ZX Spectrum Machine Hub
16K/48K (Issue 2–6) & +2/+3 workbench reference — 4116 DRAM voltage trap, membrane swaps, composite video mod, ULA/RAM diagnostics and edge connector pinouts.
4116 DRAM Voltage Trap — MEASURE ALL THREE RAILS BEFORE FITTING NEW RAM
The lower 16 KB of RAM (IC6–IC13) on every 16K/48K ZX Spectrum uses eight 4116 (16K×1) DRAM chips that require three separate power rails: −5 V, +12 V, and +5 V. If the −5 V rail is missing (usually due to a blown TR4/TR5 DC–DC converter transistor or open coil), every 4116 chip on the bus will instantly latch up and burn out the moment +12 V is present. Never insert replacement 4116 chips into sockets until you have verified all three rails with a multimeter!
1 4116 Pin Voltage Map
With the board powered on (and suspect 4116s removed if socketed), probe every lower-RAM socket (IC6–IC13) referenced to the 7805 heatsink ground tab:
| 4116 Pin | Signal | Target Voltage | Allowed Range |
|---|---|---|---|
| Pin 1 | VBB (−5 V) | −5.0 V DC | −4.5 V to −5.5 V |
| Pin 8 | VDD (+12 V) | +12.0 V DC | +11.4 V to +12.6 V |
| Pin 9 | VCC (+5 V) | +5.0 V DC | +4.75 V to +5.25 V |
| Pin 16 | VSS (GND) | 0.0 V | 0.00 V reference |
2 Step-by-Step Rail Check
- Check PSU polarity first: The original Sinclair jack is 9 V DC CENTRE-NEGATIVE (outer barrel = +9 V, centre pin = GND). Plugging in a modern guitar/router centre-positive supply destroys TR4, TR5, and the 7805 immediately.
- Measure Pin 9 (+5 V): Driven by the 7805 regulator. If dead or low, check the 9 V input, 7805 regulator, or a shorted 4116 pulling the rail down.
- Measure Pin 8 (+12 V): Generated by the onboard DC–DC inverter circuit (TR4 ZTX650, TR5 ZTX313, and coil). If missing, TR4 is almost certainly blown.
- Measure Pin 1 (−5 V): Derived from the −12 V winding via D11, R54, and 5.1 V zener D19. If Pin 1 reads 0 V while Pin 8 reads +12 V, stop immediately — that state kills 4116s in milliseconds.
3 Fixing the DC–DC Inverter & 4164 Mod
- TR4 & TR5 failure: TR4 (ZTX650) and TR5 (ZTX313 / BC184) form a self-oscillating step-up converter. If a 4116 shorts on +12 V, TR4 overheats and fails short, often taking TR5 with it. Always replace both together (ZTX651 is a rugged upgrade for TR4).
- Attention to pinout: ZTX650 has an E-B-C pinout — do not fit a standard BC337 (C-B-E) without twisting the legs and insulating them.
- Single-rail 4164 upgrade: When replacing dead 4116s, many engineers fit modified 4164 (+5 V only) DRAMs instead: isolate pin 1 and pin 8, bridge pin 8 to pin 9 (+5 V) and pin 9 to pin 1 (+5 V ties unused A7 high). This eliminates the −5 V/+12 V dependency on the lower RAM bank.
Model & Board Revision Selector
Select your ZX Spectrum board revision or model to view ULA versions, RAM architecture, and recap notes.
16K / 48K Issue 2 1982 – Early 1983 • Pale Blue / Grey Key Legend
The first mass-production board revision. Features two manual colour trimmer pots (VR1/VR2) for PAL chroma balance, a separate daughterboard or socketed upper 32 KB RAM pack on upgraded 16K units, and the early Ferranti 5C102E or 5C112E ULA. Known for the "dead cockroach" factory bodge transistor fitted upside-down on the CPU clock line.
