Sinclair ZX Spectrum +2 Troubleshooting Guide

This guide gives fault diagnosis for the grey Sinclair ZX Spectrum +2 (boards Z70500 Issue 3 and Z70700 Issue 1). Because the +2 is functionally a repackaged Spectrum 128, the 128 Service Manual is the closest official documentation; this guide combines it with the repair knowledge built up by the Spectrum community.

Do not apply +2A or +2B information to a grey +2. Those are different machines built on the +3 platform — different board, different gate array, different power supply and connector. See the comparison on the Sinclair ZX Spectrum +2 page.

⚠️ Before you start

• There are no mains voltages inside the computer. It runs from an external supply; the highest rail on the board is +12 V.
• The Amstrad-built PCB is poor quality and its pads and tracks lift easily. Great care is needed when replacing components. Read the soldering section of the Sinclair ZX Spectrum +2 General Maintenance guide before touching the board.
• Never plug or unplug anything into the expansion connector with the machine switched on.
• A current-limited bench supply is the single most useful diagnostic tool here: a healthy +2 draws roughly 800 mA–1.2 A depending on RAM type. Set a limit around 1.5 A — a board that immediately goes into current limiting has a shorted chip.

ZX Spectrum 128 circuit diagram, sheet 1. The grey +2 is functionally a repackaged 128, so these diagrams describe the same architecture. Sheets 2 and 3 are also on the wiki.

Normal power-on

A healthy machine shows the 128 menu (128 BASIC / Calculator / 48 BASIC / Tape Loader / Tape Tester). If it reaches this screen, most of the system is working.

Supply and clock checks

The on-board supply derives everything from the external unregulated input:

Rails and where they come from

Rail	Source	Feeds
+5 V	7805 regulator	All logic
+12 V	TR4/TR5 inverter, rectified and smoothed by D15/C44	Video output and the RS232 drivers
−12 V	Charge pump C111/C112 with D28/D29 off the same square wave	RS232 drivers
−5 V	Derived from the −12 V rail via zener D19	Expansion port

The expansion connector carries +5 V (pin 3A), pulsed +12 V (23B), +12 V (23A), −5 V (20B), +9 V unregulated (4A) and ground (6A, 7A, 14A).

Clocks — expected oscilloscope readings

Signal	Test point	Expected
Master crystal	IC37 pin 6	17.73447 MHz, no jitter
Z80 clock	IC2 pin 6	3.54689 MHz, no jitter
Colour encoder	IC1 pins 46, 47	8.8 MHz, no jitter
Sound carrier	IC38 pin 4	Within 2 kHz of 6.0 MHz (UK) or 5.9 MHz (Europe)

Source: 128 Service Manual. If the sound-carrier frequency is out of tolerance it can be adjusted per the setting-up instructions.

The external supply should hold up under load: at minimum mains input (215 V AC) and 1.4 A output the voltage trough should not fall below 7.0 V, and at maximum input (265 V AC) and 600 mA the peak should stay below 13 V.

If it does not initialise

With the rails and clocks proven, the 128 manual's sequence is:

1. Check +5 V and ground distribution to the ROM, Z80, ULA and RAM.
2. Check RD, WR, MREQ, D0–D7 and A0–A15 from the Z80 — all should be active immediately after a reset.
3. Check the RAS/CAS lines to the uncontended RAM area (IC15–IC22 in 128 numbering) — active immediately after reset.
4. Check the RAS/CAS lines to the contended RAM area (IC6–IC13 in 128 numbering). Press the RESET button to obtain a clear trace.
5. Confirm the ROM is enabled by an active-low signal at pin 20.
6. Check the bank register is loaded correctly — immediately after reset, pins 2, 5, 7, 10, 12 and 15 should be low.
7. Check the outputs on the RGB connector.

RAM faults

RAM is the most common electronic fault on a surviving +2.

The IC numbering trap

Most Spectrum diagnostic tools report faults using 48K Spectrum IC numbers. On the +2 board the mapping is:

+2 RAM IC numbering vs 48K Spectrum

+2 board	Equivalent 48K number	Notes
IC17–IC24	IC15–IC22	Upper set, same order
IC25–IC32	IC13–IC6	Lower set — note the reverse order

So a tool reporting "IC8 and IC13 faulty" means IC30 and IC25 on a +2; a report of "D4 on the lower RAM" corresponds to IC28.

Finding failed chips

• Shorted RAM pulls down the 5 V rail. On a current-limited supply the board goes straight into limiting and the offending chips get hot — remove the hottest first. It is common to find several.
• Diagnostic ROMs are far quicker than manual probing. Brendan Alford's ZX Diagnostics runs from the ROM socket, tests the lower 16 K, and indicates faulty bits with beeps and coloured bars even when the display RAM itself is bad. Paul Farrow's 128K RAM tester then exercises all the banks.
• Mixed RAM speeds and types on one board cause instability — a mixture of parts of different speed grades is not ideal.
• Where many chips in a bank have failed, replacing the whole bank (and socketing it) is often faster than chasing them individually.

