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The Lil Old ZX Spectrum 48k Service Manual
SECTION 4.4 - FAULT DIAGNOSIS AND REPAIR
LIST OF CONTENTS
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:
- Rails and signals taken on switch-on, computer displaying `Sinclair` logo.
- No keys pressed
- All dc levels positive wrt 0V
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.
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:
IC |
TYPE |
IC15-IC22 |
TI 4532-3 or TI 4532-4 200ns (or 250ns) |
IC23 |
74LS32 |
IC24 |
74LS00 |
IC25, IC26 |
74LS157 (NOT National
Semiconductors) |
Notes:
- All external RAM ICs must be of the same type (ie all -3 or -4 variants)
- LINK 3 on the board (IC26, pin 10) should be fitted if IC15-IC22 are type
4532-3
- LINK 4 should be fitted if IC15-IC22 are type 4532-4
Issue 3 Board
The ICs used on this board are as follows:
IC |
TYPE |
IC15-IC22 |
TI 4532-3, TI 4532-4 or OKI MSM3732 200ns (or 250ns) |
IC23 |
74LS32 |
IC24 |
74LS00 |
IC25, IC26 |
74LS157 (NOT National
Semiconductors) |
Notes:
- All extension RAM ICs must be of the same type.
- 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
Continue to Section 5
Return to Service Manual Index