Sinclair QL Machine Hub
Motorola 68008 "Quantum Leap" workbench reference — Issue 5/6 boards, QDOS & Minerva ROMs, ZX8301/8302 ULA & 4164 RAM diagnostics, Microdrive overhaul, and 8-pin RGB SCART wiring.
3-Pin Power Supply (+9V DC / 15.6V AC) & ZX8301 ULA Thermal Warning
The Sinclair QL uses a proprietary 3-blade flat power plug delivering unregulated +9 V DC (at 1.8 A) AND 15.6 V AC (at 200 mA) into the motherboard. Inside the QL, a massive rear heatsink mounts a linear 7805 (+5 V) regulator while the 15.6 V AC tap is rectified on-board by a 7812 (+12 V) and 7912 (−12 V) for RS-232 and the PAL video encoder. Because the 7805 linear regulator dumps up to 10 W of heat into the rear of the case directly behind the un-heatsinked ZX8301 Master ULA (IC22), thermal failure of the ZX8301 and 4164 DRAMs is the #1 killer of un-modified QLs!
1 Rear 3-Pin Power Socket Pinout
Viewed looking at the rear of the Sinclair QL power socket (Left, Center, Right blades):
| Blade Position | Signal | Unloaded / Loaded Spec |
|---|---|---|
| Left Blade (Nearest SER1) | 15.6 V AC | 15.5–17.5 V AC (feeds internal D9/D10 → 7812/7912) |
| Center Blade (Offset Keyed) | 0 V GND | Common DC & AC Return Ground |
| Right Blade (Nearest Joystick) | +9 V DC Unreg | ~9.0–11.5 V DC @ 1.8 A (feeds main 7805 & Microdrive motors) |
Why 15.6V AC Must Be AC (Not DC!): The QL uses a half-wave positive and negative voltage rectifier off the 15.6 V AC pin to create both +12 V (needed by the MC1377 RGB/PAL encoder!) and −12 V (RS-232). Feeding DC into the AC pin leaves you with no −12 V or no composite video!
2 Verify Internal Regulator Rails
| Regulator | Location | Target Output | Feeds |
|---|---|---|---|
| IC30 (7805) | Rear Aluminium Bar | +5.00 V DC (±5%) | 68008, ZX8301, ZX8302, 8049 IPC, all sixteen 4164 DRAMs & ROMs |
| IC31 (7812) | TO-220 on PCB | +12.0 V DC | MC1377P video encoder & SER1/2 TX drivers (1488) |
| IC32 (79L12) | TO-92 on PCB | −12.0 V DC | SER1/SER2 RS-232 negative swing |
Note: The two internal Microdrive motors run directly off the unregulated +9 V DC input rail (switched via internal transistors) so spinning up a cartridge doesn't sag the +5 V logic rail.
3 Mandatory Thermal Upgrades
- Fit Stick-on Heatsinks to ZX8301 (IC22) & ZX8302 (IC23): The 40-pin Plessey/Ferranti ZX8301 (`CLA2310`) runs hot and is difficult to source. Attach a low-profile 40-pin aluminium heatsink with thermal tape immediately!
- Swap the 7805 for a Switching Buck Regulator: Replacing the linear TO-220 7805 (IC30) with a 2 A–3 A 5.0 V DC-DC buck module (e.g., Recom R-78B5.0-2.0 or EzSBC PSU regulator) eliminates ~8 W of waste heat inside the case. (Keep the rear metal bar in place for structural rigidity!).
Board Revisions, QDOS ROMs & Gold Card Expansions
Explore the 68008 + dual-ULA + 8049 IPC architecture, compare Build D8–D13 / Issue 5 / Issue 6 boards, and identify QDOS ROM revisions.
