\section{Mains Input and Transformer}

Mains enters the printer via a IEC60320 C14 plug with integral filter on 
the rear panel. It then passes via the 1.5A fuse in the live wire to the 
double-pole on/off switch to the voltage selector switches and the mains 
transformer primary windings.

The mains transformer has 2 primary windings, each is 120V with a tap at 
100V. The  voltage selector switches connect them as follows for different 
mains input voltages : 
\begin{description}
\item[100V] :] The 2 100V sections of the primary windings are connected 
in parallel
\item[120V :] The 2 complete 120V primary windings are connected in parallel.
\item[220V :] The 100V section of one winding is connected in series with 
the 100V section of the other.
\item[240V :] Both 120V windings are connected in series.
\end{description}

In all cases, the cooling fan is connected across one of the 120V 
windings. Thus this fan operates at 120V, using the mains transformer 
primary windings as an autotransformer to obtain this voltage.

The mains transformer has 5 independent secondary windings which are 
used as follows : 
\begin{description}
\item[18V :] (Blue wires) used for the -12V logic supply
\item[18V :] (Orange wires) Used for the +12V logic supply and the 
Power-OK circuit
\item[14-0-14V :] Yellow wires with brown centre tap) This winding powers 
the motor driver circuitry as described earlier
\item[10V :] (Green wires) Used for the +5V logic power supply.
\item[39V :] (Grey wires) Used for the printhead power supply.
\end{description}

\section{Logic Power Supply}

The {\bf Logic PSU} PCB provides +12V, -12V and +5V regulated supply 
rails for the printer's control electronics using linear regulator 
circuits. It also provides the {\bf  Init/} power-on reset signal.

\subsection{+12V Supply}

One of the 18V  secondaries of the mains transformer is bridge-rectified 
and smoothed, producing approximately 24V across C1. This voltage  is 
regulated down to 12V by the 3-terminal regulator U1.

\subsection{-12V Supply}

This uses an identical circuit to the +12V supply powered from the other 
18V secondary winding. However, i nths case, the output terminal of the 
regulator is connected to ground. Since the action of the regulator IC 
is to maintain this connection at 12V above the common terminal, this 
common terminal (and the -ve side of C2, etc) is at -12V  with respect 
to logic ground, as required\footnote{There is no such thing as 
ground\ldots}

\subsection{+5V Power Supply}

The +5V supply, used for almost all the digital electronics in the 
printer, is based around the 723 IC U3 on the {\bf Logic PSU} PCB. This 
IC contains a 7.15V reference voltage source, an error amplifier and a 
drive transistor. 

The input to this regulator circuit comes from the 10V secondary winding 
of the mains transformer. This is rectified and smoothed by 
chassis-mounted components and produces about 12V DC across the 
18000$\mu$F capacitor. 

The regulator control circuitry is located on the {\bf Logic PSU} PCB. The
inverting input of the error amplifier in U3 is connected to the +5V
supply line on the {\bf Logic Backplane}. The non-inverting input is 
connected to a divided-down version of the 7.15V reference. Due to the 
values of the resistors R 1 and R3, the non-inverting input {\bf +5VRef} 
is kept at 5V. The action of the error amplifier is then to control the 
output transistor inside the IC to make these 2 voltages equal. This output 
transistor controls Q1 (on the {\bf Logic PSU} PCB, which in turn controls 
the pass transistor Q1 on the main chassis. The overall action, therefore 
,is to maintain the logic supply rail at +5V.

If the current drawn from this regulator becomes too great, the op-amp 
U4, which monitors the voltage across the 0.1$\Omega$ sense resistor R10, 
activates the current sense circuit inside U3, thus turning off the pass 
transistor and turning off the +5V power rail.

The 18V secondary winding used to supply the +12V power supply is 
rectified, clamped by the zener diode CR11 and smoothed  by C8. The 
voltage across this capacitor therefore indicates that the mains input is 
present. Should the mains fail, then transistor Q5 will turn off, allowing 
Q4  to turn on. This will short the {\bf +5VRef} signal to ground and 
thus cause U3 to reduce the output voltage to zero.

Protection for the logic ICs in the event of a failure in this regulator 
circuit is provided by the crowbar circuit based around the thyristor 
(SCR) Q2. If the +5V supply rail rises too high, zener diode CR12 
conducts and applies a gate drive current to Q2. Q2 then turns on, 
shorting the +5V line to ground protecting the logic. This will probably 
blow the 7A fuse in the regulator circuit.

