Sinclair ZX Spectrum 128 Service Manual
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Sinclair ZX Spectrum 128 Service Manual

8-bit personal home computer
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SINCLAIR
ZX Spectrum 128
Service Manual
Edited by Brendan Alford
Spectrum For Everyone Website
https://spectrumforeveryone.com/
Spectrum For Everyone Facebook Group
https://www.facebook.com/groups/spectrumforeveryone/
Revision 1.0

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Summary of Contents for Sinclair ZX Spectrum 128

  • Page 1 SINCLAIR ZX Spectrum 128 Service Manual Edited by Brendan Alford Spectrum For Everyone Website https://spectrumforeveryone.com/ Spectrum For Everyone Facebook Group https://www.facebook.com/groups/spectrumforeveryone/ Revision 1.0...

  • Page 2: Table Of Contents

    Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ ONTENTS 1 System Description ............................ 4 1.1 Introduction ............................4 1.2 Architecture ............................5 1.3 Z80A CPU ............................. 5 1.4 Memory Organisation ......................... 7 1.4.1 Read/Write Operations and Bus Arbitration ................8 1.5 Input/Output .............................
  • Page 3 Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ 4.3 Repair ..............................29 5 Parts List ..............................33 5.1 Introduction ............................33 5.2 Notes to Table 5.4 ..........................33 5.3 Case Assembly Components ......................34 5.4 Main PCB Components ........................35...

  • Page 4: System Description

    Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ YSTEM ESCRIPTION Introduction Architecture Z80A CPU Memory Organisation Read/Write Operations and Bus Arbitration Input/Output TV Picture Generation and Sound Output Keyboard Scanning Tape Interface Programmable Sound Generator RS232C/MIDI Interface Keypad Scanning Power Supplies 1.1 I...

  • Page 5: Architecture

    Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ 1.2 A RCHITECTURE The architecture of the Spectrum 128 shown in Figure 1.1 is typical of many microcomputer systems in that it comprises of a single microprocessor chip (in this case a Z80A or u780), a read only memory (ROM), a paged random-access memory (RAM) and an input-output section.
  • Page 6 Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ CPU Clock: Returning to the CPU clock mentioned earlier in this section, the ULA is able to inhibit this input bringing the CPU to a temporary halt. This mechanism gives the ULA absolute priority allowing it to access the contended RAM without interference from the CPU (see RAM description).

  • Page 7: Memory Organisation

    Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ 1.4 M EMORY RGANISATION The Spectrum 128 has 160K bytes of addressable memory - a 32K byte ROM (IC5) and 128K bytes of dynamic RAM (IC6-IC22). The latter is organised as eight 16K byte pages as indicated below.

  • Page 8: Read/Write Operations And Bus Arbitration

    Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ ZA15 = ZA14 = 0: These bits select the first 16K of Z80 address space beginning at 0000 , and result in the PAL generating ULA15 = ULA14 = 0. These are decoded by the ULA (IC1) to produce a signal /ROMCS enabling the ROM IC5.
  • Page 9 Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ An external ROM chip select input, supplied via the expansion port on pin 25A, selectively disables the on-board ROM by pulling the select input high. By virtue of R33 placed on the ULA side of the ROM the ULA /ROMCS output is effectively inhibited.
  • Page 10 Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ Figure 1.2. Uncontended RAM RAS/CAS timing (read cycle shown) Resistors R1 to R8, in series with the data bus lines, perform a similar function to the address port resistors described above. They ensure that the ULA does not 'see' CPU write data while the ULA is accessing the contended RAM.

  • Page 11: Input/Output

    Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ 1.5 I NPUT UTPUT The input/output functions are controlled by the Z80 in conjunction with the ULA (IC1) and the sound generator circuit IC32. Like its counterpart in the 48K Spectrum, the ULA handles the tape recorder read/write functions, and generates an interrupt during which it scans the main keyboard.