Key ICs & Architecture
- CPU: Zilog / NEC Z80A @ 3.50 MHz (IC1)
- ULA: Ferranti 5C102E / 5C112E / early 6C001E-6 (IC2)
- ROM: 16 KB 23128 mask ROM (IC5)
- Lower 16K RAM: 8× 4116 DRAM (IC6–IC13) — triple rail
- Upper 32K RAM: 8× 4532 / 3732 (half-good 4164s, IC15–IC22) + 4× 74LS logic chips (IC23–IC26)
- Video Encoder: LM1889N PAL encoder + VR1/VR2 trimmers
- Regulator: 7805 linear regulator on aluminium bar heatsink
Issue 2 Electrolytic Capacitors & Quirks
| Ref | Value | Voltage | Notes |
|---|---|---|---|
| C25, C46 | 22 μF | 16 V | Audio /EAR & /MIC decoupling |
| C27 | 100 μF | 25 V | +9 V DC input smoothing |
| C34 | 100 μF | 16 V | +5 V rail bulk decoupling |
| C44, C45 | 1 μF / 4.7 μF | 63 V | DC–DC inverter & −5 V rail filter (critical!) |
| C47 | 22 μF | 16 V | +12 V rail smoothing (replace first if +12 V is noisy) |
| C65 | 100 μF | 10 V | Reset circuit timing capacitor |
Issue 2 vs Issue 3 EAR Port Quirk: Issue 2 reads bit 6 of port &FE differently depending on whether the MIC output bit (bit 3) is high or low. A few games (e.g. Abu Simbel Profanation) fail to respond to keyboard input on un-modified Issue 2 boards unless a diode/resistor fix is applied.
48K Issue 3 / 3B / 4A / 4B 1983 – 1984 • Most Common Rubber-Key Boards
Issue 3 and 3B represent the peak production run of the classic rubber-key 48K Spectrum. The manual colour adjustment trimmers from Issue 2 were removed in favour of fixed resistor networks, and the board layout was cleaned up. Issue 4A/4B introduced NEC or OKI upper RAM and subtle ULA revisions.
Key ICs & Architecture
- CPU: Z80A @ 3.50 MHz (IC1)
- ULA: Ferranti 6C001E-6 or 6C001E-7 (IC2)
- ROM: 16 KB mask ROM (IC5)
- Lower 16K RAM: 8× 4116 DRAM (IC6–IC13)
- Upper 32K RAM: 8× TI TMS4532-20NL3 or OKI M3732-20 (IC15–IC22)
- Upper RAM Multiplexers: 74LS157 (IC23, IC24), 74LS32 (IC25), 74LS00 (IC26)
- PAL Encoder: LM1889N (IC14) feeding Astec UM1233 RF modulator
Upper 32K "Half-Good" RAM Jumper Links
Sinclair saved money by buying 64Kbit (4164) DRAM chips that failed factory testing in either their lower or upper 32K half, badged as 4532-L3 or 4532-L4 (or OKI 3732-L / H). A solder link near IC23 selects which half is active:
| RAM Marking | Good Half | Issue 3/3B Link | 4164 Drop-in? |
|---|---|---|---|
| TMS4532-xxNL3 / M3732-L | Pin 9 = 0 | Link set to "L" (TI) | Yes — full 4164 works in either link position |
| TMS4532-xxNL4 / M3732-H | Pin 9 = 1 | Link set to "H" (OKI) | Yes — full 4164 works in either link position |
Bench Tip: You can replace any faulty 4532 or 3732 in the upper bank (IC15–IC22) directly with a standard 4164 DRAM without changing any jumpers, because both 32K halves of a healthy 4164 are valid!
48K / Spectrum+ Issue 5 & Issue 6A 1984 – 1986 • Late Rubber-Key & Spectrum+
Found in late rubber-key 48K machines and the hard-key ZX Spectrum+ (which also adds a dedicated reset button wired to the expansion/reset pads). Issue 5 and 6A replace the four 74LSxx glue logic chips (IC23–IC26) with a single Mullard/Philips PCF1306P (or ZX8401) RAM controller IC.
Key ICs & Architecture
- CPU: Z80A @ 3.50 MHz (IC1)
- ULA: Ferranti 6C001E-7 (IC2) — runs slightly cooler than 5C series
- Upper RAM Logic: PCF1306P / ZX8401 custom gate array (IC27) replacing IC23–IC26
- Lower 16K RAM: 8× 4116 DRAM (still triple-rail!)
