Sinclair ZX Spectrum+ Capacitor Replacement Guide

Sinclair's circuit diagram for the Issue 6A board, the last 48K board fitted to the Spectrum+.

Capacitors on the Sinclair ZX Spectrum+. Sinclair describes the Spectrum+ as "basically the same as the 48k ZX Spectrum with a different keyboard and case, and a reset button". Sources differ on which board a factory-built Spectrum+ has. Television magazine's 1986 servicing series says it usually has an Issue 3B board; Spectrum for Everyone says Issues 4A and 4B were the first widely available in the Spectrum+ case, and records Issue 4S, 5 and 6A boards in it too. The table below is Sinclair's list for Issues 4A, 4B, 5 and 6A. On these boards C1–C8 are 22 nF in place of the 47 nF of Issues 2, 3 and 3B, several positions are unused on some issues, and positions up to C105 exist.

Check the issue number on the board before using the table. The Spectrum+ case was also sold as an upgrade kit for rubber-key machines, so a converted Spectrum+ can carry any earlier board; for Issues 2, 3 and 3B use the Sinclair ZX Spectrum Capacitor Replacement Guide, which has Sinclair's list for those boards. Sinclair's lists do not cover the Samsung Issue 4S, which Spectrum for Everyone describes as almost identical to Issue 3B; its electrolytics are given below.

Board handling

The computer runs from an external 9 V supply, so there is no mains voltage inside the case; the highest rail on the board is +12 V.

Sinclair's repair notes ask for recognised desoldering and heatsinking technique, a suitably earthed low-power iron, and an inspection for solder splatter afterwards; a hairline bridge can measure several ohms and cause very misleading faults. If the regulator is disturbed, it must make good contact with the heatsink, with thermal grease behind it. On some Issue 3 boards the heatsink hole allows enough play to foul the edge connector. T.I.S.'s field-service notes add a check for hairline cracks in the board radiating from the heatsink screws when chasing intermittent faults.

When to replace capacitors

Sinclair's manual gives no replacement interval for the capacitors. Its diagnostic test for an electrolytic: if the screen shows wavy lines, press down on each electrolytic in turn and change the one that makes the lines disappear.

Parts that Sinclair's fault-finding sections name:

• C50, the regulator input, and C34, the regulator output: the first two test points in the basic checks.
• C44, the +12 V smoothing in the DC-DC converter, which Sinclair says to check (with TR5) when a short persists with TR4 removed; and C45, which smooths the +12 VA supply to the colour circuits.
• Anything bulging, leaking, or that changes the symptom when pressed.

Sinclair's capacitor list

Unless otherwise stated all capacitors are axial types.

Sinclair's capacitor list, service manual section 9.13 (Issues 4A, 4B, 5 and 6A)