Audio hum and buzzing — a known +2 fault

A distinctive and very common complaint on the grey +2:

Taiwanese-built +2s suffer from poor grounding, which produces buzzing or humming on the audio output. Because the audio output is live even when the Datacorder is not running, the hum is present all the time.

The cause is that the tape deck PCB uses the metal frame of the tape mechanism to route ground paths between different parts of the board. The fix is straightforward: check that the two screws fixing the tape PCB to the mechanism are tight and making a good electrical connection. Tightening or cleaning them usually cures it completely.

Separately, the AY-to-beeper mix is poorly balanced on all 128-family machines, so ULA sounds being much louder than AY music is normal behaviour, not a fault.

Datacorder faults

The integral cassette deck is the single most common thing to fail on a +2. Symptoms include failing to load or save, jamming completely, chewing tapes, and contributing hum to the audio. Causes include dirt and dust, heat, corrosion, physical damage to the keys, perished drive belts, mechanism wear, and previous bad repairs.

Symptom	Likely cause	Action
Motor runs but tape does not move, or speed is wrong	Perished or stretched drive belt	Replace the belt — the standard first job on any unserviced +2
Won't load, but the deck runs at the right speed	Dirty head, or head azimuth out of alignment	Clean the head with isopropyl alcohol; adjust azimuth for the sharpest, brightest tone
Chews or jams tapes	Mechanism wear, dirt, damaged keys	Strip, clean and inspect the mechanism
Constant hum on audio	Tape PCB grounding through the mechanism frame	Tighten/clean the two PCB-to-mechanism screws
Loading works from an external source but not the deck	Deck electronics or head	Compare against an external tape source injected at the EAR input

Belt replacement requires removing the six case screws and separating the halves, disconnecting the keyboard ribbons and the tape deck connector.

If the deck is beyond economical repair, a common modern approach is to bypass it and load from an external source or an SD-card solution.

Keyboard faults

The +2 uses a membrane under a moulded keyboard, and membrane failure is the usual cause of dead keys. A completely unresponsive keyboard is a reported fault mode.

Diagnosis follows the same logic as the 48K machines: a dead block of keys points to one membrane tail or its connector rather than to a chip. Check the tails are fully inserted and undamaged before suspecting the ULA. Replacement +2 membranes are still manufactured.

When refitting the tails, ease them in with a slight side-to-side motion and make sure they are square and fully home.

Power circuit faults

The ±12 V inverter uses the same transistor arrangement as the 48K Spectrum. On the +2 these rails feed the video output and the RS232 drivers rather than the RAM, so a failure here shows up as video or serial problems rather than the classic white screen.

If a power transistor has been replaced previously with an under-rated part, fit a ZTX651 — the same device as the ZTX650 normally fitted, with a higher voltage rating. The ZTX213 partner is generally reliable and can usually be left alone.

Thermal faults

The ULA was fitted with an Amstrad clip-on heatsink. Machines that have lost it should have one refitted, especially if the ULA has been replaced.

An intermittent fault that appears after a few minutes and clears on cooling, with nothing obviously getting hot, can indicate hairline cracks or a bad connection triggered by thermal expansion in the board — a real risk given the known poor PCB quality of these Amstrad-built boards.

Repair notes

From the 128 Service Manual, which applies equally here:

• Use recognised desoldering and heatsinking technique; only a suitably earthed, low-power soldering iron.
• When replacing a 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 when replacing the regulator.
• Replace the modulator as a complete unit.
• Use proper IC removal and insertion tools and avoid handling the pins.
• Take normal anti-static precautions.

Common fault catalogue

• Constant hum or buzz on audio — tape PCB grounding; tighten the two screws to the mechanism.
• Tape won't load or save — perished Datacorder belt, then head cleaning and azimuth.
• Board goes into current limiting, chips hot — shorted RAM; remove the hottest first.
• Random RAM errors, diagnostic reports a 48K IC number — translate using the mapping table (IC25–IC32 map to IC13–IC6 in reverse).
• Faults that appear when warm, nothing obviously hot — hairline cracks or thermal-expansion connection faults in the board.
• Dead keyboard or a dead block of keys — membrane or its tails.
• AY music quieter than beeper effects — normal for all 128-family machines, not a fault.
• No video or serial, logic otherwise fine — the ±12 V inverter.
• Missing ULA heatsink — refit one.

Related pages

• Sinclair ZX Spectrum +2
• Sinclair ZX Spectrum +2 General Maintenance
• Sinclair ZX Spectrum +2 Capacitor Replacement Guide
• Sinclair ZX Spectrum+ Troubleshooting Guide — the 48K machine
• Sinclair ZX Spectrum Troubleshooting Guide — the wider series guide
• Sinclair ZX Spectrum Keyboard Membrane Repair