Josh: Add the relevant circuit diagram already hosted here

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Add the relevant circuit diagram already hosted here

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			[[File:Sinclair ZX Spectrum 48K circuit diagram.png\|thumb\|420px\|The 48K Spectrum circuit diagram from the Service Manual. Every designator and pin number quoted below can be located on it; the Issue 6A diagram is also on the wiki.]]
	This guide gives fault diagnosis for the '''[[Sinclair ZX Spectrum+]]'''. Because the Spectrum+ is electrically a 48K Spectrum in a different case, everything here applies equally to the late rubber-key 48K machine; the Spectrum+-specific material is the keyboard assembly and the reset switch.		This guide gives fault diagnosis for the '''[[Sinclair ZX Spectrum+]]'''. Because the Spectrum+ is electrically a 48K Spectrum in a different case, everything here applies equally to the late rubber-key 48K machine; the Spectrum+-specific material is the keyboard assembly and the reset switch.

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Josh: Create ZX Spectrum+/+2 page set (service manuals + community repair sources, cited)

2026-08-08T20:17:19Z

Create ZX Spectrum+/+2 page set (service manuals + community repair sources, cited)

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This guide gives fault diagnosis for the '''[[Sinclair ZX Spectrum+]]'''. Because the Spectrum+ is electrically a 48K Spectrum in a different case, everything here applies equally to the late rubber-key 48K machine; the Spectrum+-specific material is the keyboard assembly and the reset switch.

It draws on the official ZX Spectrum Service Manual — including its "Repair Data" field-service section — and on the repair knowledge built up by the Spectrum community over four decades.<ref name="sm48">''ZX Spectrum Service Manual'' (48K), Sinclair Research Ltd. Source for the basic-checks test-point table, the clock and address/data line checks, the RAM fault-location table, the keyboard matrix, the Repair Data symptom list (symptoms 1–11) and the capacitor parts list.</ref><ref name="ian">Worsley, Ian. ''Repairing Spectrum & Spectrum Plus'', Lil Old Sinclair Technical Information Repository. Field-service repair guide; source for the multimeter-only diagnostic sequence, the DC-DC converter procedure, the faulty-screen catalogue, Spectrum+-specific keyboard fixes, loading/sound faults, PSU repair and the RAM location chart.</ref><ref name="sfe">[https://www.spectrumforeveryone.com/technical/zx-spectrum-models/ "ZX Spectrum Models"], Spectrum for Everyone. Source for board-issue differences, ULA types and their relative reliability.</ref>

== ⚠️ Before you start ==

* '''There are no mains voltages inside the computer.''' The Spectrum+ runs from an external 9 V supply; the highest voltage on the board is +12 V. The mains transformer is inside the external PSU brick.
* '''Never plug or unplug anything into the expansion edge connector with the machine switched on.''' This is not a general caution — it is reported as the primary cause of '''80–90 % of Spectrum faults'''. The keyway locating peg on the interface plug drops out with use, allowing the connector to move sideways and short the bus lines together. Because most of the ICs hang off that bus, the damage can be extensive.<ref name="ian" />
* If the keyway peg is missing from an interface, make a replacement: cut the hinge section from an old cassette box, shape a peg from it and glue it in place.<ref name="ian" />
* Hairline solder splatter can measure several ohms and produce very misleading symptoms — inspect visually before chasing anything complicated.<ref name="sm48" />

== Start here: the physical check ==

Sinclair's own Repair Data section opens by noting that '''around half of machines returned as faulty had no real fault''' — the problems were loose or badly fitted parts. Before any measurement:<ref name="sm48" />

# Check the '''heatsink''' first on any reported fault. Is it loose? Are the screws over-tightened?
# Look for dry joints, cracks in the print, cracked boards, badly fitted ICs, and dirty or tarnished contacts, pins, holders and edge connectors.
# Check for damaged keyboard tails.
# Inspect any add-ons and interfaces for broken solder caused by the user wiggling them, and note whether an add-on is itself loose or faulty.
# Look for hairline cracks and breaks in the board, '''especially radiating from the heatsink screws'''.

