Sinclair ZX Spectrum 128 General Maintenance

The ZX Spectrum 128. The finned heatsink along the right-hand end carries the 7805 regulator.

The Sinclair ZX Spectrum 128 (the "toastrack") uses the Spectrum+ keyboard membrane and bubble mat, so keyboard work is much the same as on that model. The external heatsink, the second bank of RAM, the RS-232 and keypad circuits and the TEA2000 video encoder are specific to the 128 architecture. The parts that need regular attention are the regulator heatsink and its fixings, the two keyboard tails, the RAM, and the connectors.

Safety

The computer runs from an external pack, so there is no mains voltage inside the case. Sinclair's manual describes the pack as a mains transformer, full-wave rectifier and capacitive smoothing, with a thermal fuse at the transformer input; the 128 was supplied with a 9 V, 1.85 A unit (UK1850, spare part X-1220). The pack can be opened and repaired. Unplug it from the mains first: the primary side carries mains voltage.

Ian Worsley's repair guide for the 48K Spectrum and Spectrum+ describes the pack faults he found common to all the Spectrum supplies he saw: a wire breaking inside the moulded plug or where the lead leaves the case, and, when a bare wire has shorted the output, a blown thermal fuse or rectifier diodes.

Sinclair's repair notes call for normal anti-static precautions when handling ICs and a suitably earthed, low-power soldering iron.

Expansion connector

Never plug or unplug anything on the expansion edge connector while the machine is switched on. Worsley attributes at least 80–90 % of the 48K faults he saw to interfaces: plugged in or pulled out with the power on, or with the keyway peg missing so that the plug can move sideways and short the outputs together. He describes cutting a replacement peg from the hinge of an old cassette box. T.I.S.'s field-service notes give the same warning and name the damage: TR4 in the inverter, the RAM, the power resistors and the regulator.

Board versions

The 128 was built on five main board versions: 2, 2-1, 6K, 6U and 9G. Versions 2 and 2-1 were made for Investronica's Spanish machines and do not connect /CLK on the edge connector to the CPU clock, so peripherals that need it (a DivMMC, for example) need a wire adding. Version 6K, the first UK board, has a wrong value for R137, which leaves the MC1488 keypad driver out of specification so that a keypad will not work; fitting a 15 Ω resistor at R137 cures it. Version 6U is the most common. Version 9G, sold mostly in Europe, has a DB-9 RS-232 connector in place of the BT631W socket. Note the version before diagnosing a fault; see also the Sinclair ZX Spectrum 128 Troubleshooting Guide.

Opening the case

Sinclair's procedure:

1. Unplug all input and output connectors.
2. Turn the computer upside down and remove the eight fixing screws. Note where the two countersunk screws go.
3. Turn the computer the right way up and separate the case halves. Disconnect the two keyboard ribbon tails, KB1 (5-way) and KB2 (8-way), from the board.
4. To take the board out of the lower case, remove the board fixing screws and the fixings that hold the 7805 regulator to the finned heatsink.

If the board is to be powered up out of the case, remove it with the heatsink still attached, as one assembly. The heatsink is held to the case by two screws. Take care not to strain the regulator's legs.

On reassembly, do not overtighten the self-tapping screws, make sure the reset button sits in its cut-out in the end of the case, and check that the legs and leg springs are in place. The keyboard tails should lie in an S shape. Push each tail into its connector with a slight side-to-side movement while easing it down, and make sure it is fully home and not kinked.

Heatsink and cooling

The 7805 regulator is bolted to the external finned heatsink with an M3 screw, washer, crinkle washer and nut. Sinclair's repair notes ask for good contact between the regulator body and the heatsink, and for thermal grease on the back of a replacement regulator.

T.I.S.'s notes on the 48K tell the engineer to check the heatsink first for any reported fault (is it loose, are the screws overtightened?), to suspect the heatsink and then the ULA for faults that appear only when warm, and to look for hairline cracks in the board radiating from the heatsink screws when chasing intermittent faults.

On the 48K, Worsley notes that the ULA runs quite warm and every other chip runs cold. A RAM chip that gets hot is shorted. Keep the fins clear and do not run the machine on soft furnishings that block the vents.

RAM

The 128 has sixteen 4164 DRAMs: IC6–IC13 form the contended bank and IC15–IC22 the uncontended bank. Sinclair specifies 150 ns access time and 128-row refresh, and lists Hitachi, Intel, Mitsubishi, Mostek, Motorola, NEC, OKI, Panasonic, Toshiba and National as acceptable makers.