Ref	Issue 4A	Issue 4B	Issue 5	Issue 6A	Rating / tolerance	Type	Note
C1–C8	22 nF	22 nF	22 nF	22 nF	25 V, 10%	Ceramic
C9–C24	Not used	Not used	Not used	Not used
C25	22 µF	22 µF	22 µF	22 µF	10 V, −10% +80%	Electrolytic
C26	22 nF	22 nF	22 nF	22 nF	25 V, 10%	Ceramic
C27	1 µF	1 µF	1 µF	1 µF	50 V, 10%	Electrolytic
C28	22 µF	22 µF	22 µF	22 µF	10 V, −10% +80%	Electrolytic
C29	47 nF	47 nF	"44nf" as printed	22 nF	25 V, 10%	Ceramic
C30	22 nF	22 nF	22 nF	22 nF	25 V, 10%	Ceramic	2
C31	100 nF	100 nF	100 nF	100 nF	25 V, 10%	Ceramic
C32	100 nF	100 nF	100 nF	100 nF	25 V, 10%	Ceramic	2
C33	22 nF	22 nF	22 nF	47 nF	25 V, 10%	Ceramic
C34	22 µF	22 µF	22 µF	22 µF	10 V, −10% +80%	Electrolytic
C35	10 nF	10 nF	10 nF	10 nF (Ferranti ULA) or 100 nF (SAGA ULA)	25 V, 10%	Ceramic	2, 3
C36	Not used	Not used	Not used	Not used
C37, C38	33 pF	33 pF	33 pF	33 pF	25 V, 10%	Ceramic
C39	10 nF	10 nF	10 nF	10 nF	25 V, 10%	Ceramic
C40, C42	22 nF	22 nF	22 nF	22 nF	25 V, 10%	Ceramic
C43	100 nF	100 nF	100 nF	100 nF	25 V, 10%	Ceramic
C44, C45	100 µF	100 µF	Not used	100 µF	16 V, −10% +80%	Ceramic (as printed)
C46	1 µF	1 µF	1 µF	1 µF	50 V, −10% +80%	Electrolytic
C47	"22nf" as printed (22 µF)	"22nf" as printed (22 µF)	Not used	Not used	10 V, −10% +80%	Electrolytic
C48	22 nF	22 nF	22 nF	22 nF	25 V, 10%	Ceramic
C49	560 pF	560 pF	560 pF	560 pF	25 V, 10%	Ceramic
C50	22 µF	22 µF	22 µF	22 µF	10 V, −10% +80%	Electrolytic
C51	Not used	Not used	Not used	Not used
C52, C53	150 pF	150 pF	150 pF	150 pF	25 V, 10%	Ceramic	2
C54	Not used	Not used	Not used	Not used
C55–C60	22 nF	22 nF	22 nF	22 nF	25 V, 10%	Ceramic
C61	22 nF	Not used	22 nF	22 nF	25 V, 10%	Ceramic
C62	22 nF	22 nF	22 nF	22 nF	25 V, 10%	Ceramic
C63	47 pF	47 pF	Not used	Not used	25 V, 10%	Ceramic
C64	180 pF	180 pF	Not used	Not used	25 V, 10%	Ceramic
C65	22 µF	22 µF	22 µF	22 µF	20 V, −10% +80%	Electrolytic
C66	22 nF	22 nF	22 nF	22 nF	25 V, 10%	Ceramic
C67	100 pF	100 pF	100 pF	100 pF	25 V, 10%	Ceramic	2
C68–C71	100 nF	100 nF	100 nF	100 nF	25 V, 10%	Ceramic	2
C72, C73	16 pF	16 pF	16 pF	16 pF	25 V, 5%	Ceramic
C74	4.7 µF	4.7 µF	4.7 µF	4.7 µF	5 V minimum	Electrolytic
C75	100 nF	100 nF	100 nF	100 nF	25 V, 10%	Ceramic
C76	47 nF	47 nF	47 nF	22 nF	25 V, 10%	Ceramic	2
C77	100 nF	100 nF	Not used	Not used	25 V, 10%	Ceramic
C78	Not used	Not used	22 µF	Not used	25 V, 10%	Electrolytic
C79	Not used	1 µF	1 µF	1 µF	16 V, 10%	Electrolytic
C80	Not used	22 µF	Not used	22 µF	25 V, 10%	Electrolytic
C100	Not used	Not used	Not used	10 nF	25 V, 10%	Ceramic	4
C101	Not used	Not used	Not used	22 nF	25 V, 10%	Ceramic	4
C102, C103	Not used	Not used	Not used	Not used
C104	Not used	Not used	100 nF	100 nF	25 V, 10%	Ceramic
C105	Not used	Not used	180 pF	180 pF	25 V, 10%	Ceramic

The notes

Sinclair's note numbers in the last column, from section 9.12.1:

• Note 2: on Issue 6 boards this part is not necessary with the SAGA version of IC1.
• Note 3: on Issue 6 boards this part is not necessary with the Ferranti version of IC1.
• Note 4: on Issue 6 boards this is only required on VHF models.

