Sinclair ZX Spectrum 128 General Maintenance

The ZX Spectrum 128 "Toastrack". The external heatsink fins along the right-hand side are the machine's defining feature — and the reason cooling matters more here than on a 48K Spectrum.

This guide covers preventive maintenance for the Sinclair ZX Spectrum 128. The machine uses the Spectrum+ case and keyboard, so keyboard work is shared with that model; the heatsink, the extra RAM and the RS-232/keypad circuitry are 128-specific.

Maintenance priorities are the heatsink and its mounting, the keyboard membrane and its two ribbon tails, the RAM, and the edge connector.

Safety

There are no mains voltages inside the computer — it runs from an external supply and the highest rail on the board is +12 V. The mains transformer, full-wave rectifier and smoothing are inside the PSU brick, which also contains a thermal fuse at the transformer input.

The PSU brick can be opened and repaired by someone comfortable with mains wiring — the usual failure is the cable breaking in the moulded plug or where it leaves the case, and the thermal link can be replaced with a 160 mA slow-blow fuse.

Observe normal anti-static precautions for the ULA, Z80A, AY-3-8912A and RAM.

The golden rule

Never plug or unplug anything into the expansion edge connector while the machine is switched on. On the Spectrum family this is the leading cause of catastrophic damage: the connector shifts sideways, shorts bus lines together and takes out the power transistors and RAM. If an interface has lost its keyway peg, make a replacement before using it.

Know your board version

The 128 exists in five board versions — 2, 2-1, 6K, 6U and 9G — and two of them carry documented manufacturing defects (the Issue 6K keypad resistor and the missing /CLK connection on the Spanish boards). Identify the version before assuming a fault. Details are on the Sinclair ZX Spectrum 128 page and in the Sinclair ZX Spectrum 128 Troubleshooting Guide.

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 connects to the board by two flexible ribbon tails at KB1 (5-way) and KB2 (8-way), and these are fragile.
4. Free each tail by pinching the ribbon next to the connector and easing it straight up, not at an angle.

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

Cooling and the heatsink

The Toastrack's external heatsink is not decorative — the 128 dissipates considerably more than a 48K Spectrum, and cooling is the maintenance item most specific to this model.

• Check the heatsink is securely fastened and making good thermal contact with the regulator. Apply fresh thermal compound when refitting or replacing the regulator.
• Inspect the board around the heatsink fixings for hairline cracks. On the Spectrum family, cracks radiating from heatsink screws are a documented cause of intermittent faults.
• Do not over-tighten the fixings — over-tightened screws are specifically called out as a fault cause on these machines.
• After a few minutes' running, the ULA should be warm and the regulator hot; the RAM should stay cool. A warm RAM device is a failing one.
• Keep the fins clear of dust and do not operate the machine boxed in or on soft furnishings.

RAM

All RAM must be 4164, 150 ns access time with 128-row refresh. The Service Manual lists Hitachi, Intel, Mitsubishi, Mostek, Motorola, NEC, OKI, Panasonic, Toshiba and National as acceptable manufacturers.

Fitting slower parts, or mixing speed grades across a bank, causes instability that looks like a logic fault. If you are replacing more than one or two devices in a bank, socket the whole bank — fitting sockets once is kinder to the board than repeated desoldering, and makes future diagnosis trivial.

Keyboard

The 128 uses the Spectrum+ keyboard: a moulded key assembly over a membrane, held by a backing plate with nine screws, connected by the KB1 and KB2 tails.

• Most keyboard faults are membrane failure — the pads under the keys wear out, or the tails crack with age.
• If the tail ends are crinkled 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 can often be cured without 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 on reassembly.
• A key that repeats endlessly is the backing plate clamped too tight — slacken the screw nearest that key by a quarter turn.

Replacement Spectrum+/128 membranes are still manufactured.

Connectors and sockets

• Clean the expansion edge connector fingers with a soft eraser and contact cleaner. Tarnished contacts cause crashes that look like component faults.
• Check the power socket — a loose one causes intermittent resets when the machine is moved. Replace it; do not try to tighten it.
• The EAR and MIC sockets lose contact tension with age, causing intermittent tape loading. Re-tension by pushing the spring contact down and inward rather than replacing the socket.
• Clean the RGB, RS-232 and keypad connectors with contact cleaner on a foam swab. The RS-232 and keypad are BT-type 6-way jacks and their contacts tarnish.