Motorola 68008 + Dual ULA + 8049 Architecture 1984–1986 • 128 KB RAM
Designed by David Karlin, Tony Tebby, and Richard Altwasser, the Sinclair QL pairs a Motorola MC68008P8 (full 32-bit 68000 internal registers and instruction set over an 8-bit external data bus and 20-bit 1 MB address bus running at 7.5 MHz) with two custom semi-custom gate arrays and an Intel 8049 microcontroller:
- IC18 (CPU): Motorola MC68008P8 @ 7.50 MHz (48-pin DIP)
- IC22 (Master Chip ULA): ZX8301 (`CLA2310`) — generates 512×256 (4-colour Mode 4) and 256×256 (8-colour Mode 8 + flash) RGB video, divides the 15 MHz crystal down to 7.5 MHz CPU clock, and controls DRAM refresh/multiplexing (resulting in contended RAM access for the 68008).
- IC23 (Peripheral Chip ULA): ZX8302 (`CLA2345`) — manages dual Microdrive read/write serialization, RS-232 transmit baud rate generators, QL-NET local area network, and the battery-able 32.768 kHz real-time clock.
- IC24 (IPC): Intel 8049 / 8749 Microcontroller (11 MHz crystal) — scans the 8×8 keyboard matrix, reads the two CTL1/CTL2 joystick ports, generates beeper/pitch audio, handles RS-232 receive framing, and talks to the ZX8302 over a synchronous serial link!
- IC1–IC16 (128K RAM): 16× 4164 (64K×1, +5 V single-rail) DRAMs organized in two 64 KB banks (`IC1–IC8` = lower 64K including screen RAM `$20000–$2FFFF`; `IC9–IC16` = upper 64K `$30000–$3FFFF`).
Motherboard Issues (Issue 4 / Issue 5 / Issue 6 & Build D8–D16)
Check the serial/build sticker on the underside (`D08` to `D16`) and the copper etching near the front-right of the PCB:
| Board Issue | Build Codes | Key Hardware Differences |
|---|---|---|
| Issue 4 / Early Issue 5 | D04 – D08 (Early 1984) |
Early production. Often originally shipped with the external "kludge/dongle" 16 KB ROM cartridge hanging out the back because the 48 KB QDOS code didn't fit in two 16K ROMs! Lots of factory bodge wires underneath. |
| Issue 5 (Main Production) | D09 – D13 (1984–85) |
Most common UK QL board. Fits a 32 KB mask ROM (`IC33`) + 16 KB mask ROM (`IC34`) = 48 KB internal QDOS (`AH`, `JM`, or `JS`). |
| Issue 6 (Late / Samsung) | D13 – D16 (1985–86) |
Cleanest factory layout (some assembled by Samsung in Korea). Improved Microdrive ground plane, fewer bodge wires, and factory-fitted HAL16L8 (`IC38`) replacing discrete decode glue logic. |
Why `IC38` (HAL16L8) Matters on Issue 6: Later Issue 5 and Issue 6 boards use a programmed 20-pin PAL/HAL at `IC38` for memory decoding. If `IC38` fails, it can be replaced with a modern programmed GAL16V8 using community JEDEC equations.
QDOS & SuperBASIC ROM Versions (`PRINT VER$` in SuperBASIC)
To check which ROM is installed in a running Sinclair QL, press F1 (Monitor) or F2 (TV) at the start screen and type PRINT VER$:
| ROM Code | Release Date | Stability, Bugs & Upgrade Advice |
|---|---|---|
| FB / PM | early 1984 | Pre-production 32 KB internal + 16 KB external rear dongle ROMs (`Feng Shui` / `Pauline`). Very buggy; keep for historical preservation only. |
| AH | June 1984 | First 48 KB internal ROM release (`IC33` 32K + `IC34` 16K). Workable, but still suffers from SuperBASIC `PROC`/`FN` parameter bugs and slow Microdrive formatting. |
| JM | Late 1984 | Major bug-fix release fitted to most UK Issue 5 machines. Good compatibility with early Psion 4-pack software (Quill, Abacus, Archive, Easel). |
| JS / JSU | 1985 (Final UK/US) | Best original Sinclair QDOS ROM. Adds `WHEN ERROR` trapping and faster graphics/serial routines (`MG` is the international localized equivalent). |
| Minerva (v1.89 / v1.98) | 1989–Present (QView / Laurence Reeves) | The ultimate 48 KB drop-in OS replacement for every QL! Completely rewritten, 100% compatible, 30–50% faster execution, supports dual independent screen buffers (`$20000` & `$28000`), multitasking SuperBASIC (`MultiBASIC`), built-in RAM test with visual error codes on boot, and rock-solid I/O! |
EPROM Socket Warning (`IC33` & `IC34`): Original Sinclair mask ROMs in `IC33` (32 KB) and `IC34` (16 KB) have non-JEDEC active-high Chip Select inputs! You cannot plug a bare 27C256 or 27C512 into `IC33` without a small QL ROM adapter board (or using the rear external ROM port) that inverts `/OE` and decodes the 48 KB (`$00000–$0BFFF`) range so `$0C000–$0FFFF` remains free for peripheral cards!