- Upper 32K RAM: 8× 4532 or 4164 DRAM (IC15–IC22)
- Video Encoder: LM1889N + improved transistor buffer before modulator
Issue 5 / 6A Recapping & Reliability Notes
| Component | Spec | Failure Mode |
|---|---|---|
| Axial Electrolytics (Blue Philips) | 22 μF, 100 μF, 1 μF | Dry out badly after 40 years; cause jailbars, failure to reset (C65), or unstable +12 V/−5 V rails. Always do a full recap. |
| PCF1306P (IC27) | DIP-20 custom gate array | When it fails, basic 16K diagnostics pass (`PRINT PEEK 23733` returns 127 instead of 255). Modern CPLD/discrete replacements exist. |
| 7805 Regulator | TO-220 +5 V 1.5 A | Runs hot; swap for a drop-in switching buck replacement (e.g., Traco TSR 1-2450 or Recom R-78E5.0-1.0) to cut case heat in half. |
Switching Regulator Note: Fitting a modern 3-pin switching 7805 replacement drops internal temperatures by ~15 °C, vastly extending the life of the Ferranti ULA and TR4/TR5.
ZX Spectrum +2 (Grey) vs +2A / +2B / +3 (Black) 1986 – 1992 • Amstrad Era
After Amstrad bought Sinclair in 1986, two distinct hardware families were sold under the +2/+3 styling. The Grey +2 (Issue 3, Z70500) is essentially a Toastrack 128K with a built-in Datacorder using a single +9 V DC supply. The Black +2A/+2B and +3 (Z70830/Z70833) completely redesigned the motherboard around a single 40/64-pin gate array and require a multi-rail DIN power supply (+5 V, +12 V, −12 V).
Key ICs & Architecture
- CPU: Z80A @ 3.5469 MHz
- Grey +2 ULA: Ferranti 7K010E-5 (40-pin DIP) + TEA2000 video encoder
- Black +2A/+3 ASIC: Amstrad Gate Array 40077 (QFP-64) — integrates ULA and memory paging
- Sound: GI AY-3-8912 / Yamaha YM2149F 3-channel PSG
- RAM (Grey +2): 16× 4164 (8 per 64K bank) — +5 V single-rail only!
- RAM (Black +2A/+3): 4× 41464 (64K×4) DRAMs
- +3 Floppy Controller: NEC μPD765A + SED9420 data separator (3″ drive)
+2 / +3 Common Bench Issues
| Model | Area | Fault & Fix |
|---|---|---|
| Grey +2 | TEA2000 Video | TEA2000 encoder runs hot and dies, losing composite/RF while RGB works. Fit a heatsink or use RGB SCART. |
| Grey +2 | Audio / Tape | Drive belt stretches (0.8–1.0 mm square belt); azimuth drifts; record/playback slide switch oxidises (clean with DeoxIT). |
| Black +2A/+3 | RGB Sync | Pin 1 of the 8-pin DIN outputs composite video on +2A/+3 Only if jumper LK4/LK2 is moved, otherwise it is raw TTL CSYNC. |
| +3 Disc Drive | 3″ Belt & +12 V | 3″ Mitsumi/Hitachi drive belt perishes into black tar — clean pulleys with IPA and fit a fresh 70 mm×2.8 mm flat belt. |
No 4116 Trap on 128K/+2/+3: All 128K, +2, and +3 models use single-rail +5 V DRAM (4164 or 41464). However, never plug a Black +2A/+3 PSU into anything else, and inspect the PCB for burnt tracks near the power switch.
Keyboard Membrane Replacement & Checklist
How to replace brittle original Mylar tails and diagnose dead rows or columns on the 5×8 keyboard matrix.
1 Removal & Faceplate Prep
- Undo the casing screws underneath and lift the top shell carefully — do not yank it up or you will rip the fragile ribbon tails out of connectors KB1 (5-way) and KB2 (8-way).
- Slide the two ribbon tails straight upward out of the white Molex ZIF/friction connectors on the PCB.
- For rubber-key 16K/48K models, gently pry up a corner of the aluminium keyboard faceplate (Wait until warmed gently with a hairdryer to soften the 40-year-old contact adhesive — never bend the aluminium plate or it will permanently crease!).