Knowing what add-ons the machine was used with is a diagnostic clue in itself: the manual specifically says to '''suspect a blown TR4 and/or destroyed RAM if a Kempston interface was used'''.<ref name="sm48" />

== Supply checks ==

{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''Basic checks, in order of priority (Service Manual)'''
! Function !! Test point !! Expected
|-
| Voltage regulator input || +ve side of C50 || +9 V DC ± 2.0 V. Below +7 V the regulator may not operate correctly
|-
| Voltage regulator output || +ve side of C34 || +5 V DC ± 0.25 V, no discernible ripple
|-
| On-board +5 V || IC6 pin 9 || +5 V DC ± 0.25 V, no discernible ripple
|-
| On-board −5 V || IC6 pin 1 || −5 V DC
|-
| On-board +12 V || IC6 pin 8 || +12 V DC
|-
| +12 VA (to IC14 only) || Left end of C52 || +12 V DC
|-
| Clock || IC1 pin 32, TR3 base, IC2 pin 6 || 14 MHz square wave at +5 V amplitude
|-
| Address and data lines || Right side of R17–R23 and R1–R8 || 3.5 V amplitude
|}

Source: Service Manual.<ref name="sm48" /> If these pass, check IC1 pin 14, IC2 pin 11 and IC5 pin 28 for +5 V, and ground at IC1 pin 40, IC2 pin 29 and IC5 pin 14. If all is well and the ULA is socketed, replace IC1.<ref name="sm48" />

=== The community multimeter-only sequence ===

Ian Worsley's field guide gives a diagnostic path needing only a multimeter on the 20 V DC and 20 kΩ ranges, with the negative lead on the modulator case:<ref name="ian" />

# '''Power up briefly and measure the 7805 output''' (right-hand leg, tab away from you). '''4.85–5.2 V''' is normal.
#* Higher than that — change the regulator, and hope not too many chips have been taken out.
#* Lower, but not below 4.3 V — there is a short somewhere. Feel the chips: '''the ULA runs quite warm, everything else should run cold.'''
#* Missing altogether — replace the regulator, but do not run the machine yet.
# Switch to 20 kΩ. Measure '''pin 1 of IC6''': it should read '''above 12 kΩ'''. If lower, nick the track to the right of C4 (above IC6).
# Measure '''pin 8 of IC6''': if below 700 Ω, nick the track to the left of C44 (100 µF 16 V, bottom right of IC6).
# Leave the tracks cut, power up, and measure on 20 V DC to the right of each cut: the first should read '''−4.5 to −5.2 V''', the second '''+11.5 to +12.3 V'''.

This isolates the DC-DC converter from the RAM it feeds, which is the crux of most Spectrum faults.

== The DC-DC converter — the classic Spectrum fault ==

The 4116 lower RAM needs −5 V and +12 V, generated on-board by an inverter around '''TR4 (ZTX650)''' and '''TR5 (ZTX213)'''. When it fails, the lower RAM has no supply and the machine shows a white or patterned screen.<ref name="ian" /><ref name="sm48" />

'''If −5 V or +12 V is missing or low:'''<ref name="ian" />

* Replace '''TR4 (ZTX650)''' and '''TR5 (ZTX213)'''. Do not substitute other transistor types here — they will not work. (A ZTX651 is an acceptable TR4 substitute, being the same part with a higher voltage rating.)<ref name="tyne">Tynemouth Software, [http://blog.tynemouthsoftware.co.uk/2017/06/spectrum-128k-2-grey-repairs.html "Spectrum 128K +2 Grey Repairs"]. Source for the ZTX651 substitution, RAM IC mapping between 48K and +2 boards, typical +2 current draw and the use of diagnostic test ROMs.</ref>
* Check '''R79 (2k2, Issue 6)''', '''R62 (15 Ω, Issue 6)''' or '''R55 (56 Ω, Issue 4 and earlier)''' for discolouration. If a 56 Ω resistor is badly discoloured, change '''D15 (BA157)'''.
* Check the zener and signal diodes: on '''Issue 6/6A''' these are '''D19 (BZY88 5.1 V)''', '''D18''' and '''D11''' (1N4148); on '''Issue 3 and 2''', '''D16 (BZY88 5.1 V)''', '''D11''' and '''D12'''.
* There is roughly a '''20 % chance the coil has shorted turns'''. With the converter working you should be able to '''hear the coil buzzing'''.