Miracle Systems Gold Card, Super Gold Card & Modern Accelerators
Because the internal 68008 bus is bottlenecked to 8 bits and contended by the ZX8301 ULA, plugging an accelerator/RAM card into the left-hand 64-pin Eurocard Expansion Slot transforms the QL into a workstation:
- Trump Card (Miracle Systems / Tetroid): Adds 768 KB uncontended RAM (896 KB total), built-in Toolkit II ROM, and a WD1772 3.5″ floppy disk controller.
- Gold Card (16 MHz Motorola 68000 + 2 MB RAM): Replaces the internal 68008 dynamically over the expansion bus with a true 16-bit 16 MHz 68000 CPU, 2 MB of zero-wait-state RAM, DD/HD 1.44 MB floppy controller, and battery-backed RTC (~4× real-world speed!).
- Super Gold Card (24 MHz Motorola 68EC020 + 4 MB RAM): Full 32-bit 68EC020 processor running at 24 MHz with 4 MB 32-bit RAM, parallel printer port, and HD floppy interface (~12× original QL speed!).
- QIMSI / QIMSI Gold (Stephanener / Community): Modern open-source plug-in expansion adding SDHC card storage (QXL.WIN containers), PS/2 mouse, digital video, and FPGA 68000 acceleration!
Expansion Slot Power Caution
When fitting a Trump Card, Gold Card, or Super Gold Card into the left-hand expansion bay:
- Never rely on the QL's original linear 7805 alone for a Gold Card! A Gold Card or Super Gold Card adds 600–900 mA of +5 V current draw (or uses its own onboard 7805 tapped off the +9 V bus). Ensure your external +9 V DC supply is healthy and upgrade the internal 7805 to a 3 A switching module.
- Clean the 64-pin male Eurocard connector pins inside the left expansion tunnel with DeoxIT before inserting any expansion card.
Microdrive (`MDV1_` / `MDV2_`) Overhaul & Modern SD Storage
Restoring the twin internal endless-loop tape drives (100 KB per cartridge) and upgrading to vDriveQL, QL-SD, or Tetroid CF/SD interfaces.
1 Cartridge Felt Pressure Pad Alert!
NEVER insert an un-inspected 1980s Microdrive cartridge into a working drive!
- Inside every Microdrive cartridge sits a tiny foam/felt pressure pad glued to a copper leaf spring that presses the 1.9 mm endless magnetic tape loop against the stereo read/write head.
- After 40 years, the original foam disintegrates into sticky dust. If you run the cartridge without a pad, there is zero head contact (`not found` / `bad or changed medium`) — or worse, sticky foam residue glues the moving tape loop to the capstan roller and snaps the tape in half!
- Replacement Pad Fix: Carefully pop off the top cover of the cartridge (do not touch the delicate Mylar tape loop!) and stick a replacement high-density wool felt pad (or split-thickness 8-track / Compact Cassette replacement felt pad, ~1.5 mm thick × 3 mm wide) onto the beryllium-copper spring using cyanoacrylate gel.
2 Drive Roller, Head & ULA 8302 Checks
- Rubber Capstan Drive Roller (`2 mm shaft`): The tiny brass-hubbed rubber pinch roller that pulls the tape at 76 cm/sec (looping 5 m of tape in ~7.5 seconds) perishes, hardens, or splits. If the roller slips or has a flat spot, replace it with a modern precision polyurethane/silicone Microdrive roller (press-fit onto the motor shaft to the exact factory height!).