RAM is the commonest failure on boards of this design. Dave Curran of Tynemouth Software repaired five mid-1986 grey +2 boards, which use the same two-bank 4164 layout, and found 34 faulty RAM chips, all Samsung KM4164B. Where most of a bank had failed he replaced and socketed the whole bank, and he matched speed grades within a set where he could.

Keyboard

The keyboard is a Spectrum+ membrane over a bubble mat, held by a reaction plate and two tail clamps. Sinclair's procedure for changing the membrane, bubble mat or keys:

1. Remove the two membrane tail clamps, then the ten screws holding the reaction plate.
2. Lift out the plate, the membrane and the bubble mat. A key can be removed for cleaning by pressing it and prising the retaining sleeve off its underside with a small screwdriver.
3. To reassemble, support the upper case face down so that the keys are clear of the bench. Fit the bubble mat, membrane and reaction plate so that the hole at each end engages its locating peg.
4. Fit the screws starting with the centre row. Tighten each fully and then back it off a quarter turn.
5. Clamp the tails so that the middle, upper and lower tracks make good contact: position the packing pieces (extensions of the bubble mat) correctly, and leave the ends of the middle tracks protruding 1 mm beyond the clamps.
6. On a new membrane, bond the upper and lower tracks together near the connector end with double-sided tape to prevent short circuits.

Worsley's Spectrum+ keyboard fixes apply to the 128's membrane:

• Most keyboard faults are the membrane: the pads under the keys wear out, or the tails crack.
• A crinkled or cracked tail end can be cut back by up to an inch. Strip back the insulated layer by about another quarter inch, or the keyboard will stop working altogether.
• Keys on the bottom row that do not work can sometimes be cured without a new membrane. Remove the tail clamps, clean between the layers of each tail around the clamped area with WD-40 on a cotton bud, and add extra packing between tail and clamp when refitting.
• A key that prints repeatedly means the reaction plate is clamped too tightly. Slacken the screw nearest that key by a quarter turn.

See Sinclair ZX Spectrum Keyboard Membrane Repair for membrane testing and replacement.

Connectors and sockets

• Clean the expansion edge connector fingers. Tynemouth found dirty edge connectors on two 128-architecture boards that stopped a DivMMC from working, and cleaned them with contact cleaner and a fibreglass pencil.
• A loose power socket or a faulty power plug causes crashes and resets when the machine is moved. Replace the socket; Worsley advises against trying to tighten it. One of Tynemouth's boards that appeared dead had a bad power connector.
• EAR and MIC sockets that have lost contact tension cause intermittent loading. Re-tension the spring contact by pushing it down and inwards until it touches the case.
• Sinclair's parts list gives the RS-232 and keypad connectors as 6-way BT-type telephone jack sockets (BICC 603A) and the RGB output as an 8-way DIN or 9-way D-type. The 9G board has a DB-9 for RS-232. Clean their contacts with contact cleaner if the keypad or serial port is intermittent.

Power checks

The on-board supply takes 9 V unregulated from the pack. A 7805 produces +5 V for the logic, the ULA and the sound and UHF modulators. An inverter built round TR4 (ZTX650) and TR5 (ZTX213) produces +12 V through D15 and C44 for the RS-232 driver IC33 and the keypad; a charge pump (C111, C112, D28, D29) on the same square wave produces −12 V for IC33, and zener D19 derives −5 V for the expansion port from that rail.

ZX Spectrum 128 supply checks (Sinclair section 4.2.3)

Function	Measure at	Expected
Regulator input	+ve side of C50	+9 V ±2.0 V; below +7 V the regulator may not work correctly
Regulator output	+ve side of C34, IC15 pin 8	+5 V ±0.25 V, no discernible ripple
+12 V	IC33 pin 14	+12 V ±0.5 V
−12 V	IC34 pin 1	−12 V ±3 V
−5 V	D19 anode	−5 V

Sinclair's specification for the pack is a trough of at least 7.0 V at 1.4 A with 215 V AC mains, and a peak of no more than 13 V at 600 mA with 265 V AC mains.

A current-limited bench supply is a useful diagnostic aid. Tynemouth set the limit to 1.5 A for +2 boards, which normally draw between 800 mA and 1.2 A depending on the RAM fitted. A board that goes straight into current limiting has a shorted device, usually RAM, and the faulty chips can be found by feeling which ones get hot.

