ZX Spectrum 48K · Issue 2
The first production board of the 48K, redrawn in KiCad from photos of a real board and following the Sinclair routing, to help repair Spectrums. The author applied the known modifications (machine code, power supply, colour): some values differ from the factory ones.
- 190
- 28
- 51
- 896
Loading the board…
Drag to move; mouse wheel or two fingers to zoom. Tap a pad to follow the signal.
ZX Spectrum 48K Issue 2: the whole board, trace by trace in 3D
The video comes from YouTube (Google) and only loads when you click.
Guided diagnosis
Common faults and what to measure, step by step. Each step lights up the signal or chip it talks about on the board.
You power it on and the Sinclair message never appears. Start with the voltages: on a Spectrum, a missing rail can kill the memory chips.
Open this guide as a page →- +5VShow on board →
The 5 V rail
Measure at the output of the 7805 regulator: between 4.75 and 5.25 V. The regulator runs very hot in a Spectrum, which is normal; if the 5 V is low, switch off and look for a short.
- +12VShow on board →
The +12 V rail
The DC-DC converter (the two transistors and the coil next to them) makes the +12 V and −5 V the 4116 memory chips need. Measure the +12 V here.
- -5VShow on board →
The −5 V rail
If −5 V is missing while +12 V is present, the 4116s die. Always check it, and before powering a machine with new memory chips.
- _RESETShow on board →
Reset
At power-on, the Z80’s _RESET rises after a moment, while a capacitor charges. If it stays stuck low, the Z80 never starts.
- _CLKShow on board →
The clock
The ULA derives the Z80’s 3.5 MHz clock from the 14 MHz crystal. Without a clock nothing runs: check the crystal and the ULA.
- _M1Show on board →
The Z80 at work
_M1 pulses every time the Z80 fetches an instruction. If it pulses, the processor is alive; if not, go back to reset and the clock.
- IC5 · ROMShow on board →
The ROM
The ROM holds BASIC and the start-up code. If the Z80 is working but the Sinclair message never appears, suspect the ROM.
Didn't fix it?
We repair these boards on our bench in Loures. The request already includes the board and the fault.
About this data
Where it comes from
KiCad layout by liveboxandy, licensed CC0 1.0.
How it was worked out
Signal names and connections come from the KiCad project itself, redrawn from photos of a real board and the service manual schematic.
Before you solder
Always check with a multimeter before soldering a bodge wire. The layout may contain errors, and a repaired board may no longer match the factory one.
Rather not pick up the iron?
Traces eaten by battery acid or leaking capacitors are a common job on our bench.
Request a repair →