- Clean the Read/Write Head & Opto Write-Protect Sensor: Gently swab the miniature 2-track tape head and brass roller with 99% IPA on a pointed cotton bud. Clean dust out of the U-shaped optical write-protect slotted sensor at the side of the drive bay.
- MDV1 vs MDV2 Swap Test: Both internal Microdrives share the same data/control ribbon to the ZX8302 ULA (`IC23`) and individual local Microdrive ULA (`2C210E`) mounted under each deck. If `DIR mdv1_` fails to spin the motor at all while `DIR mdv2_` spins, swap the two drives on the internal ribbon cable to isolate whether the deck motor/ULA or mainboard ZX8302 select clock is at fault.
3 Modern SD / Microdrive Emulators
- vDriveQL (Charles Ingley): Connects directly to the external right-hand Microdrive expansion bus (or mounts internally in place of MDV2) and emulates up to six hardware Microdrives (`MDV3_` to `MDV8_`) from `.MDV` image files on an SD card with 100% ZX8302 hardware compatibility!
- QL-SD (ROM-Slot SD Interface): Plugs straight into the rear ROM Cartridge slot (or internal ROM socket via Minerva) to provide fast native `WIN1_` / `SDC1_` hard disk image access at `$0C000` without using the left expansion slot!
- Tetroid TDI / Trump Card CF/SD: Plugs into the left 64-pin expansion port, combining 768 KB RAM, Toolkit II, and QubIDE / CompactFlash / SD mass storage.
Diagnostic Checklist & IC1–IC16 4164 DRAM Locator
Decoding the QL's power-on RAM test screen ("Tweety-Pie" pattern), isolating ZX8301/8302 ULA vs 8049 IPC faults, and replacing the keyboard membrane.
Power-On "Tweety-Pie" Screen & 128KB 4164 DRAM Chip Locator (IC1–IC16)
When powered on, QDOS tests all 128 KB of RAM by writing pseudo-random patterns directly through video memory (`$20000–$27FFF` inside Bank 0). On a healthy QL, you briefly see a colourful flickering tartan pattern ("Tweety-Pie") for ~2 seconds before the screen clears to the black/red/white F1 Monitor / F2 TV prompt. If the QL freezes on the tartan screen — or shows vertical stripes — a 4164 DRAM (`IC1–IC16`) or the ZX8301 ULA (`IC22`) has failed!
128KB 4164 DRAM Bank & Data Bit Map
| Data Bit | Lower 64K Bank 0 (`$20000–$2FFFF` • Screen + Sys) |
Upper 64K Bank 1 (`$30000–$3FFFF` • User RAM) |
Visual Symptom on Screen (Bank 0) |
|---|---|---|---|
| Bit 0 (D0) | IC1 | IC9 | Rightmost pixel of even/odd word corrupted; freezes during boot test |
| Bit 1 (D1) | IC2 | IC10 | Vertical pixel stripes every 8 pixels |
| Bit 2 (D2) | IC3 | IC11 | Vertical colour/flash stripes down the display |
| Bit 3 (D3) | IC4 | IC12 | Repeating vertical bars; QDOS hangs before F1/F2 screen |
| Bit 4 (D4) | IC5 | IC13 | Vertical stripes at bit position 4 |
| Bit 5 (D5) | IC6 | IC14 | Vertical stripes at bit position 5 |
| Bit 6 (D6) | IC7 | IC15 | Green/Red bit plane stripes across screen |
| Bit 7 (D7) | IC8 | IC16 | Leftmost MSB pixel stuck on/off down every column |
Diagnosing Bank 0 (`IC1–IC8`) vs Bank 1 (`IC9–IC16`)
- Hangs with vertical stripes on the tartan screen: Because the video screen and QDOS system variables live in Bank 0 (`IC1–IC8`), any failure in `IC1–IC8` prevents QDOS from reaching the F1/F2 prompt.
- Hangs on a solid White or Green screen (or reboots during test): Often indicates a faulty chip in Bank 1 (`IC9–IC16`) detected during the second half of the RAM check.
