB (24-pin) / 16 KB | Adding JiffyDOS to both U20 (CPU board) and the 1541 FDD board accelerates internal disk loads by 6–10× with zero external cables! |
Resources & Further Reading
Blog posts, interactive bench tools, and Commodore SX-64 schematics & service manuals.
Smiggy's Commodore Posts
Bench Tools & Machine Hubs
- IC Pinout Finder — 6510, 6581, VIC-II, 6526, 82S100 & more
- Recap Planner — capacitor replacement guide
- Ohm's Law Calculator
- C64 Machine Hub — Commodore 64 reference
- C128 Machine Hub — C128, C128D & DCR reference
- Amiga Machine Hub — A500/A600/A1200 reference
- BBC Micro Hub — Model B, B+ & Master 128 reference
- ZX Spectrum Hub — 16K/48K & +2/+3 reference
- Dragon 32/64 Hub — Dragon 32 & 64 reference
- Amstrad CPC 464 Hub — CPC 464 & Datacorder reference
- Atari ST Hub — 520ST, 1040STFM & STE reference
- Apple IIe Hub — Unenhanced, Enhanced & Platinum reference
- Sinclair QL Hub — 68008, Microdrive & ULA reference
- Acorn Atom & Electron Hub — Atom & Electron reference
- Osborne 1 & Kaypro II Hub — CP/M luggable reference
- Oric-1 & Atmos Hub — Oric 6502A & ULA reference
- Amstrad CPC 6128 Hub — 128K & 3″ disc reference
- MSX Machine Hub — MSX1, MSX2, MSX2+ & Turbo R reference
- Big-Box Amiga Hub — A1000, A2000, A3000 & A4000 reference
- Atari 8-Bit Hub — 400/800, 800XL & 130XE reference
- VIC-20 Machine Hub — Commodore VIC-20 reference
External SX-64 Resources
- Zimmers.net SX-64 Schematics — official Commodore SAMS Computerfacts & PCB diagrams
- C64-Wiki: Commodore SX-64 — history, hardware specs, and Kernal ROM differences
- Ray Carlsen's SX-64 Repair Guide — multi-board fault symptoms & keyboard repair notes
- Lemon64 Hardware Forum — active SX-64 CRT & TFT mod discussions