Sinclair ZX Spectrum General Maintenance

Sinclair ZX Spectrum 48K main board, Issue 3B, with the heatsink removed to expose the regulator and the DC-DC converter area at the upper right.

Routine care for the original rubber-key Sinclair ZX Spectrum (16K and 48K), board Issues 1 to 6A. The figures on this page come from Sinclair's own ZX Spectrum Service Manual, which is hosted on this wiki, and from the Issue 3 circuit diagram.

For the later machines see Spectrum+, Spectrum 128 and Spectrum +2.

Observe ESD precautions when the board is out of the case. Sinclair's own instruction is to take normal anti-static precautions when handling ICs and to use only a suitably earthed, low-power soldering iron.

What is on the board

ZX Spectrum 16K/48K main integrated circuits

Ref	Part	Notes
IC1	Ferranti ULA, 40-pin	Video, keyboard scanning, tape I/O, chip selects and RAM multiplexing. Fitted in a socket
IC2	Z80A or µPD780	The CPU
IC3, IC4	74LS157	Address multiplexers for the lower RAM. Must not be National Semiconductor parts
IC5	Spectrum ROM, 28-pin	A single 16 KB ROM at 0000–3FFF, from NEC or Hitachi. Links marked H and N on the board select which
IC6–IC13	4116, 150 ns	Lower 16K of RAM. Eight 16k-bit DRAMs
IC14	LM1889	Colour encoder
IC15–IC22	TI 4532 or OKI MSM3732	Upper 32K of RAM, 48K machines only. Eight 32k-bit DRAMs
IC23, IC24	74LS32, 74LS00	48K machines only; not used on Issue 5 and 6A
IC25, IC26	74LS157	48K machines only, not National parts; not used on Issue 5 and 6A
IC27	Mullard ZX 84/01	Issue 5 and 6A only — a second ULA replacing six 74LS packages
IC28	74LS04	Issue 5 and 6A only
REG	7805	5 V regulator, bolted to a heatsink with a 4BA screw, shakeproof washer and nut
MOD	Astec UM1233	UHF modulator, channel E36 vision carrier (UK)
TR4 / TR5	ZTX650 or TIPP31 / ZTX213	The inverter that generates the −5 V and +12 V rails
X1 / X2	14.000000 MHz / 4.433619 MHz	Master and chrominance sub-carrier crystals
KB1 / KB2	5-way / 8-way	Keyboard tail connectors, Molex 5229
Loudspeaker	40 Ω, TV type	Driven by TR7 (ZTX450) through D9

Which ULA

The 16K/48K Spectrum was fitted with several ULA revisions, and they are not freely interchangeable with board issues.

• 5C102E — Issue 1 and a small number of Issue 2 boards. Has a timing fault cured by a 74LS00 on a small board and spider, the so-called cockroach modification. Fitted to approximately 40,000 units.
• 5C112E (and -2, -3) — Issue 2. Needs either the transistor modification (TR6, ZTX313) or the diode-and-resistor modification; TR6 became part of the board layout from Issue 3.
• 6C001E-5 — rare, late Issue 2 only.
• 6C001E-6 — late Issue 2 and most Issue 3/3B boards.
• 6C001E-7 — the final 48K revision, and the only one documented to work on every 48K board from Issue 1 to Issue 6A. Required from Issue 4A onwards.

7K010E is not a 16K/48K part. It belongs to the Spectrum 128 and the grey +2, which output RGB rather than YUV.

Several resistor values change with the ULA fitted: with a 6C001 in an Issue 2 board, R47 becomes 1 K, R49 10 K, and R56 and R63 470 R.

Regular cleaning

Case and keyboard

• Wipe the case with a damp cloth and mild detergent.
• The rubber key mat can be removed and washed in warm soapy water. Dry it completely before refitting.
• The keyboard membrane is the fragile part. Do not flex or fold it, and withdraw the tails straight out of KB1 and KB2 — Sinclair's instruction when refitting a keyboard matrix is that the ribbon connectors must be fully inserted and not kinked during insertion.
• If the keyboard template lifts at the top edge, Sinclair's fix was a 6 mm strip of double-sided tape along that edge, which early boards lacked.

See Sinclair ZX Spectrum Keyboard Membrane Repair for membrane work.

Board

• Disconnect the power and all leads first.
• Remove dust with a soft antistatic brush or dry air.
• Reseat the socketed ICs — IC1 is socketed on every board, and on a 16K machine the expansion-RAM positions are sockets too.
• Clean the edge-connector fingers and check for pitting or bent contacts.

Power supply

Inside an original Sinclair ZX Spectrum power pack: mains transformer, bridge rectifier and a single smoothing capacitor.

The Spectrum takes an unregulated 9 V DC supply through a 2.1 mm coaxial socket — not the 3.5 mm jack used by the ZX81, although provision for a 3.5 mm socket remained on the board up to Issue 4B.