- Install a Minerva Diagnostic ROM: Unlike original Sinclair AH/JM/JS ROMs (which simply halt on a bad RAM byte), a Minerva ROM flashes the border and prints the exact failing memory address and data bit mask on the screen even if Bank 0 RAM is damaged!
- Check 74LS257 Address Multiplexers (`IC19` & `IC20`): If multiple bits fail across both banks, inspect the two 74LS257 DRAM row/column multiplexers next to the RAM array.
ZX8301 / ZX8302 ULA & 8049 IPC Troubleshooting
Pure black screen, F1/F2 screen appears but keyboard does nothing, or missing colours.
- Reseat All Socketed VLSI Chips First: The Sinclair QL uses notorious low-cost single-wipe sockets on `IC18` (68008), `IC22` (ZX8301), `IC23` (ZX8302), `IC24` (8049 IPC), and `IC33/34` (ROMs). Gently lever each chip up 1 mm, spray DeoxIT D5 on the pins, and press firmly back home — this cures a huge percentage of "dead" QLs!
- Total Black Screen (No Sync):
- Probe ZX8301 (`IC22`) Pin 15 (CSYNC) and MC68008 (`IC18`) Pin 35 (CLK, 7.5 MHz). Both are generated by the ZX8301 from the 15 MHz crystal (`X1`).
- If RGB works on the 8-pin DIN but Composite/RF is dead, check that +12 V from regulator `IC31` (7812) is reaching Pin 12 of the MC1377P video encoder (`IC27`)!
- Boots to F1/F2 Prompt, But Pressing F1/F2 Does Nothing:
- If the keyboard membrane is confirmed good (test by bridging the PCB connector pins directly), check the Intel 8049 IPC (`IC24`), its 11 MHz ceramic resonator/crystal (`X3`), and the `COMDATA` link between `IC24` Pin 1/27 and ZX8302 (`IC23`) Pin 26.
- Modern programmed AT89C2051 / 8749 Hermes IPC replacements drop directly into `IC24`!
Sinclair QL Keyboard Membrane Replacement
Every original 1984 Mitsumi/Sinclair QL Mylar keyboard membrane eventually cracks where the two ribbon tails bend into the motherboard Molex connectors (`J11` & `J12`).
- Gentle Top Case Removal: Undo the bottom case screws, flip the QL right-side up, and lift the front edge of the top keyboard shell up by 45° — reach in and slide the two fragile membrane tails straight up out of the 8-way PCB connectors (`J11` & `J12`).
- Unscrew the Metal Keyboard Backplate: Unlike the rubber-key Spectrum (which is glued to the top faceplate), the QL keyboard is held inside the top shell by small self-tapping screws and plastic clips! Support the keycaps from underneath while unscrewing so the 65 individual springs and plungers stay seated in their wells.
- Install a New Polycarbonate Membrane: Lay the new modern replacement membrane flat over the rubber actuators, refit the steel backing plate evenly, and clean the tin contacts inside `J11` and `J12` with IPA before reconnecting.
8-Pin DIN RGB Video, Non-Standard SER1/SER2 & QL-NET Pinouts
Wiring the QL's TTL RGB Monitor port to SCART/OSSC/RetroTINK and navigating Sinclair's non-standard RS-232 serial sockets.
8-Pin DIN RGB Monitor Port to SCART / Upscaler DIN 262° Horseshoe
The QL's 8-pin horseshoe DIN socket (`RGB MONITOR`) outputs TTL-level (0–5 V) R, G, B, and CSYNC directly from the ZX8301 ULA alongside PAL Composite Video from the MC1377P. To connect to an analog 0.7 V SCART TV, OSSC, or RetroTINK, you must fit series attenuation resistors on the RGB and Sync lines so colours don't clip and bloom!