The Service Manual's Repair Data adds an important warning: '''TR4, TR5 and the coil often fail together'''. If you replace only TR4 it can fail again immediately because the others are still faulty — check all three.<ref name="sm48" />

'''No coil buzz and the 7805 open circuit''' — before replacing the regulator, check the heatsink for damage or bad fitting, remedy any physical defect in the print or damage around the screws, and check TR4/TR5 and the coil.<ref name="sm48" />

'''Continual failure of power supply transistors''' — verify all modifications are present and the edge connector is sound and not loose or damaged; check the peripherals are properly fitted and establish whether the owner is hot-plugging add-ons. Check the plug, socket and cable, and check the mains for interference from other equipment. If all that is satisfied, confirm 0 V is correct on earth and monitor it while running to catch an intermittent, then work systematically through the capacitors and resistors to earth.<ref name="sm48" />

== Reading the screen ==

The failure pattern on screen is the fastest route to the fault:<ref name="ian" />

{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''Faulty screen designs'''
! What you see !! Likely cause and action
|-
| '''Alternate thick black and white vertical bars''', with or without flashing || Check the CPU (IC2) is seated correctly and any cut legs resoldered; check the ROM (IC5). Test or change '''TR6''' and '''TR3''' (both ZTX313). Then re-measure legs 2–14 of IC6–IC13
|-
| '''Bright, shimmering, fine herring-bone pattern''' filling the screen || Check for −5 V on pin 1 of IC6 — it may have vanished. '''Change TR4'''
|-
| '''Black screen with one or more thin vertical dashed lines''' || Almost certainly one or more 4116 lower RAM chips. Also check the ULA for straddling, bent or damaged legs
|-
| '''Blank black screen, nothing else''' || Likely one of the upper RAM chips IC15–IC22
|-
| '''Flashing blocks''', or a mixture of the above || Check both legs of any upper-memory chip that was cut are resoldered; check the ROM is the correct type for the fitted links and correctly seated
|-
| '''Character break-up when the machine warms up''' || Nearly always the ULA (IC1)
|-
| '''Black dashes or dots appear when warm, then the machine locks up''' || Typical of the lower RAM (IC6–IC13), or the ULA. Do a resistance check on pin 1 while the machine is still warm
|}

The Service Manual adds that '''various faults which appear only when warm''' should start with a thorough heatsink check; if the heatsink is sound, the ULA is the cause, and if the ULA has been replaced before it may be worth adding extra heatsinking to it.<ref name="sm48" />

== Finding a faulty RAM chip ==

If the machine boots but a RAM fault is suspected, the Spectrum can name the failed chip itself.<ref name="sm48" /><ref name="ian" />

Type:

PRINT PEEK 23732 + PEEK 23733 * 256

A healthy machine returns '''65535''' (48K) or '''32767''' (16K). Any lower value ''n'' means the first faulty location is '''n + 1'''. A result below 32767 puts the fault in the lower 16 K (IC6–IC13); above 32767, in the upper 32 K (IC15–IC22).

Then, using that address:

POKE n+1,85 : PRINT PEEK n+1
POKE n+1,170 : PRINT PEEK n+1

85 and 170 are 01010101 and 10101010, so between them they exercise every bit. If either value reads back wrong, look it up:

{| class="wikitable styled-table" style="width:90%; text-align:center;"
|+'''RAM fault location table'''
! Reads back after POKE 85 !! Reads back after POKE 170 !! Error size !! Bit !! Lower RAM (<32767) !! Upper RAM (>32767)
|-
| 84 || 171 || 1 || 0 || IC6 || IC15
|-
| 87 || 168 || 2 || 1 || IC7 || IC16
|-
| 81 || 174 || 4 || 2 || IC8 || IC17
|-
| 93 || 162 || 8 || 3 || IC9 || IC18
|-
| 69 || 186 || 16 || 4 || IC10 || IC19
|-
| 117 || 138 || 32 || 5 || IC11 || IC20
|-
| 21 || 234 || 64 || 6 || IC12 || IC21
|-
| 213 || 42 || 128 || 7 || IC13 || IC22
|}

Values from the Service Manual.<ref name="sm48" /> If more than one location is faulty, the first must be repaired before the test can proceed past it.

If the value read back is not in the table at all, suspect bent or straddling legs on socketed upper RAM, or faults in '''IC23 (74LS32)''', '''IC24 (74LS00)''', '''IC25''' or '''IC26 (74LS157)'''.<ref name="ian" />

'''Modern alternative.''' Purpose-built diagnostic ROMs make this much faster: Brendan Alford's '''ZX Diagnostics''' test ROM will run from the ROM socket and identify bad bits even when the display RAM itself is faulty, reporting the failing data lines and IC numbers.<ref name="tyne" />

