Sinclair ZX Spectrum+ General Maintenance

A Spectrum+ Issue 6A board in the case. The 7805 and its heatsink sit centre-board; the ULA, Z80A, ROM and the two RAM rows are below, and the reset switch is wired in at the right.

This guide covers preventive maintenance for the Sinclair ZX Spectrum+. The machine is electrically a 48K Spectrum, so most of it applies to the rubber-key model too; the keyboard assembly and reset switch are Spectrum+ specific.

The maintenance priorities are the keyboard membrane and its clamped ribbons, the 7805 regulator and heatsink, the edge connector, and the ageing DC-DC converter that feeds the lower RAM.

⚠️ Safety

There are no mains voltages inside the computer — it runs from an external 9 V supply and the highest voltage on the board is +12 V. The mains transformer is inside the PSU brick, which can be opened and repaired (see below) by someone comfortable with mains wiring.

Observe normal anti-static precautions for the ULA, Z80A, ROM and RAM.

⚠️ The golden rule

Never plug or unplug anything into the expansion edge connector while the machine is switched on. This single habit is reported as the cause of 80–90 % of Spectrum faults: the keyway peg on the interface plug drops out with use, the connector shifts sideways, and the bus lines short together — taking out TR4 and often the RAM with it.

If an interface has lost its keyway peg, cut a replacement from the hinge section of an old cassette box and glue it in.

Identifying your board

The Spectrum+ shipped with several 48K board issues — 4A, 4B, 4S, 5 and 6A. The issue number is printed on the PCB. It matters because, for example, Issue 4A and 4B route /RAS through spare inverters and therefore will not run with IC24 removed, and because the DC-DC converter components differ between issues. Full detail is on the Sinclair ZX Spectrum+ page.

Samsung-built Issue 4S machines (serials beginning S01) were supplied with noticeably better keyboard membranes, many of which still work.

Opening the case

1. Unplug the power and all peripherals.
2. Turn the machine over and remove the case screws.
3. Lift the top carefully — the keyboard is connected to the board by two flexible membrane tails, and these are fragile.
4. Free the tails by pinching the ribbon next to the connector and easing it straight up. Do not pull at an angle.

When refitting, insert each tail with a slight side-to-side movement while easing it gently down into the connector, and make sure it is fully home and not kinked.

Keyboard

The Spectrum+ keyboard is a moulded key assembly over a membrane, held by a backing plate with nine screws.

• Routine care: keep the tails clean and undamaged. Most Spectrum keyboard faults are simply membrane failure — the pads under the keys wear out or the ribbon cracks with age.
• Ribbon repair: if the tail ends are crinkled, creased or cracked you can trim back up to about an inch, but strip the insulated part an extra quarter inch or the keyboard will stop working entirely.
• Bottom-row keys failing is a known Spectrum+ complaint that often does not need a new membrane. Remove the ribbon clamps, clean between the ribbon layers around the clamped area with a cotton bud and WD-40, and add extra packing under the clamp when reassembling.
• A key that repeats endlessly is caused by the backing plate being clamped too tight. Slacken the screw nearest that key by a quarter turn.

Sinclair's own view was that keyboards take a lot of battering and are usually better replaced complete, replacements being inexpensive. Replacement Spectrum+ membranes are still manufactured.

Cooling and the 7805 regulator

The 7805 does all the heavy lifting and the Spectrum runs warm by design.

• Check the heatsink is tight and undamaged. The Service Manual makes checking the heatsink the first step for any reported fault, and warns about over-tightened screws.
• Look for hairline cracks in the board radiating from the heatsink screws — a documented cause of intermittent faults.
• Apply fresh thermal compound when refitting the regulator, and ensure good contact between the regulator body and the heatsink.
• In normal running the ULA runs quite warm; every other IC should be cold. A warm RAM chip is a failing RAM chip.
• If the machine develops faults only when warm and the heatsink is sound, the ULA is the cause — and if the ULA has been replaced before, extra heatsinking on it is worthwhile.

Supply rail checks

Spectrum+ supply test points (Service Manual)

Measurement	Test point	Expected
Regulator input	+ve side of C50	+9 V DC ± 2.0 V (below +7 V the regulator may misbehave)
Regulator output	+ve side of C34	+5 V DC ± 0.25 V, no discernible ripple
+5 V to RAM	IC6 pin 9	+5 V DC ± 0.25 V
−5 V to RAM	IC6 pin 1	−5 V DC
+12 V to RAM	IC6 pin 8	+12 V DC
+12 VA (IC14 only)	Left end of C52	+12 V DC

A practical check at the regulator itself: 4.85–5.2 V at the 7805 output is normal.

