Sinclair ZX Spectrum +2 General Maintenance

The grey ZX Spectrum +2, with the Datacorder cassette deck on the right.

Preventive maintenance on the grey Sinclair ZX Spectrum +2 (main boards Z70500 Issue 3 and Z70700 Issue 1) centres on the Datacorder cassette deck, careful soldering on the Amstrad PCB, the keyboard connections and the regulator heatsink. The black +2A and +2B use Amstrad's later boards (the +3's Z70830 in some very early +2As, otherwise the Z70833) and are not covered here.

Amstrad's own 12-page Spectrum +2 Service Manual, hosted here, covers the specification, power adaptor safety tests, a fault flow chart, the parts lists, circuit diagram and PCB layouts, and the cassette mechanism. Because the +2 is functionally a Spectrum 128, the Sinclair ZX Spectrum 128 Service Manual also applies to its circuit.

Safety

There is no mains voltage inside the computer; the mains transformer is in the external power pack, and the highest rail on the board is +12 V. Take normal anti-static precautions when handling the ICs.

The grey +2 uses a 2.1 mm barrel connector for power, and the +2A and +2B a 6-pin DIN; the power supplies are not interchangeable.

Soldering on the +2 board

Spectrum for Everyone describes the PCB quality on the Amstrad-produced boards as poor, and warns that great care is needed when replacing components to avoid lifting pads or tracks. The Spectrum 128 Service Manual asks for recognised desoldering and heatsinking techniques and a suitably earthed, low-power iron, and for the board to be examined for solder splatter after any component is renewed.

• Use a temperature-controlled iron with a clean, fine tip, and keep the time on each joint short.
• Add fresh leaded solder and flux to an old joint before removing a part.
• Lever a component out only when its joint is fully molten.
• Support the board and do not flex it.
• Fit a socket when replacing a chip that may need to come out again, such as RAM.
• Clear each hole with wick or a vacuum tool before moving the part.

Opening the case

1. Unplug the power and all peripherals.
2. Remove the six screws holding the case together.
3. Separate the top and bottom halves, disconnecting the keyboard ribbons and the tape deck connector.

When reconnecting the keyboard, make sure the ribbon connectors are fully inserted and not kinked.

Datacorder servicing

Head, capstan and azimuth

If playback speed is steady, start by cleaning the tape head, capstan and pinch roller with isopropyl alcohol and cotton buds. If that does not help, adjust the head azimuth: there is a small hole in the +2's case for a No. 0 jeweller's screwdriver, and the adjustment is made with a tape playing, listening for the clearest sound. The T.I.S. Repair Data in the 1984 Spectrum servicing manual gives the same advice for external recorders: adjust for the sharpest possible note, because the Spectrum will not load from a tape whose sound is at all muffled, then clean the head and recheck.

Drive belts

If playback speed varies noticeably, or the deck struggles to fast-forward or rewind, replace the belts. The grey +2 mechanism has two belts; the +2A and +2B mechanism has one. Replacement belt kits are sold by retro suppliers, and the belts are also available from Farnell. Lay the top half of the case keys-down to reach the mechanism, lift the old belts off with tweezers and fit the new ones untwisted, the smaller (lower) belt first.

Pause mechanism

If speed still varies with new belts, turn the large flywheel by hand and check whether it fouls the white plastic latch of the pause mechanism underneath it. The white "hat" on the latch's pillar can crack and let its spring push the latch against the flywheel. To fix it, remove the circlip on the top of the mechanism, take the flywheel out from underneath (keeping the small washers, hat, spring and latch), glue the hat back on its pillar, and refit.

Audio hum

Taiwanese-built grey +2s are prone to buzzing or humming on the audio output, present all the time because the audio output is live even when the Datacorder is stopped. The tape PCB uses the metal frame of the mechanism as a ground path, so check first that the two screws holding the PCB to the mechanism are tight and conductive. A ground wire between two points on the PCB is a more reliable fix. The tape PCB capacitors are listed in the Sinclair ZX Spectrum +2 Capacitor Replacement Guide.