  • Page 12: Keyboard Scanning

    Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ The RGB colour, bright-up and composite sync signals (Figure 1.6) are coupled to the RGB output socket via 68-ohm resistors and are suitable for direct input to a wide range of colour monitors. The same signals are also applied to the encoder IC36 to produce a composite video output at pin 6.
  • Page 13 Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ When the keyboard scanning routines are entered, the CPU performs successive I/O read cycles setting the /IORQ and /RD lines to the ULA low. At the same time, the I/O port addresses placed on the upper half of the address bus are modified with each cycle such that each of the address lines A15 through A8 is set low in turn, the other lines remaining high.
  • Page 14 Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ Figure 1.4. Keyboard Lower Membrane The row signal(s) is subsequently buffered by the ULA and placed on one of the five low order data bus lines. For example, if the CAPS SHIFT key is pressed, row one output drives data bus D0 high, and so on.

  • Page 15: Tape Interface

    Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ For example, pressing TRUE VIDEO closes the switch contact at row 1, column 6 on the upper membrane (CAPS SHIFT) and row 3, column 1 on the lower membrane (TRUE VIDEO).

  • Page 16: Programmable Sound Generator

    Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ 1.7 P ROGRAMMABLE OUND ENERATOR The audio from the sound generator IC32 is derived from a master clock input supplied by the ULA, controlled and shaped in accordance with instruction codes loaded by the Z80 into 14 internal byte wide registers (see below).

  • Page 17: Rs232/Midi Interface

    Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ 1.8 RS232/M NTERFACE The RS232C/MIDI interface is implemented using the Port A Data Store in the sound generator chip IC32. The data store is a special register at octal address 16 which accesses an 8-bit bi-directional port A7-A0.

  • Page 18: Reset Protocol

    Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ a space in response to the START signal from the Z80. In this case the PIC and Z80 determine that the next START signal is a call for the result of the row scan at the next column address. If the scan indicates that there has been a change in status since the previous scan, the PIC responds positively by sending a mark in response to the START signal.

  • Page 19: Bit Transfer Protocol

    Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ c) The 0.6ms delay between the PIC returning to the idle state (CORRECT) and the Z80 setting TXD high (GO). If the delay is exceeded the keypad assumes that the Z80 has been reset and resets itself (i.e.

  • Page 20: Power Supplies

    OWER UPPLIES The on-board power supply unit receives a 9V unregulated supply from the external Sinclair ZX power pack and derives the following internal supply rails: a) Regulated +5V for the IC logic circuits, the ULA and the sound/UHF modulators...
  • Page 21 Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ Figure 1.7. Keypad Circuit...

  • Page 22: Disassembly/Assembly

    Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ ISASSEMBLY SSEMBLY Disassembly Assembly 2.1 D ISASSEMBLY Unplug all input/output connectors and turn the computer upside down to reveal eight fixing screws. Release the screws (noting the position of two countersunk screws for re-assembly), turn the computer right side up and separate the case halves.
  • Page 23 Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ When replacing the PCB, ensure that the reset pushbutton is correctly located in the cut-out provided in the end of the case. Before final assembly reconnect the keyboard ribbon cables (they should lie in an ‘S’ shape) and ensure...

  • Page 24: Setting Up And System Test

    Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ ETTING P AND YSTEM Setting Up Instructions Sound Carrier Frequency System Test 3.1 S ETTING NSTRUCTIONS 3.2 S YSTEM (note: the above is as per the source manual and is intentionally reproduced as is)

  • Page 25: Fault Finding And Repair

    Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ AULT INDING AND EPAIR Introduction Test Equipment Fault Diagnosis Techniques Power Supply Unit Initialisation Symptomatic Faults Repair Illustrations Test Oscillograms 4.1 I NTRODUCTION 4.1.1 Test Equipment Section 4 is intended as a guide to fault diagnosis and repair of the SPECTRUM 128. it is assumed that users have a reasonable knowledge of electronic servicing, theory and standard fault-finding techniques and have access to the test equipment and tools required to carry out the task.
  • Page 26 Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ Blank tape Open Market Double-sided adhesive tape 12mm and 6mm wide, Tesafix 959 (B.D.F. Tesa) or 3M equivalent...

