Editing Sinclair ZX Spectrum+ General Maintenance

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[[File:Sinclair ZX Spectrum Plus main board Issue 6A.jpg|thumb|420px|A Spectrum+ Issue 6A board in the case. The 7805 and its heatsink sit centre-board; the ULA, Z80A, ROM and the two RAM rows are below, and the reset switch is wired in at the right.]]

This guide covers preventive maintenance for the '''[[Sinclair ZX Spectrum+]]'''. The machine is electrically a 48K Spectrum, so most of it applies to the rubber-key model too; the keyboard assembly and reset switch are Spectrum+ specific.

The maintenance priorities are the '''keyboard membrane and its clamped ribbons''', the '''7805 regulator and heatsink''', the '''edge connector''', and the ageing '''DC-DC converter''' that feeds the lower RAM.

== ⚠️ Safety ==

There are '''no mains voltages inside the computer''' — it runs from an external 9 V supply and the highest voltage on the board is +12 V. The mains transformer is inside the PSU brick, which can be opened and repaired (see below) by someone comfortable with mains wiring.<ref name="ian">Worsley, Ian. ''Repairing Spectrum &amp; Spectrum Plus'', Lil Old Sinclair Technical Information Repository. Source for the PSU repair procedure, keyboard ribbon handling and Spectrum+-specific keyboard fixes, socket re-tensioning and the multimeter diagnostic approach.</ref>

Observe normal anti-static precautions for the ULA, Z80A, ROM and RAM.<ref name="sm48">''ZX Spectrum Service Manual'' (48K), Sinclair Research Ltd. Source for the repair notes, supply test points, keyboard matrix, mandatory checks and the Repair Data section.</ref>

== ⚠️ The golden rule ==

'''Never plug or unplug anything into the expansion edge connector while the machine is switched on.''' This single habit is reported as the cause of '''80–90 % of Spectrum faults''': the keyway peg on the interface plug drops out with use, the connector shifts sideways, and the bus lines short together — taking out TR4 and often the RAM with it.<ref name="ian" />

If an interface has lost its keyway peg, cut a replacement from the hinge section of an old cassette box and glue it in.<ref name="ian" />

== Identifying your board ==

The Spectrum+ shipped with several 48K board issues — '''4A, 4B, 4S, 5''' and '''6A'''. The issue number is printed on the PCB. It matters because, for example, Issue 4A and 4B route /RAS through spare inverters and therefore '''will not run with IC24 removed''', and because the DC-DC converter components differ between issues.<ref name="sfe">[https://www.spectrumforeveryone.com/technical/zx-spectrum-models/ "ZX Spectrum Models"], Spectrum for Everyone.</ref> Full detail is on the [[Sinclair ZX Spectrum+]] page.

Samsung-built '''Issue 4S''' machines (serials beginning S01) were supplied with noticeably better keyboard membranes, many of which still work.<ref name="sfe" />

== Opening the case ==

# Unplug the power and all peripherals.
# Turn the machine over and remove the case screws.
# Lift the top carefully — the keyboard is connected to the board by '''two flexible membrane tails''', and these are fragile.
# Free the tails by pinching the ribbon next to the connector and easing it straight up. Do not pull at an angle.

When refitting, insert each tail with a '''slight side-to-side movement while easing it gently down''' into the connector, and make sure it is fully home and not kinked.<ref name="ian" /><ref name="sm48" />

== Keyboard ==

The Spectrum+ keyboard is a moulded key assembly over a membrane, held by a backing plate with nine screws.

* '''Routine care''': keep the tails clean and undamaged. Most Spectrum keyboard faults are simply membrane failure — the pads under the keys wear out or the ribbon cracks with age.<ref name="ian" />
* '''Ribbon repair''': if the tail 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" />
* '''Bottom-row keys failing''' is a known Spectrum+ complaint that often does '''not''' need 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 when reassembling.<ref name="ian" />
* '''A key that repeats endlessly''' is caused by the backing plate being clamped too tight. Slacken the screw nearest that key by a quarter turn.<ref name="ian" />

Sinclair's own view was that keyboards take a lot of battering and are usually better replaced complete, replacements being inexpensive.<ref name="sm48" /> Replacement Spectrum+ membranes are still manufactured.

== Cooling and the 7805 regulator ==

The 7805 does all the heavy lifting and the Spectrum runs warm by design.

* Check the '''heatsink is tight and undamaged'''. The Service Manual makes checking the heatsink the '''first''' step for any reported fault, and warns about over-tightened screws.<ref name="sm48" />
* Look for '''hairline cracks in the board radiating from the heatsink screws''' — a documented cause of intermittent faults.<ref name="sm48" />
* Apply fresh thermal compound when refitting the regulator, and ensure good contact between the regulator body and the heatsink.<ref name="sm48" />
* '''In normal running the ULA runs quite warm; every other IC should be cold.''' A warm RAM chip is a failing RAM chip.<ref name="ian" />
* If the machine develops faults only when warm and the heatsink is sound, the ULA is the cause — and if the ULA has been replaced before, extra heatsinking on it is worthwhile.<ref name="sm48" />