\subsection{Init/ Signal}

This signal provides a  power-on reset to the control state machine to 
ensure the printer starts in the correct state. Capacitor C9 is charged 
from the +5V line at power-on via a 39k resistor. When the voltage across 
it exceeds the input threshold of U5a (and providing the mains input is 
present), U5a's output goes low, causing the output of U5b to go high. 
Feedback form this output to U5a's input provides hysteresis to prevent 
the circuit from oscillating.

The output of U5b is therefore low at power-on and goes high a short time 
later when C9 is charged. This output is logically ORed with the {\bf 
Initin/} signal from the test connector (which can be used to reset the 
printer during testing) by U5c and U5d to provide the {\bf Init/} signal 
to the printer  logic.

The power supply for U5 comes from the +12V via a simple linear regulator 
circuit consisting of zener diode CR15 buffered by the emitter follower Q3.

\section{Printhead Power Supply}

The 20V power supply needed for the printhead elements is produced by a 
switching regulator contained on the {\bf Printhead PSU} PCB along with 
the associated chassis-mounted components. 

The 39V secondary winding of the mains transformer is bridge-rectified 
and smoothed, resulting in approximately 52V across the 10000$\mu$F 
chassis-mounted capacitor. This voltage is used to power the step-down 
switching regulator

The chopper transistor for this regulator is Q2 on the main chassis. When 
it is turned on, current flows from the 52V supply to the load via the 
inductor on the {\bf Printhead PSU} PCB. This inductor causes the load 
voltage to change relatively slowly as the magnetic field builds up in 
its core. When the chopper transistor is turned off, the magnetic field 
in the inductor core collapses  and current  flows from the inductor via the 
chassis-mounted flyback diode, thus again supplying power to the load. By 
varying the on:off ratio of the chopper transistor base drive  signal, the 
output voltage of the power supply can be controlled.

The chopper transistor is controlled by the {\bf PHPSUDrv} signal. When 
it is high, transistor Q6 is saturated. This turns on Q2, and in turn Q4, 
which provides base current to the chopper transistor. When {\bf 
PHPSUDrv} is low, then all these transistors are cut off. Should the 
chopper transistor base current rise too high, the excessive voltage drop 
across R25 (0.15$\Omega$) will turn on transistor Q5, thus preventing Q2 
from turning on and thus disabling the chopper transistor.

The chopper control circuitry therefore controls the high:low 
ratio\footnote{Or mark:space ratio, a term from telegraph signalling 
instruments.} of the {\bf PHPSUDrv} signal depending on the output 
voltage. This circuit operates as follows: U4 is wired as a relaxation 
oscillator which charges and discharges the 6.8nF capacitor on its 
inverting input. The {\bf osc} signal is thus a non-linear ramp signal. 
This is fed to the inverting input of U5, which is used as a comparator. 
The non-inverting input of U5 comes from the error amplifier U2 which 
compares the actual printhead voltage ({\bf PH+}), divided down to 
produce {\bf PHSense} with the reference voltage from a 6.4V zener diode. 

The output of U5 is high when the ramp voltage is less than the output of 
U2. If the printhead supply voltage rises too high, the output of U2 is 
becomes lower so the output of U5 is high for a shorter time. Similarly 
if the output voltage is too low, the output of U5 is high for a longer time.

The output of U5 is fed into the AND gate  produced by inverting the 
output of the NAND gate U6a using U6b. The other 2 inputs of this NAND 
gate shut the power supply down if the logic power supplies are not 
present (that is, if Q7 is turned on or if the +5V lien is totally 
missing) or if the printhead current is too high (comparator U3's output 
goes low). 

As in the case of the +5V logic supply, the printhead supply is protected 
by a crowbar circuit. Should the output voltage rise too high, Op-amp U1, 
again sued as a comparator) will change state. The inverting input of 
this amplifier is kept at a stable voltage by a zener diode, the 
non-inverting input is a divided down version of the printhead power 
supply voltage {\bf PH+}. When the latter rises too high, the output of 
U1 goes high, turning on transistor Q1 and providing gate drive to the 
thyristor Q3. This then shorts the input to the chopper circuit to 
ground, blowing the 10A fuse and disabling the power supply. 