Power checks

The on-board supply produces +5 V from a 7805, +12 V from the TR4/TR5 inverter, −12 V from a charge pump, and −5 V from the −12 V rail via zener D19.

The external pack should not sag below 7.0 V at 1.4 A with low mains, nor peak above 13 V at 600 mA with high mains.

A current-limited bench supply is a very useful diagnostic aid: a board that goes straight into current limiting has a shorted device, usually RAM, and catching that before applying full power avoids further damage.

Capacitor health

The 128's electrolytics are ordinary leaded parts and the machine is not a systematic recap case. The Service Manual publishes a full parts list, so exact values are known — see the Sinclair ZX Spectrum 128 Capacitor Replacement Guide.

Two capacitor jobs are worth knowing about as maintenance rather than repair: the jailbars modification (C28, C7 and C8) which improves RGB picture quality, and the wavy lines test — press each electrolytic in turn and see whether the artefact clears.

Repair notes

From the Service Manual, applicable to any component work:

• Use recognised desoldering and heatsinking technique; only a suitably earthed, low-power soldering iron.
• When replacing the keyboard matrix, make sure the ribbon connectors are fully inserted and not kinked.
• Ensure good contact between the voltage regulator body and its heatsink, and apply thermal grease.
• Replace the modulator as a complete unit.
• Use proper IC removal and insertion tools; avoid contaminating pins by handling.
• Inspect the board for solder splatter after any work.

Common failure points

ZX Spectrum 128 — failure-prone items

Item	Symptom	Action
RAM (4164, 150 ns)	Crashes, corrupt display, excess current draw	Test with a diagnostic ROM; socket and replace
Heatsink mounting	Warm-only intermittents	Check fastening and thermal compound; inspect for hairline cracks around fixings
Keyboard membrane and tails	Dead keys, dead block of keys, repeating key	Clean/re-pack the tails; slacken backing-plate screws; replace membrane
±12 V inverter (TR4/TR5/coil)	No video, dead serial port, dead keypad	Replace TR4, TR5 and the coil together
Edge connector	Random crashes	Clean fingers; never hot-plug
Power socket and PSU lead	Resets when moved	Replace the socket; repair the PSU lead
EAR/MIC sockets	Intermittent tape loading	Re-tension the contacts
MC1488 / MC1489	Dead RS-232 or keypad	Check the −12 V rail first, then substitute

Recommended tools

• Small Philips/Pozidriv screwdrivers and an anti-static strap.
• Digital multimeter; a current-limited bench supply is very useful.
• Oscilloscope for the clock and RAS/CAS checks.
• Temperature-controlled soldering iron, solder wick, fresh leaded solder and flux.
• IC sockets (for RAM) and an IC extraction tool.
• Isopropyl alcohol, cotton buds, contact cleaner, a soft eraser for the edge connector.
• Thermal compound for the regulator.
• Replacement keyboard membrane.
• Optionally a diagnostic test ROM (for example ZX Diagnostics) and a 128K RAM tester.

See Recommended Tools for the general toolkit.

Preventive maintenance checklist

1. Check the heatsink is tight, well-mated and re-greased; inspect the board around its fixings for hairline cracks.
2. Confirm current draw is sane on a limited supply and that no RAM device runs warm.
3. Verify the +5 V, ±12 V and −5 V rails.
4. Clean the edge connector, EAR/MIC, RGB, RS-232 and keypad sockets; replace a loose power socket.
5. Inspect, clean and correctly reseat both keyboard tails.
6. Note the board version and whether the 6K R137 or Spanish /CLK corrections have been applied.

Related pages

• Sinclair ZX Spectrum 128
• Sinclair ZX Spectrum 128 Troubleshooting Guide
• Sinclair ZX Spectrum 128 Capacitor Replacement Guide
• Sinclair ZX Spectrum+ General Maintenance — the shared case and keyboard
• Sinclair ZX Spectrum +2 General Maintenance
• Sinclair ZX Spectrum Keyboard Membrane Repair
• Capacitor Failure Symptoms · Recommended Tools