The Lil Old ZX Spectrum 48k Service Manual

SECTION 4.4 - FAULT DIAGNOSIS AND REPAIR

LIST OF CONTENTS

• IC14 (LM1889) Pin Signals
• Repair
• 16k to 48k Upgrade
• Integrated Circuits/Link Settings
• Issue 2 Board
• Issue 3 Board

IC14 (LM1889) PIN SIGNALS

Pin No	Signal
1	(CHROMA LEAD) SINEWAVE 500mV, 0.2us, dc 9.5V
2	(R-Y INPUT)
3	(CHROMA BIAS) dc 4.8V
4	(B-Y INPUT)
5	0V (GROUND)
12	(VIDEO INPUT) dc 12V unreg
13	(CHROMA SUBCARRIER)
14	(RF SUPPLY)
15	(SOUND TANK) `LINKED` 12V dc
16	(CHROMA SUPPLY)
17	(CHROMA OSCILLATOR OUTPUT) SQUAREWAVE 0.2us 4V p-p dc 0.8V
18	(CHROMA LAG) SINEWAVE 500mV p-p 2us dc 9.5V
6,7,8,9,10,11	Not Connected

Notes:

1. Rails and signals taken on switch-on, computer displaying `Sinclair` logo.
2. No keys pressed
3. All dc levels positive wrt 0V

REPAIR

Renewal of components should be carried out using recognised desoldering/heatsinking techniques to prevent damage to the component or to the printed circuit board. Other points to be noted are:

• When replacing a keyboard matrix, take care that the ribbon connectors are fully inserted into the board connectors,and are not kinked during insertion.
• Make sure there is a good contact made between the voltage regulator body and the associated heatsink in order to ensure adequate heat conduction. The heatsink hole in certain Issue 3 boards allows excessive play which could cause fouling of the edge connector. Take care in re-assembly that the heatsink is fitted away from the edge connector.
• When the regulator is being replaced it is recommended that a suitable proprietary thermal grease is applied to the rear surface of the component body.
• The modulator should be replaced as a complete unit.
• When replacing plug-in ICs it is advisable to use the correct removal and insertion tools. Avoid contaminating the connection pins by handling.
• When handling ICs take normal anti-static precautions. It is recommended that only a suitably earthed, low power soldering iron be used.
• After any component has been renewed the circuit board should be examined carefully, to ensure that there are no solder `splatters` which may cause short circuits between tracks or connector pins.

16k to 48k UPGRADE

Integrated Circuits/Link Settings

For both the Issue 2 and the Issue 3 boards the memory upgrade is accomplished by plugging four logic ICs and eight memory ICs into existing board sockets. Depending on the types of memory ICs used, appropriate circuit links must be fitted on the board.

Issue 2 Board

The ICs used on this board are as follows:

Notes:

1. All external RAM ICs must be of the same type (ie all -3 or -4 variants)
2. LINK 3 on the board (IC26, pin 10) should be fitted if IC15-IC22 are type 4532-3
3. LINK 4 should be fitted if IC15-IC22 are type 4532-4

Issue 3 Board

The ICs used on this board are as follows:

Notes:

1. All extension RAM ICs must be of the same type.
2. Two links must be fitted on the board (grid located between edge connector and mic jack socket) depending on memory manufacturer and type (high or low enable).

ISSUE 3 LINKS vs IC MANUFACTURER AND TYPE

EXPANSION MEMORY IC LOCATIONS