Three entries are misprints. C44 and C45 are printed as "Ceramic", but a 100 µF part with a −10% +80% tolerance is an electrolytic, and section 8.5 lists them as electrolytics. C47 is printed "22nf" with an electrolytic rating; section 8.5 gives 22 µF. C29 on Issue 5 is printed "44nf", which is not a preferred value; check the part on the board.

The list puts C47 on Issue 4B, but section 9.6 says the 4B board includes the DC-DC converter modification, which removes C47 (section 9.2), and Spectrum for Everyone's Issue 4B list has no C47. Check the board before ordering a part for that position. Spectrum for Everyone's lists otherwise give the same electrolytic positions as Sinclair's for Issues 4A, 4B and 6A.

Issue 4S

Sinclair published no list for the Samsung Issue 4S. Spectrum for Everyone's community list gives twelve electrolytics: C25, C28, C34, C47, C50, C65 and C78 (22 µF, 10 V), C27 and C46 (1 µF, 50 V), C44 and C45 (100 µF, 16 V) and C74 (4.7 µF, 5 V). The voltages are minimum ratings for replacements. Read each value and polarity from the board before removing it.

C27 and the reset switch

C27 is a 1 µF, 50 V electrolytic on every board issue Sinclair lists. On a Spectrum+ conversion the reset switch is connected across C27 on the top side of the board, and Spectrum for Everyone notes that factory boards before Issue 6A in Spectrum+ cases have the switch there too; Issue 6A has a dedicated connection. Note the polarity of C27 and the switch wiring before removing it.

Two transistor traps

Two notes in the same section of the parts list concern transistors near the capacitors:

• If TR4 is replaced with a KTC2236A-Y, its pin connections differ from every other replacement type: pin 1 is the emitter, pin 2 the collector and pin 3 the base, so base and collector are reversed. TR4 is the switching transistor of the DC-DC converter that makes the −5 V and +12 V rails.
• If TR3 is a KSC839, R24 should be 15 kΩ.

Choosing replacements

• Use 105 °C aluminium electrolytics of the same capacitance and the same or higher voltage rating. Modern 16 V or 25 V parts replace the 10 V and 16 V originals.
• Several originals are specified at −10% +80%, so a modern ±20% part is within Sinclair's specification.
• The originals are axial. Axial parts are still made; radial replacements need their leads formed and insulated so that nothing shorts to adjacent tracks.

Replacement procedure

1. Unplug the power, open the case and disconnect the keyboard tails carefully (see Sinclair ZX Spectrum Keyboard Membrane Repair).
2. Record the value, voltage and polarity of each electrolytic before removal.
3. Add fresh flux and solder to both joints and let it mix with the old solder.
4. Lift one lead at a time with the joint fully molten, then clear the hole with braid.
5. Fit the replacement the right way round, solder and trim.
6. Inspect for bridges and splatter before reassembly.

Post-recap checks

1. Confirm the copyright screen appears.
2. Measure +9 V ±2.0 V at C50 and +5 V ±0.25 V at C34, with no discernible ripple.
3. Measure −5 V (−5.5 V to −4 V) at IC6 pin 1 and +12 V ±1.2 V at IC6 pin 8.
4. Listen for the faint buzz of the DC-DC converter coil.
5. Check for a stable display with no wavy lines.
6. Test sound, a tape load and every key.

If something that worked before the recap has stopped, check the polarity of every replaced electrolytic first.

Related pages

• Sinclair ZX Spectrum+
• Sinclair ZX Spectrum+ General Maintenance
• Sinclair ZX Spectrum+ Troubleshooting Guide
• Sinclair ZX Spectrum
• Sinclair ZX Spectrum Capacitor Replacement Guide
• Sinclair ZX Spectrum Keyboard Membrane Repair
• Sinclair ZX Spectrum Service Manual
• Capacitor Failure Symptoms