- Lift out the grey silicone rubber keymat, wash it in warm soapy water, and dry completely before reassembly.
2 Physical Inspection Checklist
- Inspect KB1 & KB2 Molex fingers: Look inside both white PCB connectors with a magnifier. Tarnished or flattened tin contacts cause intermittent dead rows/columns even with a brand-new membrane. Clean with IPA on a strip of stiff card.
- Trimmed tail Rescue (emergency only): If an original tail has cracked right at the connector mouth and 3 mm of clean silver track remains, you can trim 2 mm squarely off the end with scissors as a temporary test — though a modern polycarbonate replacement membrane is strongly recommended.
- Verify ULA keyboard matrix diodes (D1–D8): Check the eight 1N4148 signal diodes next to the CPU/ULA. A shorted or open diode kills an entire half-row of 5 keys.
- Check 10 kΩ pull-up array (RP1): Pins 18, 19, 20, 29, 30 of the ULA (D0–D4 keyboard sense lines) are pulled to +5 V via a 10 kΩ resistor pack. If one key column appears permanently held down at boot, test RP1.
3 Keyboard Matrix Pin Map (KB1 & KB2)
You can test whether a dead key is caused by the membrane or the ULA/CPU by momentarily bridging one pin of KB1 (5-pin data lines D0–D4) to one pin of KB2 (8-pin address lines A8–A15) via a 680 Ω resistor while at the BASIC prompt:
| KB2 (8-way) | Address Line | Keys (KB1 Pin 1 → Pin 5) |
|---|---|---|
| Pin 1 | A11 | 1, 2, 3, 4, 5 |
| Pin 2 | A10 | Q, W, E, R, T |
| Pin 3 | A9 | A, S, D, F, G |
| Pin 4 | A12 | 0, 9, 8, 7, 6 |
| Pin 5 | A13 | P, O, I, U, Y |
| Pin 6 | A8 | CAPS SHIFT, Z, X, C, V |
| Pin 7 | A14 | ENTER, L, K, J, H |
| Pin 8 | A15 | SPACE, SYM SHIFT, M, N, B |
Simple Composite Video Mod (16K / 48K / +)
Bypass the Astec UM1233 RF modulator for crisp, plug-and-play composite video through the existing RCA phono socket.
Schematic BC547 Emitter-Follower Buffer Mod
While you can connect the disconnect wire straight to the RCA jack, using an NPN emitter-follower buffer (BC547 / 2N3904) + decoupling capacitor protects the LM1889 video encoder, eliminates dark/crushed levels on 75 Ω modern TVs, and avoids vertical ghosting bars.
Inside Astec UM1233 RF Modulator Can:
=======================================================
+5V Rail o--------------------+
(former RF +5V feed) |
| Collector (C)
|/
Comp Video In o--------------| BC547B / 2N3904 (NPN)
(from LM1889) |\
| Emitter (E)
+----||----+----o RCA Centre Pin
| 100uF | (Video Out)
[ ] 16V + |
75R [ ] |
(or 68R) | |
| |
Modulator Case GND o-----------+----------+----o RCA Outer Shield
=======================================================
Why the 100 μF series capacitor matters: Modern LCD/OLED TVs and upscalers terminate composite inputs to ground via 75 Ω and expect an AC-coupled signal. Adding a 100 μF (positive leg toward the transistor emitter) prevents DC offset clipping.
Step-by-Step Installation
- Open the RF modulator lid: Pry off the top metal cover of the silver Astec UM1233 modulator box on the back-left corner of the PCB.
- Disconnect the +5 V RF feed: Two wires enter the bottom-left of the modulator box from the motherboard: the left wire is +5 V power to the RF oscillator, and the right wire is Composite Video In. Desolder the +5 V wire from the modulator PCB pad so the RF circuit is powered down (stops RF interference!).
- Disconnect the RCA centre pin from the RF coil: Snip or desolder the tiny fine copper inductor wire/resistor connecting the internal RF circuit to the centre pin of the phono jack. Verify with a multimeter in continuity mode that the RCA centre pin is no longer shorted to GND (the stock RF choke reads ~0.5 Ω to ground!).
