The Lil Old ZX Spectrum 48k Service Manual

SUPPLEMENT No 1

LIST OF CONTENTS

• Introduction
• DC-DC Converter Modification
• Issue 3B Board
• Issue 4 Board
• Issue 4A Board
• Issue 4B Board
• Issue 5 Board
• Issue 6 Board
• Issue 6A Board
• Fault-Finding - Servicing Addenda
• Warranty Seal
• Parts Lists
• Notes to parts lists
• Capacitors
• Coil
• Connectors
• Crystals
• Resistors
• Diodes
• Integrated Circuits
• Transistors
• Miscellaneous

Introduction

In line with Sinclair`s policy of continual product improvement a number of changes to the Spectrum Printed Circuit board have been implemented. Since the Service Manual was published the printed circuit board has been upgraded through Issues 4A, 4B, 5 and is currently at Issue 6A. This supplement details the modifications, board by board and also provides additional servicing information and spare parts lists.

A substantial change has been introduced in the area of the dc-dc converter and the method of implementing this modification is detailed under its own heading. It is recommended that this modification is embodied in units on which either TR4 or the 16k DRAM has blown. Some Issue 3B and 4A units, and all later issues have this modifications embodied.

DC-DC CONVERTER MODIFICATION

The following new materials are required:

Diodes

• D17 - BR157
• D18 - IN4148
• D19 - 5V1 Zener

Capacitors

• C78 - 1uF 25V WKG Electrolytic
• C79 - 1uF 16V WKG Electrolytic

Resistors

• R60 - 220R 5%, 0.25W
• R79 - 2k2 5%, 0.25W

Remove the following components from the printed circuit board:

• D12- IN4148 Diode
• D16 - 5V1 Zener
• C47 - 22uF Electrolytic
• R54 - 100k
• R55 - 56R
• R60 - 270R

Embody the modification as follows:

• Break the connection between D15 and C49
• Connect C78 between C49 and D15 (positive end of C78 to C49)
• Connect D17 between ground (positive end of D17) and the new connection of C78 to D15

Substitute components as follows:

Circuit Operation

The modification is confined to adding/changing protection components in the rectification circuit to improve reliability. Circuit function does not change.

ISSUE 3B BOARD - (Already covered in existing manual)

ISSUE 4 BOARD - None issued

ISSUE 4A BOARD

This differs from the issue 3B follows:

• R32 deleted
• Two spare gates from IC24 have been used to improve ULA timing. It is important that ULA 6C001-7 or later issues (the issue number is designated by the number after the hyphen) are used in this circuit. The gates are fitted in series with the RASL output from the ULA and the input to the display RAM multiplexers.
• Provision is made for fitting interference suppression coils and a modulator channel switch, as required on VHF units.

ISSUE 4B BOARD

The Issue 4B board is the same as that for the Issue 4A with the addition of the dc-dc converter modification. The board layout was changed at this stage as shown here. Due to lack of board space, C61 (22uF decoupler) was omitted. It is reinstated on subsequent board issues. The circuit diagram is shown here.

ISSUE 5 BOARD

1000 of these boards were supplied. They are restricted to the UK market only. A single ULA, IC27, (Mullard ZX 84/01) replaces the six 74LS chips. The circuit diagram is shown here. An inverter chip IC28 has been added and is used in the decoupling of the RAS/CAS function. Circuit functions do not change. Some discrete components were added/changed as reference to the parts list show. The board layout is shown here.

ISSUE 6 BOARD - None issued

ISSUE 6A BOARD

This board is similar to Issue 5 but can be fitted with either a Ferranti or a Saga main ULA. The circuit diagram is shown here and the board layout is shown here.

FAULT FINDING - SERVICING ADDENDA

The following are additions to Section 4 of the servicing manual:

• Picture quality and possible RAM failures. Test at elevated temperature to show up faults.
• Interface 1 fault. Inspect the edge connector to check for damaged pins. Check the M1 signal from the CPU; this signal is only used with Interface 1 and so a failure is not evident when the Spectrum is used on its own.
• SAVE/LOAD failure. Inspect the jack sockets for deformation of contact springs.

WARRANTY SEAL

From August 1984 a new warranty seal label is fitted. The label is applied over the central screw hole at the rear of the base and has a four digit code. Breaking of this seal invalidates the warranty.

PARTS LISTS

As alternatives to ZTX 213 in TR5 position, transistors BC213, BX214 or FRB925 may be used.

As alternatives to ZTX 313 in TR1, TR2, TR3 and TR6 positions, transistors MPD 2369 or MPS 2713 (both in TO-92 packages) can be used.

The parts lists follow. Some components on this table are not always fitted. Each is identified by reference to the note number in the NB column. The notes are explained below.

Notes:

1. With Issue 5 boards, if R80-R86 are seen to be fitted, the value o R17-R23 is 470R. If R80-R86 are fitted, the values of R17-R23 are 330R.
2. With Issue 6 boards, this is not necessary with the SAGA version of IC1.
3. With Issue 6 boards, this is not necessary with the FERRANTI version of IC1.
4. With Issue 6 boards, this is only required on VHF models.
5. With Issue 6 boards, this is only required on the 48k model.
6. With all issues of Spectrum, if TR3 is type KSC839, resistor R24 should be 15k.
7. With all Issue Spectrums, if TR4 is to be replaced by type KTC2236A-Y, the pin connections differ to all other replacement types (Pin 1 is emitter, Pin 2 is collector and Pin 3 is base, ie the base and collectors are reversed).

Capacitors - Unless otherwise stated all are axial types

COIL

CONNECTORS

CRYSTALS

RESISTORS - 0.25W, 5% tolerance unless otherwise stated

DIODES

INTEGRATED CIRCUITS

TRANSISTORS

MISCELLANEOUS