Sinclair ZX Spectrum +2 Capacitor Replacement Guide

The grey ZX Spectrum +2. The Datacorder tape PCB sits under the cassette deck on the right.

Capacitor replacement on the grey Sinclair ZX Spectrum +2 (main boards Z70500 Issue 3 and Z70700 Issue 1) is covered here. The black +2A and +2B use Amstrad's later boards: the +3's Z70830 in some very early +2As, and the Z70833, made only for later +2A and +2B machines. They have no DC-DC converter and a different power supply, so none of this applies to them.

Safety and board handling

There is no mains voltage inside the computer: it runs from an external power pack, and the highest rail on the board is +12 V.

Spectrum for Everyone describes the PCB quality on the Amstrad-produced boards as poor, and warns that great care is needed when replacing components to avoid lifting pads or tracks. The soldering notes in the Sinclair ZX Spectrum +2 General Maintenance guide apply.

When to replace capacitors

Replace capacitors where there is a reason to:

• Visible failure: bulging, leakage, residue or a split sleeve.
• An unstable supply: random crashes, or rails that measure out of tolerance or show ripple.
• Poor Datacorder audio that cleaning the head and setting the azimuth do not cure. Spectrum for Everyone's next step is to recap the tape PCB (table below).
• A constant hum on the audio output. On Taiwanese-built +2s this is usually poor grounding: the tape PCB uses the metal frame of the mechanism as a ground path, so first check that the two screws holding the PCB to the mechanism are tight and conductive. A ground wire between two points on the PCB is the more reliable fix. Later machines had a 10 nF ceramic and a 1 µF electrolytic added to the tape PCB at the factory, and these can be fitted if the ground wire does not cure it.

Sinclair's service manual for the 48K Spectrum gives a quick test for wavy lines across the screen: press down on each electrolytic in turn, and if the lines disappear, change that capacitor.

Datacorder tape PCB capacitors

The tape PCB differs between the UK-built and Taiwanese-built grey +2. The values are the same but the UK board uses mostly axial parts, and C318 and C324 swap values between the two. Spectrum for Everyone gives the voltage ratings as minimums:

Grey +2 tape PCB electrolytics (Spectrum for Everyone)

Ref	UK grey +2 (main PCB Z70700 Issue 1; tape PCB Timex 377 211002)	Taiwanese grey +2 (main PCB Z70500 or 0500 Issue 3; tape PCB Z70501 C3-4494(A))
C315	100 µF 25 V axial	100 µF 25 V radial
C318	22 µF 25 V axial	10 µF 25 V radial
C322	470 µF 16 V radial	470 µF 16 V radial
C324	10 µF 25 V axial	22 µF 25 V radial

Amstrad's parts list gives C315 100 µF 16 V, C318 22 µF 10 V, C322 470 µF 10 V and C324 10 µF 25 V for the Z70501 tape PCB. Its C318 and C324 values match Spectrum for Everyone's UK column, not its Taiwanese one, so read those two parts on the board before ordering.

To reach the tape PCB, take the mechanism out of the case: four screws hold it to the top casing, two more hold the motor and one holds the LED board, which is attached to the tape PCB. Two further screws hold the PCB to the mechanism; remove them to turn the PCB over.

Main board capacitors

Amstrad's Spectrum +2 Service Manual lists every electrolytic by designator in its electrical parts list. Its layouts are drawn for the Taiwanese-built Z70500 Issue 3 main board and the Z70501 tape PCB; the C3xx positions are on the tape PCB.