- Wire the transistor buffer:
- Solder the Collector of a BC547B (or 2N3904) to the disconnected +5 V feed wire.
- Solder the Base to the Video In pad (where the right-hand motherboard wire enters).
- Solder a 68 Ω or 75 Ω resistor from the Emitter to the metal modulator case (GND).
- Solder the positive (+) leg of a 100 μF 16 V electrolytic capacitor to the Emitter, and its negative (−) leg to the RCA centre pin.
- Test & close up: Connect a standard yellow RCA composite cable to your TV or upscaler. Once confirmed sharp and stable, snap the metal lid back onto the modulator can. Completely reversible!
Common Fault Checklist
Symptom-by-symptom diagnostics for Ferranti ULA failures, lower/upper RAM faults, and boot screen patterns.
Ferranti ULA Failure Symptoms
The Ferranti ULA (Uncommitted Logic Array, IC2) handles video generation, dynamic RAM refresh/multiplexing, keyboard scanning, tape I/O, beeper audio, and the CPU clock.
Symptoms & Isolation Steps
- Thermal check: A healthy Ferranti ULA runs noticeably warm (~45–55 °C). If it gets scalding hot within 5 seconds of power-on (or stays stone cold), it has likely suffered an internal short or open bond wire.
- No CPU Clock (Pin 32): The ULA divides the 14 MHz master crystal down to 3.5 MHz for Z80 pin 6. Check ULA pin 32 with a scope or frequency counter. Check TR3 (buffer transistor) if clock is present at the ULA but missing at the Z80.
- White border with black paper & repeating vertical stripes: Often caused by dead lower RAM or CPU, not necessarily a dead ULA — the fact that you have a valid TV sync and border means the ULA's video counter section is alive!
- No sync / rolling black screen: Check ULA pin 15 (Video Out) and LM1889 power (+12 V on LM1889 pin 14 — remember: if the +12 V inverter is dead, composite/RF video dies too!).
- Modern ULA Replacements: vLA82, Nebula, Retroleumitus / Harlequin ULA replacements drop straight into the 40-pin socket and run cool.
RAM Testing Routines & Bit Mapping
Because screen memory ($4000–$5AFF) lives inside the lower 16 KB bank (IC6–IC13), visual artifacts on the boot screen immediately point to which specific 4116 chip has failed.
Bit-to-IC Mapping & BASIC Tests
| Data Bit | Lower 16K (4116) | Upper 32K (4532/4164) | Screen Attribute Symptom if Stuck |
|---|---|---|---|
| Bit 0 (D0) | IC6 | IC15 | Blue component of INK stuck on/off; 1px vertical lines |
| Bit 1 (D1) | IC7 | IC16 | Red component of INK stuck on/off |
| Bit 2 (D2) | IC8 | IC17 | Green component of INK stuck on/off |
| Bit 3 (D3) | IC9 | IC18 | Blue component of PAPER stuck on/off |
| Bit 4 (D4) | IC10 | IC19 | Red component of PAPER stuck on/off |
| Bit 5 (D5) | IC11 | IC20 | Green component of PAPER stuck on/off |
| Bit 6 (D6) | IC12 | IC21 | BRIGHT attribute stuck on in random blocks |
| Bit 7 (D7) | IC13 | IC22 | FLASHing attribute squares across the screen |
Quick BASIC RAM Top Check: If the machine boots to `© 1982 Sinclair Research Ltd`, type:
PRINT PEEK 23732 + 256 * PEEK 23733
• 65535 = Full 48 KB passed boot test.
• 32767 = Only 16 KB detected (Upper 32K RAM IC15–IC22 or multiplexer IC23–IC27 fault).
• Anything between 32768 and 65534 = Subtract (PEEK 23732 + 256 * PEEK 23733 + 1) from the value in that failing address via PEEK to find the exact stuck bit!
Black Screen vs. Border-Only Hang
Distinguishing power/video loss from a bus crash before the ROM clears the screen.
- Pure black screen (no sync at all): Verify +5 V and +12 V rails first. Without +12 V, the LM1889 video encoder cannot output composite or RF even if the CPU and ULA are running fine.
