Sinclair
Interactive Flowchart Sinclair QL
Issue 5 & Issue 6 Mainboard — 68008 Quantum Leap dual-ULA mainboard
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
The Sinclair QL carries an internal linear 7805 regulator mounted to the rear heatsink (fed by 9V DC and 15.6V AC from the custom 3-pin power brick) plus +12V and -12V rails for RS-232 ports. Drying axial capacitors cause Microdrive read errors and RAM parity crashes.
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
Sinclair QL SER1 / SER2 ports output +12V DC on Pin 6!
Never plug standard RS-232 devices into the QL's British BT-style serial sockets without checking Pin 6 (+12V out).
Is This Your Board?
- Look near the front-right corner by the Microdrive units for "ISSUE 5" or "ISSUE 6" silkscreened on the PCB.
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.
Sinclair QL Mainboard (Issue 5 / Issue 6)
| Ref | Value | Voltage | Type | Function | Notes | Confidence |
|---|---|---|---|---|---|---|
C8 |
10 uF | 25 V | Axial | 68008 /RESET timing | Verified | |
C11 |
100 uF | 25 V | Axial | +12V regulator filter | Verified | |
C12 |
100 uF | 25 V | Axial | -12V regulator filter | Verified | |
C14 |
220 uF | 16 V | Axial | +5V logic rail reservoir | Verified | |
C15 |
22 uF | 25 V | Axial | Microdrive motor / amplifier decoupling | Verified | |
C16 |
22 uF | 25 V | Axial | 8049 IPC audio coupling | Verified | |
C21 |
47 uF | 16 V | Axial | MC1377 RGB/PAL video encoder filter | Verified |
Sinclair QL Mainboard (Issue 5 / Issue 6) — shopping list
- 1 x 10 uF 25 V Axial
- 2 x 100 uF 25 V Axial
- 1 x 220 uF 16 V Axial
- 2 x 22 uF 25 V Axial
- 1 x 47 uF 16 V Axial
Recap Tips
- Replace the fragile brittle keyboard membranes while the upper lid is off.
Related Symptoms
- White/green/red stipple pattern freeze on power-on Indicates a failed 4164 DRAM chip in IC1-IC16 or oxidised ZX8301 ULA socket.
Sources & References
- Sinclair QL Service Manual (Issue 5) Official Sinclair Research component schedule.