External supply

Property	Value	Source
Connector	2.1 mm coaxial socket
Polarity	Centre negative, outer positive. This is the opposite of most modern adaptors, and of the ZX81 — getting it wrong will damage the machine
Type	DC, unregulated. Mains transformer, full-wave rectifier and capacitive smoothing, with a thermal fuse at the transformer input
Original model	Sinclair UK1400, rated 1.4 A. Reads about 12.5 V unloaded, because it is unregulated
Sinclair's own load specification	At 215 V ac mains input and 1.4 A output, the supply should not fall below 7.0 V; at 265 V ac and 600 mA, the peak should be less than 14 V
Measured at the board	+9 V ±2.0 V at the positive side of C50. Below +7 V the regulator will not function correctly

A high reading on an unloaded original pack is normal, not a fault. An unregulated supply sags under load; about 12.5 V off-load falling to around 9 V in use is what a healthy UK1400 does. Judge it against the +9 V ±2.0 V figure measured at C50 with the machine running, not against its off-load reading.

The UK1400 is not suitable for a Spectrum 128 or a +2, even though the plug fits: those machines draw more current.

The pack contains a mains transformer and rectifier and is perfectly repairable — that is ordinary work. The primary side is at mains potential and the smoothing capacitor holds charge after unplugging, so unplug it, wait, and confirm the capacitor is discharged before putting a hand or a probe inside. Replace a perished or damaged lead rather than patching it: a shorted lead on these packs has been known to set the rectifier diodes alight.

The rails the machine makes for itself

The board derives three supplies from that single 9 V input:

• +5 V from the 7805, for the logic, the ULA and the modulator.
• −5 V and +12 V for the 4116 lower RAM, generated by the TR4/TR5 inverter, which lifts the 9 V input above 12 V; D15 and C44 rectify and smooth it.
• +12 VA for the colour circuits, taken through R62/C45 so it is free of the switching noise the RAM makes, and also presented on the expansion port.

Failure of TR4 is, in Sinclair's own words, a significant cause of problems, and it does not fail alone: the DC-DC converter modification is recommended for units in which either TR4 or the 16K DRAM has blown.

Supply, clock and test points

Voltage and signal checkpoints

Point	Expect	Source
IC6 pin 9 (lower RAM)	+5 V, ±0.25 V, no discernible ripple
IC6 pin 1	−5 V (acceptable range −5.5 V to −4 V)
IC6 pin 8	+12 V, ±1.2 V
LT side of C52	+12 VA, ±1.2 V
Positive side of C50	+9 V ±2.0 V — the regulator input
Positive side of C34	+5 V ±0.25 V — the regulator output
IC1 (ULA) pin 14	+5 V. This is Vcc
IC1 (ULA) pin 40	Ground — not a supply pin
IC1 (ULA) pin 13	Vcc2, about +3 V, fed through R34 (15 R, 1 W) and decoupled by C30. It is not a 5 V pin
IC2 (Z80A) pin 11 / pin 29	+5 V / ground
IC5 (ROM) pin 28 / pin 14	+5 V / ground
IC1 (ULA) pin 39	14 MHz, from crystal X1
IC1 pin 32, TR3 base and collector, IC2 pin 6	The CPU clock, a square wave at +5 V amplitude
Right-hand side of R1–R8 and R17–R23	Data and address waveforms of 3.5 V amplitude

One caution about the manual's own figures. Section 5.3.1 labels IC1 pin 32 as a 14 MHz square wave, while section 6.3.2 gives pin 39 as the 14 MHz point and pin 32 as the CPU clock. The two cannot both be right; section 6.3.2 is the self-consistent reading, because pin 39 is where crystal X1 connects and pin 32 feeds TR3 and then the CPU clock input at IC2 pin 6. The manual's fault list refers to the CPU clock loosely as "3 MHz"; X1 is specified as 14.000000 MHz.

Edge connector supplies

From Sinclair's Issue 3 diagram: 3A is +5 V, 4A is +9 V, 6A, 7A and 14A are 0 V, 20B is −5 V, 22B is +12 V and 23B is −12 V. RESET is 20A, ROMCS is 25A and VIDEO is 15A.

Capacitor health

The Spectrum is not a recap-on-sight machine, and there is no service interval for its capacitors. Replace one because it has failed or because it changes a symptom when you press it, not because of its age.

Sinclair's own test for a suspect electrolytic: with wavy lines on the picture, press each electrolytic in turn — if the lines go while you press one, that is the fault.

Which capacitors are fitted depends on the board issue; the full per-issue list is in the Sinclair ZX Spectrum Capacitor Replacement Guide. See also Capacitor Failure Symptoms.