- Check Z80 /RESET (Pin 26): Should pulse low (0 V) briefly at power-on, then rise cleanly to +5 V. If stuck low, capacitor C65 (100 μF) or transistor TR6 has failed.
- Check Z80 /WAIT (Pin 24), /INT (Pin 16), /NMI (Pin 17), /BUSRQ (Pin 25): All must sit high (~4.5–5 V). If /WAIT is pulled low, the ULA is stalling the CPU indefinitely.
- Use a Diagnostic ROM Cartridge: Plugging an external diagnostic board (e.g., Retroleum DiagROM, Brendan Alford's ZX Diagnostics, or a Dandanator with diag ROM) into the rear edge connector overrides a dead internal ROM (IC5) and flashes border colour codes + beeps to pinpoint the exact dead 4116 chip even when all 8 are corrupt.
Dead Beeper / Tape Loading Errors
R Tape loading error, 0:1 or silent internal piezo/moving-coil speaker.
- Silent internal speaker: The 40 Ω miniature loudspeaker on 16K/48K boards is driven from ULA pin 28 via transistor TR7 (ZTX313 / BC184) and diode D9. Open-circuit voice coils are very common — measure across the speaker terminals with an ohmmeter (~32–40 Ω is normal; ∞ = dead speaker).
- Tape won't load (no cyan/red loading stripes): Inspect the 3.5 mm EAR socket contacts for corrosion or cracked solder joints on the PCB, and check coupling capacitor C25 (22 μF) and diode D13 into ULA pin 28.
- Modern TZXDuino / Smartphone levels: The Spectrum EAR input expects a comparatively high signal swing (~1–2 V p-p). Many EU volume-capped phones cannot drive it directly without a small inline headphone amplifier.
ZX Spectrum IC Pinouts
Click any chip to view the full interactive pinout diagram and signal reference in the IC Pinout Finder.
Zilog Z80A CPU
8-bit microprocessor running at 3.50 MHz (48K) or 3.5469 MHz (128K/+2/+3). Drives all system addressing, I/O, and refresh.
View Pinout →4116 DRAM (16K×1)
Lower 16 KB contended RAM (IC6–IC13). Triple-rail (−5 V, +12 V, +5 V) dynamic RAM — most common failure on 48K boards.
View Pinout →4164 / 4532 DRAM
Single-rail (+5 V) 64K×1 DRAM used in 128K/+2 boards, upper 32 KB replacements, and modified lower-RAM conversions.
View Pinout →AY-3-8912 / 8910 PSG
3-channel programmable sound generator and I/O controller fitted to the ZX Spectrum 128K, +2, +2A, and +3.
View Pinout →Expansion Edge Connector & Video Pinouts
28-way (56-contact) rear expansion bus for 16K/48K/128K/+2 and 8-pin DIN RGB video socket (+2/+3).
28-Way Expansion Edge Connector 0.1″ Pitch • Viewed from Rear of Machine
Pin 1 is on the left when looking at the rear of the ZX Spectrum. Position 5 contains the polarising key slot between pins 4 and 6. Top side (A) faces up; bottom side (B) faces the desk. Always power off before inserting or removing any peripheral!