  • Page 27: Fault Diagnosis

    Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ Engineers who are already familiar with the Sinclair SPECTRUM+ will find some similarities in the SPECTRUM 128. The SPECTRUM 128, however, is a more sophisticated device with improved colour and sound circuitry.
  • Page 28 Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ Function Circuit Reference Voltage/Waveform Voltage regulator input +ve side of C50 +9V DC +/- 2.0V. At less than +7V the regulator may not operate correctly Voltage regulator output +ve side of C34 and IC15 pin 8 +5V DC +/- 0.25V, no...

  • Page 29: Symptomatic Faults

    Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ 4.2.4 Symptomatic Faults As with any complex digital equipment the possible permutations are vast, thus the following table is not intended to be an exhaustive list of the faults that might occur on the Spectrum. It is intended as a guide only to possible courses of action to follow when faults show up in particular areas of the circuit.
  • Page 30 Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ (a) TR4 Collector (b) TR4 Base (c) IC37 Pin 6 (d) TR3 Collector...
  • Page 31 Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ (e) IC1 Pin 42 (f) IC1 Pin 2 (g) IC36 Pin 7 (h) IC38 Pin 4...
  • Page 32 Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ (j) TR11 Emitter...

  • Page 33: Parts List

    Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ ARTS Introduction Notes to Table 5.4 Spectrum 128 PCB Component Layout (Editor’s note: missing in available documentation) Keypad PCB Component Layout (Editor’s note: missing in available documentation) 5.1 I NTRODUCTION Parts lists for the SPECTRUM 128 are provided in table form; one for the case assembly (Table 5.3), one for the main PCB assembly (Table 5.4) and another for the keypad PCB assembly (Table 5.5).

  • Page 34: Case Assembly Components

    Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ 5.3 C SSEMBLY OMPONENTS Description Manufacturer Base Assembly Final PCB Assembly – Table 5.4 Heatsink Retractable legs – 2 off Leg Springs – 2 off Bottom Case Moulding Fixings – ¼ in self tap screw – 3 off Fibre washer –...

  • Page 35: Main Pcb Components

    Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ 5.4 M PCB C OMPONENTS Capacitors (Unless otherwise stated, all capacitors are axial types): Circuit Reference Value Rating/Tolerance Manufacturer/Type Notes C1-C8 22nF 25V, 10% Ceramic C9-C24 Not used 100uF 10V, -10%+80%...
  • Page 36 Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ Coils: Circuit Reference Value Rating/Tolerance Manufacturer/Type Notes COIL Spectrum TFR N Devon L1/2 N Devon, Toroidal/2- winding Toko, 7KL (PF291ACS- 1885Z) Toko, 7KL ( ? ) Not used L6, 7 100Uh...
  • Page 37 Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ Integrated Circuits: Circuit Reference Device Manufacturer/Type Notes IC1 (ULA) 7C001 Ferranti IC2 (CPU) Z80A/u780 Zilog/NEC IC3, 4 Not used IC5 (ROM) SPECTRUM 128 IC6-IC13 4164 150ns IC14 Not used IC15-IC22 4164...
  • Page 38 Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ Resistors (1/4W, 5% unless otherwise stated): Circuit Reference Value Rating/Tolerance Manufacturer/Type Notes R1-R8 470R R9-R16 R17-R23 470R 180R R26, R27 470R R28-R30 10K0 220K Not used 680R 0.5W or 1W 10K0...

  • Page 39: Keypad Pcb Components

    Sinclair ZX Spectrum Service Manual Spectrum For Everyone https://spectrumforeveryone.com/ Resistors (Continued): Circuit Reference Value Rating/Tolerance Manufacturer/Type Notes R128 R129 R130, R131 R132 39K0 R133 R134 R135 R136 Not used R137 R138 470R Transistors: Circuit Reference Device Alternative Notes TR1, TR2...

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