When the Spectrum 48K won't start
A black screen, lines, or random squares instead of the copyright message: the Spectrum 48K has a set of typical faults that can be diagnosed with a multimeter and a bit of patience.
Difficulty: WorkbenchTime: 1 h
⚠ Before you start
- If you have no experience taking measurements on a powered board, stick to looking and bring the machine to us.
Common symptoms
Black screen or border only: often there is no power (power supply, 5 V regulator) or the processor is not starting. Screen full of squares or coloured lines: usually memory or the ULA. It starts but the keyboard doesn't respond: the keyboard membrane or the ribbons that connect it.
The memory's three voltages
The first 16 KB of memory on the 48K use 4116 chips, which need +5 V, +12 V and −5 V. The +12 V and −5 V voltages are generated by a small circuit on the board itself. If those voltages are missing, the machine won't start and the memory chips can fail.
So, before replacing memory, it is worth measuring all three voltages. If any of them is missing, the problem is in the circuit that generates them, and replacing the memory first will only damage the new memory.
Diagnostic boards
There are modern diagnostic boards that plug into the expansion port and run tests on the memory, the ULA and the keyboard, showing which chip is suspect. They save many hours of measurements.
When to bring it to us
If the voltages are fine and it still won't start, the next step is to test chip by chip, and that requires a bench, replacement parts and, often, desoldering components.
Request a repairThese guides are for information. Working on old electronics carries risks: do it at your own risk, with the machine unplugged, and never open power supplies or CRT monitors. If in doubt, bring the machine to us.