=== Checking for shorted or leaky RAM ===

* After the supply rails are proven, feel each lower RAM chip. '''None should be even warm''' — any that is should be removed.<ref name="ian" />
* The manual's method is to switch on and check whether any RAM gets hot; a very hot chip is a sure sign it is short circuit. For open-circuit failures, bypass each RAM in turn with a known-good device — and note that after switching off you should '''wait about ten seconds before removing the test chip''', or it will damage itself.<ref name="sm48" />

== Keyboard faults ==

The matrix is eight blocks of five keys horizontally and five blocks of eight vertically. '''A dead block of five''' points to the KB2 circuitry or the 8-way membrane tail; '''a dead block of eight''' points to KB1 or the 5-way tail.<ref name="sm48" />

* '''Entire keyboard dead''' — check the 5 V supply is reaching it before condemning the ULA. The commonest cause is a faulty '''10 kΩ resistor in the R65–R69 group''', identified by which keyboard column has failed.<ref name="sm48" />
* '''Keys fail when the machine is warm''', with a known-good ULA — replace the membrane.<ref name="ian" />
* Most keyboard faults are simply membrane failure: the pads under the keys wear out, or the ribbon cracks with age.<ref name="ian" />

=== Spectrum+ specific keyboard fixes ===

Two faults belong to the Spectrum+ keyboard assembly in particular:<ref name="ian" />

* '''Some keys not working, especially on the bottom row.''' Both ribbon cables are clamped to the keyboard case. Remove the clamps and carefully clean between the layers of each ribbon — around the clamped area — using a cotton bud and WD-40. When re-clamping, add extra packing between the ribbon and the clamp. This often avoids a membrane replacement entirely.
* '''A character printing repeatedly.''' This is caused by the keyboard membrane backing plate being clamped too tightly. Remove the keyboard top to expose the plate and its nine screws, then '''slacken by a quarter turn the screw or screws nearest the offending key'''.

When handling the tails, insert them with a slight side-to-side movement while easing the ribbon gently down into the connector. If the 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.<ref name="ian" />

== Loading faults ==

{| class="wikitable styled-table" style="width:100%; text-align:left;"
! Symptom !! Action
|-
| Loading bars appear but no program name || Almost always the tape deck. Clean the head and '''adjust the azimuth''' — with a tape playing, turn the azimuth screw for the sharpest possible note. If the note sounds at all muffled the Spectrum will not load from it
|-
| Program name shows as '''????????''' || Volume set too high — reduce it
|-
| Intermittent loading || Check the tension of the EAR/MIC socket contacts; they can be re-tensioned by pushing the spring contact down and inward rather than replacing the socket
|-
| No loading bars at all, tape deck known good || Change the ULA (IC1) first, then check the MIC socket, then '''D9 (1N4148)''' behind the MIC socket
|}

Sources: Service Manual Repair Data and the community repair guide.<ref name="sm48" /><ref name="ian" /> The manual notes that a recorder can play music perfectly and still be misaligned for the Spectrum, and that it is better to set volume for maximum treble than to use the midpoint.<ref name="sm48" />

== Sound faults ==

Loads correctly but no sound:<ref name="ian" />

# Check the ULA (IC1) first — faulty or missing sound most often identifies the ULA, which is the likeliest suspect in any case.<ref name="sm48" />
# Enter <code>10 BEEP 1,1 : GOTO 10</code> and RUN it, then disconnect the keyboard and hold a replacement speaker across the existing one. If you hear a tone, replace the speaker.
# On Issue 3 and later, change '''TR7 (ZTX450''', ZTX650/1 also acceptable) and '''D9 (1N4148)''', found to the left of the speaker.

== Video and colour faults ==

* '''No colour.''' On Issue 2, adjust TC2, VR1 and VR2 first. On other issues, change the '''LM1889 (IC14)''' — though it is a very reliable part. Some later boards were fitted with an '''SN-series device in the IC14 position, and this was always the cause of no colour'''; it should be replaced with an LM1889, which the board still has provision to accept despite the different pin count.<ref name="ian" /><ref name="sm48" />
* '''No video input at IC14 pin 12''' — check the waveform at IC14 pin 13; if correct, check TR1 and TR2. Check the waveforms at IC1 pins 15, 16 and 17; if correct, check VR1 and VR2 on Issue 1 and 2 boards. Then change IC1. If +5 V and video input are both correct, change the modulator.<ref name="sm48" />
* '''Dark screen''' — check tuning, then change the modulator.<ref name="sm48" />
* '''Dim Sinclair logo''' — check +12 V, check C65 for high impedance, check R52, and check +5 V on IC14 pin 3.<ref name="sm48" />
* '''Corrupt paper after a ULA change''' — check the ULA socket, check the power socket is not intermittent, and check IC1 pin 14 for +5 V with no discernible ripple.<ref name="sm48" />
* '''No 3 MHz at IC2 pin 6''' — if the amplitude is low check R25; check TR3; check IC1 pin 32 for 3 MHz and IC1 pin 39 for 14 MHz; check crystal X1; check IC1 pin 13 for +3 V.<ref name="sm48" />
* '''Wavy lines across the screen''' — press down on each electrolytic capacitor in turn and see whether the lines disappear; if so, that capacitor is the culprit.<ref name="sm48" />

