Canon PC770 Service Manual - Page 91

detected temperature

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Ik!;,,, ,",-i./ 011111111111M1,MII IM0PERATI0NS AND TIMING 5. Checking for Overheating at the End of the Fixing Heater The rear end of the copier's fixing heater is equipped with a sub thermistor (TH2) to check for overheating. The thermistor serves to prevent damage to the fixing film otherwise caused by overcurrent occurring in areas where paper does not come into contact while copies are made continuously on paper of A4 or small- er. In multifeeder manual mode, if the detected temperature of the main thermistor (TH1) is 190°C or less for the black developing assembly or 195°C or less for the color developing assembly, the distance between sheets of paper is increased (12 to 8 cpm), thereby decreasing the supply power to the fixing heater to prevent overheating at the end of the fixing heater. Once changed, the distance between sheets of paper is maintained until copying is over. Should the temperature at the end of the fixing heater still keep rising after increasing the distance between sheets and the sub thermistor (TH2) detects 255°C or more, copying operation is stopped after discharging the ongoing copy; at the same time, 'H' is indicated in the display of the control panel, disabling the Copy Start key. The 'H' indication turns off automatically when the detection temperature of the sub thermistor (TH2) falls below 240°C, and a press on the Copy Start key then will make the remaining number of copies. 6. Protection Mechanisms The copier is equipped with the following protection mechanisms to prevent malfunction of the fixing heater: a. Thermistor (TH1, TH2) The microprocessor on the DC controller PCB monitors the voltage of the thermistor (TH1, TH2); if it detects a high temperature error or a low temperature error, it sends the fixing heater error signal (HTNG) to the timer circuit and, at the same time, indicates an error code (E00 through E003) on the control panel. In response, the timer circuit sends the relay off signal (RLOFF*) to the auxiliary power supply about 1.5 after it has received the fixing heater error signal (HTNG). The auxiliary power supply in turn turns off the relay (RL601) to turn off the power supply. The data for problems associated with `E0', i.e., related to the heater, is backed up and `E0' will be indicated on the control panel next time the power is turned on. The data, however, is retained only about 5 min; when the power is turned off while 'E000' through `E003' is indicated on the control panel, 'EU will be indicated on the control panel if the power switch is turned on within 5 min, and the Copy Start key will not be enabled. If you have turned on the power switch about 5 min or more after you turned off the power switch while 'E000' through `E003' is indicated on the control panel, the copier will respond to a press on the Copy Start key. If an error associated with the heater is detected during copying operation, the copier once again will indicate `E000' through 'E003' on the control panel and will turn itself off. b. Thermal Fuse (FU1) If the area around the thermal fuse exceeds 226°C and remains so for a specific period of time, the thermal fuse melts to cut off the power to the fixing heater. COPYRIGHT 0 1994 CANON INC. CANON PC720/740/7501770 REV.() AUG. 1994 PRINTED IN JAPAN piPoit AU JAPAN) 3 - 51

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Ik!;,
,,,
,,
"
-
i./
011111111111M1
,
MII
IM
0PERATI0NS
AND
TIMING
5.
Checking
for
Overheating
at
the
End
of
the
Fixing
Heater
The
rear
end
of
the
copier's
fixing
heater
is
equipped
with
a
sub
thermistor
(TH2)
to
check
for
overheating.
The
thermistor
serves
to
prevent
dam-
age
to
the
fixing
film
otherwise
caused
by
overcurrent
occurring
in
areas
where
paper
does
not
come
into
contact
while
copies
are
made
continuously
on
paper
of
A4
or
small-
er.
In
multifeeder
manual
mode,
if
the
detected temperature
of
the
main
thermistor
(TH1)
is
190°C
or
less
for
the
black
develop-
ing
assembly
or
195°C
or
less
for
the
color
developing
assembly,
the
distance
between
sheets
of
paper
is
increased
(12
to
8
cpm),
thereby
decreasing
the
supply
power
to
the
fixing
heater
to
prevent
overheating
at
the
end
of
the
fixing
heater.
Once
changed,
the
distance
between
sheets
of
paper
is
maintained
until
copying
is
over.
Should
the
temperature
at
the
end
of
the
fixing
heater
still
keep
rising
after
increasing
the
distance
between
sheets
and
the
sub
thermistor
(TH2)
detects
255°C
or
more,
copying
operation
is
stopped
after
discharg-
ing
the
ongoing
copy;
at
the
same
time,
'H'
is
indicated
in
the
display
of
the
control
panel,
disabling
the
Copy
Start
key.
The
'H'
indication
turns
off
automatically
when
the
detection
temperature
of
the
sub
thermistor
(TH2)
falls
below
240°C,
and
a
press
on
the
Copy
Start
key
then
will
make
the
remaining
number
of
copies.
6.
Protection
Mechanisms
The
copier
is
equipped
with
the
following
protection
mechanisms
to
prevent
malfunc-
tion
of
the
fixing
heater:
a.
Thermistor
(TH1,
TH2)
The
microprocessor
on
the
DC
controller
PCB
monitors
the
voltage
of
the
thermistor
(TH1,
TH2);
if
it
detects
a
high
temperature
error
or a
low
temperature
error,
it
sends
the
fixing
heater
error
signal
(HTNG)
to
the
timer
circuit
and,
at
the
same
time,
indicates
an
error
code
(E00
through
E003)
on
the
con-
trol
panel.
In
response,
the
timer
circuit
sends
the
relay
off
signal
(RLOFF*)
to
the
auxiliary
power
supply
about
1.5
after
it
has
received
the
fixing
heater
error
signal
(HTNG).
The
auxiliary
power
supply
in
turn
turns
off
the
relay
(RL601)
to
turn
off
the
power
supply.
The
data
for
problems
associ-
ated
with
`E0',
i.e.,
related
to
the
heater,
is
backed
up
and
`E0'
will
be
indicated
on
the
control
panel
next
time
the
power
is
turned
on.
The
data,
however,
is
retained
only
about
5
min;
when
the
power
is
turned
off
while
'E000'
through
`E003'
is
indicated
on
the
control
panel,
'EU
will
be
indicated
on
the
control
panel
if
the
power
switch
is
turned
on
within
5
min,
and
the
Copy
Start
key
will
not
be
enabled.
If
you
have
turned
on
the
power
switch
about
5
min
or
more
after
you
turned
off
the
power
switch
while
'E000'
through
`E003'
is
indicated
on
the
control
panel,
the
copier
will
respond
to
a
press
on
the
Copy
Start
key.
If
an
error
associated
with
the
heater
is
detect-
ed
during
copying
operation,
the
copier
once
again
will
indicate
`E000'
through
'E003'
on
the
control
panel
and
will
turn
itself
off.
b.
Thermal
Fuse
(FU1)
If
the
area
around
the
thermal
fuse
exceeds
226°C
and
remains
so
for
a
specific
period
of
time,
the
thermal
fuse
melts
to
cut
off
the
power
to
the
fixing
heater.
COPYRIGHT
0
1994
CANON
INC.
CANON
PC720/740/7501770
REV.()
AUG.
1994
PRINTED
IN
JAPAN
piPoit
AU
JAPAN)
3
-
51