Known failure-prone components

Where Spectrum faults actually come from

Component	Symptom	First check
TR4 / TR5 inverter	Dead with smoke on switch-on; no −5 V or +12 V; lower RAM damaged	Sinclair names TR4 failure as a significant cause of problems. If TR4 is short base-to-collector, remove it; if the short persists, check TR5 and C44. If a replacement TR4 blows again, change TR4 and TR5 together and confirm the TR5 circuit is to the latest specification
ULA (IC1)	No video, wrong colours, dead or wrong keyboard response	It is socketed, so substitution is easy. Check the socket itself and pin 14 for +5 V with no ripple before condemning the chip
Lower RAM (IC6–IC13, 4116)	Will not initialise; corrupt display	These are the chips that need all three rails. Check +5 V, −5 V and +12 V at IC6 first — a rail fault looks exactly like a dead RAM
Upper RAM (IC15–IC22)	48K machine behaves as 16K, or crashes	Can be isolated for testing. All eight must be the same type, and the board links must match
Keyboard membrane	Whole rows or columns dead	An 8 × 5 matrix: if a block of five keys fails, the fault is KB2 or the 8-way membrane; if a block of eight fails, it is KB1 or the 5-way membrane
7805 regulator	Overheating, resets	Check it is making good contact with its heatsink and that thermal grease was used. On some Issue 3 boards the heatsink hole allows enough play to foul the edge connector
Regulator coil	Audible whine	A faint buzz is normal. Sinclair's remedy for an excessively noisy coil is to change it
Jack sockets	SAVE or LOAD fails	Sinclair's servicing addendum: inspect the jack sockets for deformation of the contact springs
Edge connector	Interface 1 not recognised	Inspect for damaged pins, and check the M1 signal — it is used only with Interface 1, so a failure is invisible when the Spectrum is used alone
Speaker	No sound	Driven by TR7 through D9. Check the input waveforms, then TR7, then D9, then the speaker itself

Preventive maintenance

1. Measure the supply and confirm its polarity is centre-negative before plugging it in.
2. Remove the five self-tapping screws from the base, then the single board screw.
3. Inspect the board for corrosion, cracked tracks, burnt components and dry joints, particularly around the regulator, the power socket and the edge connector.
4. Look for solder splatter as well as dry joints. Sinclair is explicit that hairline splatter can measure several ohms and produce very misleading symptoms.
5. Inspect the electrolytics for bulging or leakage.
6. Reseat IC1 and any other socketed devices. Use the correct removal and insertion tools and avoid handling the pins.
7. Clean the edge-connector fingers.
8. Check the regulator is still firmly bolted to its heatsink, and that the heatsink cannot touch the edge connector.
9. Refit the keyboard tails fully and without kinking them.
10. Run the machine and confirm the rails against the table above.

Things that are not faults

• The regulator and its heatsink run warm. They are meant to. What matters is that the regulator is properly bolted down with thermal grease behind it.
• The inverter coil buzzes faintly. That is the DC-DC converter working. Only an excessively noisy coil is a fault.
• An original power pack reads well above 9 V off-load. It is unregulated; it is supposed to.
• Some ICs are socketed and others are not. On a 16K machine the expansion-RAM positions are sockets from the factory, waiting for the upgrade.
• A picture shifted one character to the left on a machine with a 6C001 ULA. That timing change was deliberate, to improve compatibility with certain television sets.
• A four-digit seal over the rear centre screw hole. From August 1984 Sinclair fitted a warranty seal there.

Tools

See Recommended Tools. Sinclair's own recommended list for this machine is an oscilloscope with a ×10 probe (rise time 0.02 µs/cm), a variable 0–30 V bench supply, a general-purpose multimeter, a mono cassette recorder with record and playback, a colour television and a blank tape.

Common faults and first checks

Symptom	Where to look first
Dead, smoke on switch-on	TR4 shorted base to collector; then TR5 and C44
Does not initialise	Work Sinclair's basic-check table in order: regulator input at C50, regulator output at C34, the three rails at IC6, the clock, then the address and data lines
No video	Change IC1; check TR1 and TR2; check the modulator's +5 V and the video input at IC14 pin 12
Wavy lines	Press each electrolytic in turn; then crystal X2, TR1/TR2, IC14
Dim copyright logo	Check +12 V, then C65 for high impedance, then R52 and IC14 pin 3
Self-resetting	Check TR6
Random dynamic squares	Change C54
Dead or wrong keyboard response	Change IC1 — but first check for a short across two or more membrane tracks, which holds a key permanently on
Corrupt display, suspected RAM	Use Sinclair's memory check and the bit-to-IC table in the Sinclair ZX Spectrum Troubleshooting Guide

For the full symptom-by-symptom treatment, see the Sinclair ZX Spectrum Troubleshooting Guide.

Related pages

• Sinclair ZX Spectrum
• Sinclair ZX Spectrum Capacitor Replacement Guide
• Sinclair ZX Spectrum Troubleshooting Guide
• Sinclair ZX Spectrum Keyboard Membrane Repair
• Sinclair ZX Spectrum Service Manual
• Capacitor Failure Symptoms
• Recommended Tools