== Power supply faults ==

The external PSU is a common failure point, and it is repairable:<ref name="ian" />

* The usual fault is the wire snapping or going intermittent '''in the moulded plug or where the lead leaves the case'''.
* To fix the plug end, fit a replacement 2.1 mm plug with '''negative to the centre''' (the traced wire), then fill the plastic cover with silicone rubber before screwing it up — this prevents internal shorts and takes strain off the solder joints.
* At the PSU end, split the case, cut off the faulty section, tie a double knot about three inches down the cable, slide protective sleeving on, and resolder. The knot inside the case stops the joints being strained.
* A bare wire shorting the output blows the '''thermal fuse''' and/or takes out the '''1N4007 rectifiers'''. The thermal fusible link can be replaced by spot-soldering a '''160 mA slow-blow 20 mm fuse''' across the old device. A faulty transformer, however, means the unit is scrap.

'''Intermittent crashes or resets, especially when the machine is moved''', are usually the power plug or the power socket. Replace the socket — do not try to tighten it.<ref name="ian" />

== Intermittent and warm-only faults ==

On a comparatively young machine, intermittent faults are most likely poor contacts or soldering from manufacture. Check all holders and pins for tarnish and dry joints; replace faulty holders by cutting them out and fitting new ones. Look for parts that were poorly soldered or not soldered at all. Finally check the board for hairline cracks and breaks, '''especially radiating from the heatsink screws'''.<ref name="sm48" />

== If the power supply is fine but the machine still misbehaves ==

Work through single then multiple RAM failure as above. If the RAM is sound, or the fault is too severe to test RAM, check that '''+5 V is reaching all the ICs in the order IC1, IC2, IC24, IC23, IC3, IC4, IC26, IC25 and IC5''', and replace whatever open-circuit component is cutting the supply.<ref name="sm48" />

== Common fault catalogue ==

* '''Machine dead after using an interface''' — edge-connector damage from hot-plugging; check TR4 first, then the RAM.
* '''White or patterned screen''' — DC-DC converter (TR4/TR5/coil) or lower RAM.
* '''Herring-bone pattern''' — −5 V missing; change TR4.
* '''Thin vertical dashes on black''' — lower RAM, or ULA legs.
* '''Blank black screen''' — upper RAM.
* '''Character break-up when warm''' — ULA.
* '''Won't load, bars but no filename''' — tape azimuth.
* '''No sound''' — ULA, then speaker, then TR7/D9.
* '''No colour''' — LM1889, or an SN-series substitute in IC14.
* '''Dead keyboard''' — 5 V supply, then the R65–R69 10 kΩ resistors, then the membrane.
* '''Bottom-row keys dead (Spectrum+)''' — clean and re-pack the clamped ribbons.
* '''A key repeats endlessly (Spectrum+)''' — slacken the backing-plate screws nearest that key.
* '''Random resets when moved''' — power plug or socket.

== Related pages ==

* [[Sinclair ZX Spectrum+]]
* [[Sinclair ZX Spectrum+ General Maintenance]]
* [[Sinclair ZX Spectrum+ Capacitor Replacement Guide]]
* [[Sinclair ZX Spectrum Troubleshooting Guide]] — the wider Spectrum series guide
* [[Sinclair ZX Spectrum Keyboard Membrane Repair]]
* [[Sinclair ZX Spectrum +2 Troubleshooting Guide]]

== References ==

<references />

[[Category:Sinclair Computers]]
[[Category:Troubleshooting Guides]]

Sinclair ZX Spectrum+ Troubleshooting Guide - Revision history

Josh: Add the relevant circuit diagram already hosted here

Josh: P5 verification: correct figures and re-attribute the Repair Data citations to the T.I.S. document now hosted here; remove emoji headings

Josh: Create ZX Spectrum+/+2 page set (service manuals + community repair sources, cited)