| Pin | Top Side (A) — Component Side | Bottom Side (B) — Solder Side | Notes |
|---|---|---|---|
| 1 | A15 | A14 | Address bus MSB |
| 2 | A13 | A12 | Address bus |
| 3 | D7 | +5 V DC | Regulated +5 V rail on 3B |
| 4 | NC (or +9V on +2A/+3) | +9 V DC Unregulated | Input rail before 7805 (48K/+2) |
| 5 | KEYWAY SLOT | KEYWAY SLOT | Plastic polarising key |
| 6 | D0 | GND (0 V) | Data bit 0 / System Ground |
| 7 | D1 | GND (0 V) | Data bit 1 / System Ground |
| 8 | D2 | /CLK (3.5 MHz CPU Clock) | Z80 clock signal on 8B |
| 9 | D6 | A0 | Data bit 6 / Address bit 0 |
| 10 | D5 | A1 | Data bit 5 / Address bit 1 |
| 11 | D3 | A2 | Data bit 3 / Address bit 2 |
| 12 | D4 | A3 | Data bit 4 / Address bit 3 |
| 13 | /INT | /IORQGE (ULA I/O Disable) | Pull 13B high to override internal ULA I/O |
| 14 | /NMI | GND (0 V) | Non-maskable interrupt (used by Multiface) |
| 15 | /HALT | VIDEO (or NC on +2A/+3) | Composite video out on 48K Issue 2–6 |
| 16 | /MREQ | /Y (Luminance) / NC | Memory Request active-low |
| 17 | /IORQ | V (Chroma) / NC | I/O Request active-low |
| 18 | /RD | U (Chroma) / NC | Read strobe active-low |
| 19 | /WR | /BUSRQ | Write strobe / DMA Bus Request |
| 20 | −5 V DC (48K only) | /RESET | Pull 20B to GND to reset CPU |
| 21 | /WAIT | A7 | CPU Wait line / Address bit 7 |
| 22 | +12 V DC | A6 | +12 V DC inverter rail (low current!) |
| 23 | −12 V AC/DC (48K) | A5 | Unregulated negative inverter rail |
| 24 | /M1 | A4 | Machine Cycle 1 (opcode fetch) |
| 25 | /RFSH | /ROMCS (ROM Disable) | Pull 25B high (+5 V via diode) to disable internal ROM |
| 26 | A8 | /BUSACK | Address bit 8 / DMA Bus Acknowledge |
| 27 | A10 | A9 | Address bus |
| 28 | NC | A11 | Address bit 11 |
Note on Pin 25B (/ROMCS): Diagnostic cartridges, Interface 1, and DivMMC interfaces pull pin 25B high toward +5 V to disable the internal 16K ROM (IC5) and map their own external EEPROM/Flash into $0000–$3FFF. On +2A/+3 models, two separate ROMCS lines exist (pins 18B and 25B).
128K / +2 / +3 RGB Video 8-Pin DIN (262°)
All 128K, +2, +2A, and +3 models feature an 8-pin horseshoe DIN socket outputting sharp RGB video alongside mono audio.
| Pin | Signal | SCART Wiring Note |
|---|---|---|
| 1 | CSYNC / Composite | To SCART Pin 20 (via 330 Ω if TTL sync on +2A/+3) |
| 2 | GND | Signal ground (SCART pins 4, 5, 9, 13, 17, 18, 21) |
| 3 | BRIGHT | TTL brightness bit (combine into R/G/B via diodes/resistors) |
| 4 | +12 V DC Output | Can feed SCART Pin 8 (aspect) & Pin 16 (RGB blanking via resistor) |
| 5 | +5 V (or VSYNC) | +5 V on +2A/+3; VSYNC on 128K Toastrack |
| 6 | GREEN | To SCART Pin 11 via resistor network |
| 7 | RED | To SCART Pin 15 via resistor network |
| 8 | BLUE | To SCART Pin 7 via resistor network |
Grey +2 vs Black +2A/+3 Cable Difference: Because the Grey +2 and Black +2A/+3 have slightly different sync and BRIGHT output levels, buy or build an RGB SCART cable equipped with passive BRIGHT mixing diodes so high-intensity colours display accurately on both.
Resources & Further Reading
Blog posts, bench tools, and essential ZX Spectrum hardware references.
Smiggy's ZX Spectrum Posts
Bench Tools & Machine Hubs
- IC Pinout Finder — Z80, 4116, 4164, AY-3-8910 & more
- Recap Planner — capacitor replacement guide
- Ohm's Law Calculator
- C64 Machine Hub — Commodore 64 reference
- Amiga Machine Hub — A500/A600/A1200 reference
- BBC Micro Hub — Model B, B+ & Master 128 reference
- Dragon 32/64 Hub — Dragon 32 & 64 reference
External Resources
- Spectrum For Everyone — technical articles, schematics, and board issue guides
- Retroleum Repair Guides — Phil Ruston's definitive Spectrum fault-finding notes
- World of Spectrum — software, manuals, and hardware documentation archive
- ZX Diagnostics (GitHub) — open-source diagnostic ROM for testing RAM & ULA
- Break Into Program — Z80 assembly & Spectrum architecture deep-dives