Grey +2 electrolytic capacitors, Z70500 main board and Z70501 tape PCB (Amstrad parts list, p. 5)

Value	Main board	Tape PCB	Amstrad part
1 µF 16 V	C29, C36, C49, C50	–	20056
4.7 µF 16 V	C39	–	150520
10 µF 25 V	C24	C324	200528
22 µF 10 V	C81	C318	172073
22 µF 16 V	C23, C52, C62, C63	–	200530
47 µF 16 V	C22, C25	–	172074
100 µF 16 V	C6, C41	C315	20028
220 µF 16 V	C5, C68	–	20029
470 µF 10 V	–	C322	20044

That is seventeen electrolytics on the main board and four on the tape PCB. The rest of the list is ceramics and one 50 pF polycarbonate part, C66. Amstrad's manual has no list for the UK-built Z70700. Spectrum for Everyone's community list gives the same seventeen main-board positions on both boards, with the same values except C68, which it gives as 100 µF on the Z70700 and 220 µF on the Z70500. Its Z70500 parts are all radial; on the Z70700 all but C5 are axial. Read each fitted part's value, voltage and polarity before removing it.

The +2's power circuit works as the Spectrum 128's does. A 7805 gives +5 V; an inverter raises the 9 V input above +12 V, and the inverter output is rectified and smoothed for the +12 V rail; a charge pump from the same square wave gives −12 V, and a zener takes −5 V from that. The designators on the +2 board are not the 128's, however: on a grey +2, TR4, TR5 and TR7 are small video transistors near the modulator, whereas on the 128 TR4 and TR5 are the inverter transistors. Trace the circuit on the board itself; do not work from the 128's designators. Tynemouth Software identifies the +2's inverter transistors as a ZTX650 and a ZTX213; Amstrad's parts list puts them at TR9 and TR8, and lists TR4, TR5 and TR7 as 2N3904s.

The UK-built Z70700 uses mostly axial capacitors, the Taiwanese Z70500 mostly radial, so check the board before ordering parts.

Jailbars on the grey +2

For faint vertical banding ("jailbars") on plain, light colours, Spectrum for Everyone's fix on a Z70500 Issue 3 board is to replace C52, just to the right of the ULA, with a radial electrolytic of 47 µF to 100 µF (Amstrad's list gives C52 as 22 µF 16 V). Stuart Brand improved the picture further by adding 1 nF ceramic capacitors across the C72 to C79 footprints on the underside of the board.

Choosing replacements

• Use aluminium electrolytics of the same capacitance and the same or a higher voltage rating, rated for 105 °C.
• Match the form factor to the board: axial where axial was fitted, radial where radial was fitted.
• Leave the small ceramic and film capacitors alone unless there is a specific fault.

Replacement procedure

1. Unplug the power, remove the six case screws and separate the halves, disconnecting the keyboard ribbons and the tape deck connector.
2. Lift the board out so it can be supported flat.
3. Record the value, voltage and polarity of every electrolytic before removing it.
4. Add fresh flux and solder to both joints.
5. Lift one lead at a time with the joint fully molten, and clear the hole with wick.
6. Fit the replacement the correct way round, solder it and trim the leads.
7. Inspect every pad under magnification for lifting or cracks before reassembly, and repair any broken track with an insulated wire link.

Post-recap checks

1. Power up on a current-limited bench supply. Tynemouth Software sets 1.5 A; a working +2 draws between about 800 mA and 1.2 A depending on the RAM fitted, and a board that goes straight into current limiting has a short.
2. Check that the 128 menu appears.
3. Check the +5 V, +12 V, −12 V and −5 V rails.
4. Check the clocks. The Spectrum 128 Service Manual expects 3.54689 MHz at the Z80's clock pin, with no jitter. The +2's crystal is not the 128's 17.734475 MHz part: Amstrad's parts list gives X1 as 35.4690 MHz.
5. Test the AY sound and the beeper, a tape load and save, both joystick ports and the whole keyboard.

If a function that worked before the recap now fails, check the polarity of every replaced electrolytic first.

Related pages

• Sinclair ZX Spectrum +2
• Sinclair ZX Spectrum +2 General Maintenance
• Sinclair ZX Spectrum +2 Troubleshooting Guide
• Sinclair ZX Spectrum 128 Capacitor Replacement Guide
• Sinclair ZX Spectrum+ Capacitor Replacement Guide
• Capacitor Failure Symptoms