Sinclair ZX Spectrum Power Supply Repair

A Sinclair "ZX Power Supply", the European Euro 1400 version, rated 220 V AC in and 9 V DC at 1.4 A out

The Sinclair ZX Spectrum (16K and 48K) and the Sinclair ZX Spectrum+ run from an external, unregulated power pack: Sinclair's UK1400, rated 1.4 A, with a mains transformer, a full-wave rectifier and capacitive smoothing, and a thermal fuse at the transformer input. The Spectrum+ was sold with a pack in a different case, but the packs are interchangeable between the 16K, 48K and Spectrum+; they are not suitable for the Spectrum 128 or the grey +2, which draw more current, even though the plug fits. Most failures are in the lead and plug; a short in the lead can also take out the thermal fuse and the rectifier diodes.

Symptoms

• The machine is dead or resets when the lead is moved. Worsley says the pack suffers mainly from wires snapping or going intermittent inside the moulded plug or where the lead leaves the box.
• No output at all after a lead fault: a bare wire shorting the output blows the thermal fuse, the rectifiers (1N4007), or both. Television magazine's 1986 repair column saw several packs whose leads had gone short-circuit and whose diodes had caught fire and burnt the panel.
• The Spectrum misbehaves although the pack gives a voltage: check the voltage at the board under load (below), and the power socket on the Spectrum.

Safety

The pack's primary side is at mains voltage. Unplug it from the mains and from the computer before opening it.

• In 1983 Sinclair recalled some early power packs because mains voltage could appear on the outer, positive part of the DC barrel plug. PopeyMon Games reproduces the recall notice from Personal Computer News of March 1983, which lists the affected units; packs checked after the recall may carry a sticker underneath.
• Have an old pack electrically safety tested (PAT) before use, and replace an old or damaged mains plug with a modern fused plug; Adam's restorations fit a 3 A fused plug.
• The output is centre negative, outer positive: the opposite of most modern adaptors. Many "replacement" supplies are centre positive, and getting this wrong can seriously damage the Spectrum.

Specification

UK1400 figures

Item	Figure
Rating	9 V DC nominal, 1.4 A maximum
Off load	About 12.5 V (PopeyMon), 13.5 V (Retroleum) or 15 V (Adam's restoration); all normal for an unregulated pack
Sinclair's design limits	At 215 V AC mains and 1.4 A, the trough must not fall below 7.0 V; at 265 V AC and 600 mA, the peak must be below 14 V (13 V in the 1984 Thorn EMI edition)
At the Spectrum, under load	+9 V ±2.0 V at the positive side of C50; below +7 V the 7805 will not work correctly
Spectrum's current	About 500 mA to 700 mA (16K) and 700 mA to 900 mA (48K) on a 9 V bench supply
Connector	2.1 mm coaxial plug, centre negative

Tools and parts

• A multimeter.
• A replacement 2.1 mm plug, or a complete new lead.
• Silicone rubber to fill the plug cover, and a short length of protective sleeving for the lead exit.
• Rectifier diodes (1N4007) and, for a blown thermal fuse, a 160 mA slow-blow 20 mm fuse.

Procedure

Test the pack

1. With the pack plugged into the mains but not into the Spectrum, measure the output: expect well over 9 V, roughly 12.5 V to 15 V, because the pack is unregulated.
2. Check the polarity: the centre pin is negative.
3. Plug it into the Spectrum and measure at the positive side of C50: +9 V ±2.0 V.

Repair the plug

1. Cut off the old moulded plug and fit a replacement 2.1 mm plug, with the negative wire, the traced one, to the centre.
2. Fill the plug's plastic cover with silicone rubber before screwing it up. Worsley does this to stop internal shorts in the plug and to take the strain off the solder joints when the lead is pulled.

Television magazine's repairers simply replaced the whole lead, which they called a five-minute job.

Repair the lead where it leaves the box

1. Disconnect the pack from the mains and from the computer, then split the box.
2. Cut off the damaged section of lead and tie a double knot about three inches from the new end.
3. Slide about an inch of protective sleeving onto the lead.
4. Solder the wires back where the old ones came from, noting which terminal the traced wire went to.
5. Close the box with the knot inside, so that it takes any pull on the lead, and with the sleeving trapped at the exit to protect the wire.

Repair the inside

1. Check the rectifier diodes. Some packs use a bridge of four 1N4007s, others two diodes in a full-wave circuit.
2. Check the thermal fuse on the transformer's primary. On some units it is on the circuit board. Worsley replaces a blown thermal fuse by spot-soldering a 160 mA slow-blow 20 mm fuse across the old device.
3. If the transformer itself has failed, the pack is not worth repairing. Television magazine also found that a pack whose diodes had burnt the panel was only economic to replace.

Using a modern supply instead

Adam's restorations recommend a modern supply for vintage computers generally, while noting that the Spectrum regulates its own 5 V, so a failing original pack is unlikely to damage it. Any replacement must be centre negative; the original is rated 9 V at 1.4 A.

Testing

1. Measure the off-load output and the polarity again before connecting the Spectrum.
2. With the Spectrum running, check +9 V ±2.0 V at C50 and +5 V ±0.25 V at C34.
3. Run the machine for a while and move the lead gently at both ends; the picture should not drop out.

Related pages

• Sinclair ZX Spectrum
• Sinclair ZX Spectrum+
• Sinclair ZX Spectrum Voltage Regulator Replacement
• Sinclair ZX Spectrum General Maintenance
• Sinclair ZX Spectrum Troubleshooting Guide