== Supply rail checks ==

{| class="wikitable styled-table" style="width:95%; text-align:left;"
|+'''Spectrum+ supply test points (Service Manual)'''
! Measurement !! Test point !! Expected
|-
| Regulator input || +ve side of C50 || +9 V DC ± 2.0 V (below +7 V the regulator may misbehave)
|-
| Regulator output || +ve side of C34 || +5 V DC ± 0.25 V, no discernible ripple
|-
| +5 V to RAM || IC6 pin 9 || +5 V DC ± 0.25 V
|-
| −5 V to RAM || IC6 pin 1 || −5 V DC
|-
| +12 V to RAM || IC6 pin 8 || +12 V DC
|-
| +12 VA (IC14 only) || Left end of C52 || +12 V DC
|}

A practical check at the regulator itself: '''4.85–5.2 V''' at the 7805 output is normal.<ref name="ian" />

The −5 V and +12 V rails come from the on-board DC-DC converter (TR4/TR5 and the coil), which is the most failure-prone circuit in the machine. With it running you should be able to '''hear the coil buzzing faintly'''.<ref name="ian" />

== Edge connector and sockets ==

* Clean the '''expansion edge connector''' fingers with a soft eraser and contact cleaner. Tarnished contacts cause crashes and phantom faults.<ref name="sm48" />
* Check the '''power socket''' — it is a common source of intermittent resets when the machine is moved. If it is loose, '''replace it; do not attempt to tighten it'''.<ref name="ian" />
* The '''EAR and MIC sockets''' lose contact tension with age, causing intermittent loading. They can be re-tensioned by carefully pushing the spring contact downwards and inwards until it touches the case, rather than replacing the socket.<ref name="ian" />

== External power supply ==

The PSU brick is a mains transformer, rectifier and smoothing capacitor delivering 9 V DC through a '''2.1 mm centre-negative''' plug. It is repairable:<ref name="ian" />

* The usual failure is the cable breaking in the moulded plug or where it exits the case.
* Refit a new 2.1 mm plug with '''negative to the centre''' (the traced wire) and fill the cover with silicone rubber for strain relief.
* At the PSU end, split the case, cut back the damaged cable, tie a double knot about three inches down so the knot sits inside the case, and resolder.
* A shorted output blows the '''thermal fuse''' and can take out the '''1N4007 rectifiers'''. The thermal link can be replaced with a '''160 mA slow-blow 20 mm fuse''' soldered across it. A failed transformer means the unit is scrap.

== Capacitor health ==

The Spectrum's electrolytics are ordinary leaded parts and the machine is not a systematic "recap" case, but ageing capacitors do cause real symptoms — the Service Manual's own test for '''wavy lines across the screen''' is to press down on each electrolytic in turn and see whether the lines disappear.<ref name="sm48" /> Full detail: [[Sinclair ZX Spectrum+ Capacitor Replacement Guide]].

== Common failure points ==

{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''Spectrum+ — failure-prone items'''
! Item !! Symptom !! Action
|-
| Edge connector / hot-plugged interfaces || Sudden death, blown TR4, dead RAM || Never hot-plug; fit a keyway peg; inspect the connector
|-
| TR4 / TR5 / coil (DC-DC converter) || White or patterned screen; no −5 V or +12 V || Replace TR4 (ZTX650) and TR5 (ZTX213) together with the coil if suspect
|-
| Lower RAM (IC6–IC13, 4116) || Patterned screen, warm chips || Locate with the POKE test; replace
|-
| ULA (IC1) || Faults when warm, no sound, character break-up || Substitute; add heatsinking
|-
| Keyboard membrane and tails || Dead keys, repeating keys || Clean/re-pack the clamped ribbons; slacken backing-plate screws; replace membrane
|-
| 7805 and heatsink || Low or missing +5 V; warm-only faults || Check mounting, re-grease, replace regulator
|-
| Power socket and PSU lead || Random resets when moved || Replace the socket; repair the PSU lead
|-
| EAR/MIC sockets || Intermittent loading || Re-tension the contacts
|}

== Recommended tools ==

* Small Philips/Pozidriv screwdrivers and an anti-static strap.
* Digital multimeter (20 V DC and 20 kΩ ranges cover most of the diagnostic sequence).
* Temperature-controlled soldering iron, solder wick, fresh leaded solder and flux.
* IC extraction tool.
* Isopropyl alcohol, cotton buds, contact cleaner, a soft eraser for the edge connector.
* Thermal compound for the 7805.
* Replacement keyboard membrane.
* Optionally a diagnostic test ROM (for example ZX Diagnostics) for fast RAM testing.

See [[Recommended Tools]] for the general toolkit.

== Preventive maintenance checklist ==

# Check the heatsink is tight, undamaged and well-mated; inspect the board around its screws for hairline cracks.
# Verify +5 V, −5 V and +12 V at IC6.
# Listen for the DC-DC converter coil buzzing.
# Clean the edge connector, EAR/MIC sockets and power socket; replace a loose power socket.
# Inspect and clean the keyboard tails; re-pack the ribbon clamps.
# Feel the ICs after a few minutes' running — only the ULA should be warm.

== Related pages ==

* [[Sinclair ZX Spectrum+]]
* [[Sinclair ZX Spectrum+ Troubleshooting Guide]]
* [[Sinclair ZX Spectrum+ Capacitor Replacement Guide]]
* [[Sinclair ZX Spectrum Keyboard Membrane Repair]]
* [[Sinclair ZX Spectrum General Maintenance]]
* [[Capacitor Failure Symptoms]] · [[Recommended Tools]]

== References ==

<references />

[[Category:Sinclair Computers]]
[[Category:Maintenance Guides]]