Sinclair ZX Spectrum Service Manual
Spectrum For Everyone https://spectrumforeveryone.com/
27
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.
4.2 FAULT DIAGNOSIS
4.2.1 Techniques
In a closed loop system such as a computer, because of the interdependence of numerous component
parts, fault diagnosis is not necessarily straight-forward. In addition, because of the high speed cyclic
operation, interpretation of any waveforms on control, data and address lines as being valid depends to
a large extent on practical experience of the system. There are however, certain checks with valid
waveforms and levels that can be carried out before substituting any integrated circuits. Experience has
shown that the best method of initially checking waveforms and levels can be to compare with the same
point in a known serviceable board. The following pages provide a basic fault-finding procedure and
furnish a list of possible faults along with suggested ways of curing them.
With a densely populated board such as the SPECTRUM 128, a careful physical examination of the board
can sometimes indicate an obvious fault. Burnt-out discrete components or an overheated track show
up immediately, as do the attentions of an enthusiastic amateur. Bearing in mind the latter, short
circuits caused by hairline solder 'splatter' can be of several ohms resistance and can cause some very
misleading fault symptoms.
Provided first principles are adhered to and a common-sense approach is adopted, it will be found after
a short space of time that fixing a faulty Spectrum is very much a routine operation.
4.2.2 Power Supply Unit
The unstabilised external power supply unit is a source of some problems. The design is such that, at
minimum input voltage (215V AC) and 1.4A output, the voltage trough should not be less than 7.0V; at
maximum input voltage (265V AC.) and 600mA output, the voltage peak should be less than 13V.
4.2.3 Initialisation
At switch-on the computer should automatically 'initialise' and produce a clear screen with the
statement.
© 1985 Sinclair Research Ltd
- displayed in the lower left section of the screen. This indicates that most of the system is working. If
the SPECTRUM 128 does not initialise, carry out the following basic checks.
Basic Checks: It is difficult to be specific in a fault-finding guide because of the large variety of fault
conditions which can occur, but the following procedure, starting with a table of checks set out in order
of priority, will however isolate the major fault area. The oscillograms reproduced on pages 4.6 and 4.7
are measured at points referenced on the circuit diagrams.
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