Regulator and ULA heatsinks

• Make good contact between the voltage regulator body and its heatsink, and apply thermal grease to the back of the regulator when replacing it.
• The ULA was fitted with an Amstrad clip-on heatsink. Tynemouth Software fits a replacement heatsink where the original is missing.
• The ULA is IC14, which Amstrad's parts list gives as "ULA7K010E5/AMS40056", the Ferranti 7K010 of the Spectrum 128 or Amstrad's 40056; see Sinclair ZX Spectrum 128 ULA. When Ferranti tested 7K010 parts returned from early production in 1985, those that were marginal on speed failed only when warm.

Power checks

A working +2 draws between about 800 mA and 1.2 A, depending on the type of RAM fitted. Tynemouth Software tests boards on a bench supply current-limited to 1.5 A; on one board that went straight into current limiting, several RAM chips were getting hot and three of them were shorting the 5 V rail.

The on-board rails are +5 V from the 7805 regulator, +12 V and −12 V from the inverter and its charge pump, and −5 V for the expansion port from the −12 V rail via a zener. On one board Tynemouth Software found an inverter transistor replaced with a lower-rated part and fitted a ZTX651, which is the ZTX650 with a higher voltage rating.

The Spectrum 128 Service Manual's design limits for the external power pack are a voltage trough of no less than 7.0 V at 215 V AC mains and 1.4 A load, and a peak of less than 13 V at 265 V AC and 600 mA.

Cleaning

• Case: a damp cloth with a little mild detergent.
• Board: a soft brush and low-pressure air. A dirty edge connector stopped a DivMMC loading on one of Tynemouth Software's boards; contact cleaner and a fibreglass pencil cured it.
• Datacorder: clean the head, capstan and pinch roller with isopropyl alcohol whenever the case is open.

Common failure points

ZX Spectrum +2: failure-prone items

Item	Symptom	Action
Datacorder belts	Speed varies; slow or no fast-forward and rewind	Replace both belts
Tape head and azimuth	Tape loading errors	Clean the head; adjust the azimuth
Pause mechanism	Speed varies after new belts	Repair the cracked latch "hat"
Tape PCB grounding	Constant hum or buzz on the audio output	Tighten the two PCB-to-mechanism screws; add a ground wire
RAM	RAM faults reported by test software; excess current draw; hot chips	Replace the faulty chips, fitting sockets
Video transistors TR4, TR5, TR7	Poor composite video	Check that any 2N3904 fitted in place of a BC237 is turned round to suit its reversed pinout

Recommended tools

• Screwdrivers, including a No. 0 jeweller's screwdriver for the azimuth, and an anti-static strap.
• Temperature-controlled soldering iron with a fine tip, solder wick or a vacuum tool, leaded solder and flux.
• IC sockets for RAM, and an IC extraction tool.
• Digital multimeter and a current-limited bench supply.
• Oscilloscope for clock work.
• Isopropyl alcohol, cotton buds, contact cleaner and a fibreglass pencil.
• Replacement Datacorder belts.
• A RAM test program or diagnostic ROM.

See Recommended Tools for the general toolkit.

Maintenance checklist

1. Replace the Datacorder belts; clean the head, capstan and pinch roller; set the azimuth.
2. Tighten the two tape-PCB grounding screws.
3. Check the regulator heatsink and its thermal grease, and that the ULA heatsink is fitted.
4. Check the current draw on a current-limited supply.
5. Clean the edge connector and reseat the keyboard ribbons.
6. Inspect the board for lifted pads and cracked tracks, especially around earlier repairs.

Related pages

• Sinclair ZX Spectrum +2
• Sinclair ZX Spectrum +2 Troubleshooting Guide
• Sinclair ZX Spectrum +2 Capacitor Replacement Guide
• Sinclair ZX Spectrum+ General Maintenance
• Sinclair ZX Spectrum Keyboard Membrane Repair
• Capacitor Failure Symptoms
• Recommended Tools
• Sinclair ZX Spectrum 128 ULA