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L
D
OGIC
ESCRIPTION
The LED and the photodiode are positioned on opposite sides of an apertured encoder disc. As the disc
rotates, the beam of light from the LED is interrupted, thus providing a pulsed signal, the timing of which
is governed by the speed of rotation of the disc. This, pulsed signal is applied to one input of an AND-gate
whose1 output is connected, via a divide-by-four circuit, to a block input of a buffer store and decoder
(see Figure 6).
The buffer store and decoder receives and stores information relating to one line of print, i.e. six scanned
lines, from the computer; it responds to clock pulses at its clock input to selectively enable the high-
voltage (50V) switch to permit current to flow from the power supply to the conductor strip and thence,
via the stylus to the paper. The circuit is earthed via the conducting rubber roller of the paper feed and a
resistor. Where the 50V is applied to the surface of the paper, the metallic coating is burned off, leaving
the dark undercoating exposed to provide the permanent image. The conductor strip is also connected to
the paper detect circuit which detects when the stylus has run off the edge of the insulating wafer and
has commenced tracking across the surface of the paper.
The encoder disc produces 1024 pulses across the printing width of the paper; these are reduced to 256
by the divide-by-four circuit to ensure that the register of each line of dots within the matrix is maintained
to within a quarter of the width of a dot.
C
D
IRCUIT
ESCRIPTION
9.1 TR1 and TR2 form a self-regulating 50V power supply. The 50V may be measured conveniently on C4
(see Figure 7).
9.2 When the stylus first makes contact with the paper at the left-hand margin, TR5 is switched on via R8.
This raises the level at pin 4 of the ULA and results in the encoder signal being passed to the associated
micro-computer. C6 and R11 prevent contact being lost momentarily and are critical in value. Many
problems have been experienced with devices giving too much gain and TR5 which on some early printers
was a ZTX550 should be replaced by a ZTX551.
TR3 and TR4 switch the 50V to the stylus in order to generate printing.
The phototransistor TR9 is soldered to the board, while the LED is on flying leads stuck into the base insert.
These items are subject to a lot of tolerances but there should be greater than 200 mV p-p at pin 2 of the
ULA when operating. Common faults encountered include, over-etched encoder disc, encoder disc
working loose, incorrectly formed slot m the base frame in front of the phototransistor (some have metal
shims with photo-etched slots). Dirt in the encoder disc or in front of the phototransistor also give
misleading results. The operation of transistor TR9 can be readily checked using an oscilloscope probe on
the emitter.
7
Sinclair ZX Spectrum Service Manual
Spectrum For Everyone https://spectrumforeveryone.com/
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