The −5 V and +12 V rails come from the on-board DC-DC converter (TR4/TR5 and the coil), which is the most failure-prone circuit in the machine. With it running you should be able to hear the coil buzzing faintly.

Edge connector and sockets

• Clean the expansion edge connector fingers with a soft eraser and contact cleaner. Tarnished contacts cause crashes and phantom faults.
• Check the power socket — it is a common source of intermittent resets when the machine is moved. If it is loose, replace it; do not attempt to tighten it.
• The EAR and MIC sockets lose contact tension with age, causing intermittent loading. They can be re-tensioned by carefully pushing the spring contact downwards and inwards until it touches the case, rather than replacing the socket.

External power supply

The PSU brick is a mains transformer, rectifier and smoothing capacitor delivering 9 V DC through a 2.1 mm centre-negative plug. It is repairable:

• The usual failure is the cable breaking in the moulded plug or where it exits the case.
• Refit a new 2.1 mm plug with negative to the centre (the traced wire) and fill the cover with silicone rubber for strain relief.
• At the PSU end, split the case, cut back the damaged cable, tie a double knot about three inches down so the knot sits inside the case, and resolder.
• A shorted output blows the thermal fuse and can take out the 1N4007 rectifiers. The thermal link can be replaced with a 160 mA slow-blow 20 mm fuse soldered across it. A failed transformer means the unit is scrap.

Capacitor health

The Spectrum's electrolytics are ordinary leaded parts and the machine is not a systematic "recap" case, but ageing capacitors do cause real symptoms — the Service Manual's own test for wavy lines across the screen is to press down on each electrolytic in turn and see whether the lines disappear. Full detail: Sinclair ZX Spectrum+ Capacitor Replacement Guide.

Common failure points

Spectrum+ — failure-prone items

Item	Symptom	Action
Edge connector / hot-plugged interfaces	Sudden death, blown TR4, dead RAM	Never hot-plug; fit a keyway peg; inspect the connector
TR4 / TR5 / coil (DC-DC converter)	White or patterned screen; no −5 V or +12 V	Replace TR4 (ZTX650) and TR5 (ZTX213) together with the coil if suspect
Lower RAM (IC6–IC13, 4116)	Patterned screen, warm chips	Locate with the POKE test; replace
ULA (IC1)	Faults when warm, no sound, character break-up	Substitute; add heatsinking
Keyboard membrane and tails	Dead keys, repeating keys	Clean/re-pack the clamped ribbons; slacken backing-plate screws; replace membrane
7805 and heatsink	Low or missing +5 V; warm-only faults	Check mounting, re-grease, replace regulator
Power socket and PSU lead	Random resets when moved	Replace the socket; repair the PSU lead
EAR/MIC sockets	Intermittent loading	Re-tension the contacts

Recommended tools

• Small Philips/Pozidriv screwdrivers and an anti-static strap.
• Digital multimeter (20 V DC and 20 kΩ ranges cover most of the diagnostic sequence).
• Temperature-controlled soldering iron, solder wick, fresh leaded solder and flux.
• IC extraction tool.
• Isopropyl alcohol, cotton buds, contact cleaner, a soft eraser for the edge connector.
• Thermal compound for the 7805.
• Replacement keyboard membrane.
• Optionally a diagnostic test ROM (for example ZX Diagnostics) for fast RAM testing.

See Recommended Tools for the general toolkit.

Preventive maintenance checklist

1. Check the heatsink is tight, undamaged and well-mated; inspect the board around its screws for hairline cracks.
2. Verify +5 V, −5 V and +12 V at IC6.
3. Listen for the DC-DC converter coil buzzing.
4. Clean the edge connector, EAR/MIC sockets and power socket; replace a loose power socket.
5. Inspect and clean the keyboard tails; re-pack the ribbon clamps.
6. Feel the ICs after a few minutes' running — only the ULA should be warm.

Related pages

• Sinclair ZX Spectrum+
• Sinclair ZX Spectrum+ Troubleshooting Guide
• Sinclair ZX Spectrum+ Capacitor Replacement Guide
• Sinclair ZX Spectrum Keyboard Membrane Repair
• Sinclair ZX Spectrum General Maintenance
• Capacitor Failure Symptoms · Recommended Tools