Heathkit H14 Manual page 19

Page 17
The signal from U202 is coupled to U203F, which
turns on 0205. Q205 supplies 35 volts to the paper
drive motor when it is required to turn. R216 and
R217 limit the cunent to the motor. The operation of
U203F. Q206. and 0205 is identical to the head driv-
ers.
PRINT HEAD TEMPERATURE CONTROL
0205 supplies a holding current to the paper drive
motor while the motor is at rest.
U117A and B are comparators which are configured
to measure the resistance of the copper winding of a
print head solenoid. The temperature coefficient of
copper wire is approximately OA%/"C. Therefore, this
method is very effective in determining the solenoid
temperature.
The temperature is sensed in two levels. When the
first level is exceeded, U117 A's output will change to
a "0" logic level. This signal is connected to the
microcontroller through U109 and is tested during
the time the head is returning home. If the head temp-
erature continues to rise due to:high ambient temper-
atures or very dense printing, U117B will be triggered
and the controller will cause printing to stop and
LE02 (High Temp lamp on the front panel) wiII light
continuously. Note: It is normal for this lamp to
flicker during active printing.
The voltage which the comparators are referenced to
appears across a precision voltage divider formed by
R191 through R194. The head solenoid is placed in
series with R189 and R195 across the same supply as
divider R191 through R194. This makes the circuit
insensitive to supply changes. To calibrate this cir-
cuit, the comparator reference is temporarily con-
nected to a point in the divider string which corres-
ponds to the cold resistance of the head solenoid. The
current through this solenoid is adjusted by R195 to
produce a voltage across it which is equal to the
calibration reference. This is indicated by High Temp
lamp LE02.
PAPER OUT/JAM DETECTOR
~~--------------
A logic "1" at the input of Ul15A causes its output to
be held at less than 0.4 volts. This causes diode 0111
to be ofr. no gate currellt flows in triac 0117, so 0117
is off or turns off when the motor current goes through
zero. With a logic "0" at the input of Ul15A, its
output goes high. This allows current to flow through
0111 to the gate of Ql17 and turn it on. Capacitor
Cl09 makes Ql17 less susceptible to noise pulses.
Because the three drivers are identical, only one of
them will be described.
PRINT HEAD DRIVERS
HEAD AND RIBBON MOTOR DRIVERS
PAPER MOTOR DRIVERS
Because the seven driver circuits are identical, only
one driver circuit will he described.
Buffer U1140 is an open collector TTL inverter. It
interfaces the microcontroller output with driver
transistor Ql03. With logic ''1'' on the input of
U1140, its output will be less than 0.4 volts. This
turns
Ql03
off, which in turn, turns Ql04 off. A logic
"0" at the input of U1140 turns on Ql03 and Ql04,
and current flows through the print head solenoid.
Diodes Z0101 and 0103 provide arc suppression.
Z0101 also makes the solenoid current decay more
quickly by forcing the solenoid voltage to reach sev-
eral volts before it zeners. Current peaks are supplied
by capacitor C4.
Again, because the four drivers are identical. only one
of them will be described.
A logic "1" at the input of Ul16B causes its output to
be a logic "0". Its output is connected to the input of
U203E, which causes the output of U203E to be high.
This allows current to flow in the emitter-base circuit
of Q204, turning it on. When a "0" is input to Ul16B.
the logic levels are reversed and Q204 turns off. 0204
is used for arc suppression.
•
•
Inputs to U2010 and U201A are always complements
of each other. Whenever the inputs change state. one
of the inputs to U2010 or A is delayed by the RiC
circuit (R203/C201 or R205/C202). This delay causes
the inputs to be the same for a short time. When this
occurs, a pulse is output from U2010 or A. These
pulses are coupled to U202 by U201C. U202 is a
retriggerable, one-shot multivibrator with a time con-
stant of 10 milliseconds.
LE03 and Q2 form an optical switch. LE03 is on one
side of the paper and 02 is on the other side. and in
line with the sprocket holes. Therefore. Q2 changes
state when a sprocket hole appears and detects that
the paper is moving. The logic levels from 02 are
tested by U106 (the microprocessor) through input
buffer U109. If a level change is not detected by the
time the processor has output enough pulses to the
paper drive stepper motor to move the paper approx-
imately one inch. the processor stops all printing.