Sinclair
Interactive Flowchart ZX Spectrum 128K / +2 / +3
128K Toastrack, +2 Grey (Issue 3) & +2A/+3 (Z70830) — Sinclair / Amstrad 128K AY-3-8912 mainboards
Complete Diagnostic Flowchart: ZX Spectrum 48K Triple-Rail (-5V/+12V/+5V) & ULA / 4116 RAM Isolation (Sinclair ZX Spectrum 16K / 48K (Issue 2 / Issue 3 / 3B))
STEP 1: Measure 4116 Lower RAM Triple Rails: +5V (Pin 9), +12V (Pin 8) & -5V (Pin 1)
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Verify 9V DC CENTRE-NEGATIVE PSU polarity before powering on. Place black multimeter probe on the 7805 heatsink tab (0V GND). Probe any 4116 Lower RAM chip (IC6–IC13): check Pin 9 (+5V), Pin 8 (+12V), and Pin 1 (-5V).
Expected Reading: Pin 9 = +4.85V to +5.15V; Pin 8 = +11.4V to +12.6V; Pin 1 = -4.5V to -5.5V.
STEP 2: Probe Z80A CPU (IC1) Pin 6 (3.5 MHz CLK) & Pin 27 (/M1 Opcode Fetch)
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Scope Z80A (IC1) Pin 6 (CLK driven by ULA Pin 32): verify a 3.5 MHz clock pulse. Scope Z80A Pin 26 (/RESET) for HIGH (+5V) and Pin 27 (/M1) for continuous active-low opcode fetch pulses.
Expected Reading: 3.5 MHz clock on Pin 6 and active-low /M1 pulses every instruction cycle on Pin 27.
[ISOLATED FAULT: RES-ZX-INVERTER] Fault Isolated: Failed TR4 (ZTX650) / TR5 (ZTX450) DC-DC Converter or Recapped C44/C47
VERDICT
The ZX Spectrum's internal coil inverter is not generating the +12V and -5V rails required by the 4116 lower RAM and LM1889 video encoder.
[ISOLATED FAULT: RES-ZX-4116] Fault Isolated: Faulty 4116 Lower RAM (IC6–IC13) — Colored Bars / Attribute Squares
VERDICT
All three rails and the Z80 clock/M1 are alive, so colored vertical bars or flashing attribute squares on screen indicate one or more dead 4116 lower RAM chips (IC6–IC13).
[ISOLATED FAULT: RES-ZX-ULA] Fault Isolated: Ferranti ULA (IC2 6C001E-6/7) or Z80A CPU Bus Contention
VERDICT
Missing 3.5 MHz CPU clock or dead video sync with valid +5V rail points to the Ferranti ULA (IC2) or oxidised ULA socket contacts.
Summary
Unlike the 48K Issue 2/3, the 128K/+2/+3 models use single-rail +5V 4164/41464 DRAM and an AY-3-8912 3-channel sound chip. On the Grey +2 and Toastrack, drying capacitors around the TEA2000 video encoder cause dark or jailbarred composite video.
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
Grey +2 is 9V Centre-Negative; Black +2A/+3 uses multi-rail 6-Pin DIN!
Never connect a 48K/+2 power supply to a +2A/+3 or vice versa.
Is This Your Board?
- Grey ZX Spectrum +2 uses PCB Issue 3 (single 9V DC centre-negative barrel jack, TEA2000 video IC).
- Black ZX Spectrum +2A and +3 use PCB Z70830 / Z70833 with a 6-pin DIN power supply (+5V, +12V, -12V) and gate-array ASIC.
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.
ZX Spectrum +2 Grey (Issue 3 Mainboard)
| Ref | Value | Voltage | Type | Function | Notes | Confidence |
|---|---|---|---|---|---|---|
C1 |
22 uF | 25 V | Axial | TEA2000 video decoupling | Verified | |
C4 |
100 uF | 16 V | Axial | +5V logic rail decoupling | Verified | |
C9 |
100 uF | 25 V | Axial | 9V input filter | Verified | |
C12 |
22 uF | 25 V | Axial | Reset timing capacitor | Verified | |
C26 |
47 uF | 16 V | Axial | +12V DC-DC converter output | Verified | |
C31 |
22 uF | 25 V | Axial | AY-3-8912 audio mix coupling | Verified | |
C32 |
22 uF | 25 V | Axial | Tape audio amplifier coupling | Verified | |
C47 |
100 uF | 16 V | Axial | +5V RAM bank filter | Verified |
ZX Spectrum +2 Grey (Issue 3 Mainboard) — shopping list
- 4 x 22 uF 25 V Axial
- 2 x 100 uF 16 V Axial
- 1 x 100 uF 25 V Axial
- 1 x 47 uF 16 V Axial
ZX Spectrum +2A / +3 Mainboard (Z70830 / Z70833)
| Ref | Value | Voltage | Type | Function | Notes | Confidence |
|---|---|---|---|---|---|---|
C1 |
100 uF | 16 V | Radial | +5V input filter | Verified | |
C2 |
47 uF | 25 V | Radial | +12V input filter | Verified | |
C9 |
10 uF | 25 V | Radial | Z80A /RESET timing | Verified | |
C15 |
22 uF | 25 V | Radial | AY-3-8912 audio out coupling | Verified | |
C18 |
100 uF | 16 V | Radial | TEA2000 / RGB encoder rail | Verified |
ZX Spectrum +2A / +3 Mainboard (Z70830 / Z70833) — shopping list
- 2 x 100 uF 16 V Radial
- 1 x 47 uF 25 V Radial
- 1 x 10 uF 25 V Radial
- 1 x 22 uF 25 V Radial
Whole-Machine Shopping List
- 4 x 22 uF 25 V Axial
- 2 x 100 uF 16 V Axial
- 1 x 100 uF 25 V Axial
- 1 x 47 uF 16 V Axial
- 2 x 100 uF 16 V Radial
- 1 x 47 uF 25 V Radial
- 1 x 10 uF 25 V Radial
- 1 x 22 uF 25 V Radial
Recap Tips
- On the ZX Spectrum +3, always replace the 3-inch floppy drive belt at the same time as recapping the mainboard.
Related Symptoms
- Muffled AY-3-8912 3-channel sound Dried-out 22 uF coupling capacitors around the AY-3-8912 mixing stage.
Sources & References
- Console5 TechWiki - ZX Spectrum 128 / +2 / +3 Per-issue capacitor schedules.