| DIN-8 Pin | Sinclair QL Signal | Resistor Required | SCART Target Pin |
|---|---|---|---|
| Pin 1 | Composite PAL Video (75 Ω) | Direct (optional fallback) | Not used if wiring CSYNC on Pin 4 |
| Pin 2 | GND (0 V Shield) | Direct | SCART Pins 4, 5, 9, 13, 17, 18, 21 |
| Pin 3 | MONO Video (Luma+Sync) | — | Monochrome monitor input |
| Pin 4 | CSYNC (TTL Composite Sync) | 470 Ω – 680 Ω series | SCART Pin 20 (Sync In) |
| Pin 5 | VSYNC (TTL Vertical Sync) | — | Used for RGBtoHDMI / multisync only |
| Pin 6 | GREEN (TTL 5 V) | 330 Ω – 470 Ω series | SCART Pin 11 (Green In) |
| Pin 7 | RED (TTL 5 V) | 330 Ω – 470 Ω series | SCART Pin 15 (Red In) |
| Pin 8 | BLUE (TTL 5 V) | 330 Ω – 470 Ω series | SCART Pin 7 (Blue In) |
512-Pixel Overscan Note (`F1 Monitor` vs `F2 TV`): In F1 Monitor Mode (512×256), the QL outputs a very wide active line that gets clipped on the left and right edges of many 4:3 consumer CRT TVs! On an OSSC, RetroTINK 5X/4K, or RGBtoHDMI, F1 Monitor Mode displays all 85 character columns edge-to-edge in razor-sharp detail.
SER1 & SER2 RS-232C Pinouts BT630W (UK) vs DB9 (Export)
UK Sinclair QLs use 6-pin British Telecom modular jacks (`SER1` = DTE/Modem, `SER2` = DCE/Printer) with TX and RX swapped between the two ports! Samsung/US export QLs use 9-pin D-Sub sockets, which are NOT IBM PC AT DB9 standard (and carry +12 V DC on Pin 6!):
| Pin | UK 6-Pin BT Plug (`SER1` / `SER2`) | Export DB9 (`SER1` / `SER2`) | Warning / Function |
|---|---|---|---|
| 1 | GND (0 V) | GND (0 V) | Signal Ground |
| 2 | TXD (`SER1`) / RXD (`SER2`) | TXD (`SER1`) / RXD (`SER2`) | Transmit / Receive Data (swapped on SER2!) |
| 3 | RXD (`SER1`) / TXD (`SER2`) | RXD (`SER1`) / TXD (`SER2`) | Receive / Transmit Data |
| 4 | DTR (`SER1`) / CTS (`SER2`) | DTR (`SER1`) / CTS (`SER2`) | Hardware Handshake Out / In |
| 5 | CTS (`SER1`) / DTR (`SER2`) | CTS (`SER1`) / DTR (`SER2`) | Hardware Handshake In / Out |
| 6 | +12 V DC Output! | +12 V DC Output! | DANGER: Do NOT connect Pin 6 to a PC serial port! |
CTL1 & CTL2 Joystick Ports: Also use 6-pin BT plugs on UK QLs (mapped as extra keyboard keys: `CTL1` = Cursor Up/Down/Left/Right + Space; `CTL2` = F1/F2/F3/F4/Enter).
QL-NET Ports (2× 3.5 mm Mono Jacks): Tip = Active Data/Scout signal (`0 V / +5 V` via ZX8302), Sleeve = GND. Allows up to 64 QLs (and ZX Spectrums with Interface 1!) to network at 87.5 kbaud.
Chip & EPROM Reference
Interactive IC Pinout Finder cards plus Sinclair QL custom silicon (MC68008, ZX8301, ZX8302, 8049 IPC) and ROM adapter reference.
Motorola MC68008P8
IC18 — 7.5 MHz 16/32-bit 68000 core with an 8-bit external data bus (`D0–D7`) and 20-bit address bus (`A0–A19`, 1 MB range).
View 68000 Family Pinout →4164 DRAM (×16)
IC1–IC16 — Single-rail +5 V 64K×1 dynamic RAM organized into two 64 KB banks (128 KB total).