Capacitors

Sinclair's Table 5.4 lists the value and rating of the capacitors on the main board. UK boards (Versions 6K, 6U and 9G) also have C131, a 47 µF electrolytic that is not in the table, according to Spectrum for Everyone's community list. See the Sinclair ZX Spectrum 128 Capacitor Replacement Guide.

Two capacitor changes are worth knowing about:

• Vertical "jailbars" on the RGB output (not over RF or composite) are reduced by replacing C28 (22 µF, to the right of the 5-way keyboard connector) with a 47 µF axial capacitor and C7 and C8 (next to IC12 and IC13) with 1 µF ceramics.
• For wavy lines across the screen, Sinclair's 48K manual says to press down on each electrolytic in turn and change the one that makes the lines disappear. The 128 manual's own symptom table (section 4.2.4) was never filled in.

Repair notes

Sinclair's repair notes for the 128:

• Use proper desoldering and heatsinking technique so that neither the component nor the board is damaged.
• When fitting a keyboard membrane, make sure the ribbon tails are fully inserted and not kinked.
• Replace the modulator as a complete unit.
• Use proper removal and insertion tools for socketed ICs and avoid handling the pins.
• After replacing any component, check the board for solder splatter that could short tracks or connector pins.

Common failure points

ZX Spectrum 128 failure-prone items

Item	Symptom	Action
4164 RAM	Crashes, corrupt display, high current draw, chips getting hot	Test with a diagnostic ROM; replace and socket
Heatsink and regulator fixing	Faults that appear only when warm	Check the fixing and thermal grease; look for cracks round the screws
Keyboard membrane and tails	Dead keys, a dead block of keys, a repeating key	Clean and re-pack the tails; slacken the reaction plate screw; replace the membrane
Inverter (TR4, TR5, coil)	+12 V or −12 V missing; RS-232 and keypad dead	When TR4 has failed, check TR5 and the coil too
ULA (IC1, Ferranti 7K010)	No reset, picture faults, crashes after warming up	Check +5 V on pin 14, R34 (15 Ω) feeding pin 13, and the clocks; the ULA is socketed, so substitute a known good one. See Sinclair ZX Spectrum 128 ULA
R137 on version 6K boards	Keypad does not work	Fit 15 Ω at R137
Edge connector	Interfaces fail to work	Clean the fingers; never hot-plug
Power socket and pack lead	Resets when moved	Replace the socket; repair the lead
EAR and MIC sockets	Intermittent tape loading	Re-tension the contacts

Tools

Sinclair's minimum list for servicing the 128 is a storage oscilloscope with a ×10 probe, a 0–30 V variable DC supply, a general-purpose multimeter, a mono cassette recorder, a colour TV and monitor, a test tape and blank tape. Add to that:

• small cross-head screwdrivers and an anti-static strap
• a temperature-controlled soldering iron, desoldering braid or pump, leaded solder and flux
• 16-pin IC sockets for the RAM and an IC extraction tool
• isopropyl alcohol, cotton buds and contact cleaner
• thermal grease for the regulator
• a diagnostic ROM such as ZX Diagnostics, and Paul Farrow's 128K RAM tester, both used by Tynemouth on these boards

See Recommended Tools for the general toolkit.

Preventive maintenance checklist

1. Note the board version, and whether the R137 change (6K) or the /CLK wire (2 and 2-1) has been made.
2. Check that the regulator is tight on the heatsink with thermal grease, and inspect the board around the fixings for cracks.
3. Check current draw on a limited supply and feel for any RAM chip running warm.
4. Measure the +5 V, +12 V, −12 V and −5 V rails against the table above.
5. Clean the edge connector and the EAR, MIC, RGB, RS-232 and keypad sockets. Replace a loose power socket.
6. Inspect, clean and reseat both keyboard tails.
7. Run the built-in colour test by holding BREAK while pressing reset: it shows vertical bands of the eight paper colours, bright and normal, with the eight ink colours on them. Use the Tape Tester from the start-up menu to set the playback volume of the cassette recorder.

Related pages

• Sinclair ZX Spectrum 128
• Sinclair ZX Spectrum 128 Troubleshooting Guide
• Sinclair ZX Spectrum 128 Capacitor Replacement Guide
• Sinclair ZX Spectrum 128 Service Manual
• Sinclair ZX Spectrum+ General Maintenance, for the shared keyboard
• Sinclair ZX Spectrum +2 General Maintenance
• Sinclair ZX Spectrum Keyboard Membrane Repair
• Sinclair ZX Spectrum 128 ULA
• Capacitor Failure Symptoms
• Recommended Tools