View Pinout →Sinclair QL Custom Silicon & CPU Reference
| Board Ref | Chip & Part Number | Key Pins & Modern Replacements |
|---|---|---|
| IC18 (CPU) | Motorola MC68008P8 / P10 (DIP-48) | Pin 14 = `V_CC (+5V)`, Pin 37 = `GND`, Pin 35 = `CLK (7.5MHz)`, Pin 32 = `/RESET`, Pin 31 = `/HALT`, Pin 30 = `/BERR`, Pin 29 = `/DTACK`. Direct drop-in: any 48-pin DIP MC68008P8 or P10. |
| IC22 (Master ULA) | Plessey ZX8301 (`CLA2310` / `CLA2311`) (DIP-40) | Generates 7.5 MHz CPU clock, DRAM `/RAS` & `/CAS0`/`/CAS1`, and TTL RGB + Sync. Modern CPLD replacement: Q68 / ZX8301 CPLD Replacement Module. |
| IC23 (Peripheral ULA) | Ferranti/Plessey ZX8302 (`CLA2345`) (DIP-40) | Handles Microdrive read/write tracks, RS-232 TX clocks, QL-NET, RTC (`32.768 kHz` crystal `X2`), and IPC serial link. |
| IC24 (IPC) | Intel 8049H / 8749H (DIP-40) | Mask-programmed 8049 microcontroller scanning keyboard & joysticks. Replace with a programmed 8749 EPROM MCU or the enhanced Hermes IPC (which fixes keyboard bounce and improves serial handshake!). |
| MDV1 / MDV2 ULA | Ferranti 2C210E (DIP-22 under each drive) | Analogue/digital read/write amplifier and motor switch on the underside of each Microdrive mechanism (shared part with ZX Interface 1!). |
QDOS ROM Sockets (`IC33` & `IC34`) & EPROM Conversion Guide
The Sinclair QL maps 48 KB of system ROM at `$00000–$0BFFF` across two 28-pin sockets: `IC33` (32 KB, `$00000–$07FFF`) and `IC34` (16 KB, `$08000–$0BFFF`), leaving the 16 KB window at `$0C000–$0FFFF` open for plug-in ROM cartridges and peripheral interfaces:
| Upgrade Method | Chips Used | Wiring / Adapter Details |
|---|---|---|
| Single 27C512 (64 KB) in `IC33` + Adapter | 1× 27C512 (with `IC34` left empty) | Program the 48 KB QDOS (`JS` or `Minerva`) image into `$0000–$BFFF` of the 27C512. Use a small 28-pin adapter PCB in `IC33` that uses a 74HCT00/02 to enable the EPROM only when `ROMOEH` is high AND `A15+A14` are NOT both high (preserving the `$0C000–$0FFFF` cartridge space!). |
| External Rear ROM Port Cartridge | 27C128 (16 KB) | The rear 30-way (`2×15`) ROM cartridge edge connector uses standard active-low `/ROMOEH` logic — allowing a standard 27C128 EPROM (Toolkit II, Diagnostic ROM, or Ice ROM) to map directly at `$0C000–$0FFFF`! |
Why You Must Preserve `$0C000–$0FFFF`: If you wire a 64 KB 27C512 into `IC33` without decoding out the top 16 KB (`$0C000–$0FFFF`), any external ROM cartridge, QL-SD, or disk interface mapped in the `$0C000` slot will cause a bus collision with the internal EPROM!
Resources & Further Reading
Blog posts, interactive bench tools, and essential Sinclair QL communities & archives.
Smiggy's Sinclair Posts
Bench Tools & Machine Hubs
- IC Pinout Finder — 68000, 4164 & more
- Recap Planner — capacitor replacement guide
- Ohm's Law Calculator
- C64 Machine Hub — Commodore 64 reference
- C128 Machine Hub — C128, C128D & DCR reference
- SX-64 Machine Hub — Executive Portable C64 reference
- Amiga Machine Hub — A500/A600/A1200 reference
- Atari ST Hub — 520ST, 1040STFM & STE reference
- Apple IIe Hub — Unenhanced, Enhanced & Platinum 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
- 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 Sinclair QL Resources
- QL Forum — the main active Sinclair QL hardware repair, QIMSI, and software community
- Dilwyn Jones's Sinclair QL Pages — the definitive archive of QL ROMs, manuals, schematics, and software
- Sinclair QL Service Manual — complete factory schematics, build standards, and fault flowcharts
- RWAP Software (SellMyRetro) — replacement QL membranes, Microdrive felt pads, and ROM adapters