SERVICE MANUAL
Medi-Therm II
THEORY OF OPERATION
w
w
w
.g
aymar
.com P/N 10187-000 7/98
The control latch U51 on the control/display board is the interface between
the microprocessor (via the data bus at address 0FF7FH) and the refrigeration
compressor relay driver on the power supply board. A high signal on the
appropriate data line causes a low signal at pin 6 of U51 on the control/display
board, which then activates Q12 on the power supply board. Q12 on the
power supply board is the interface between the control/display board and the
coil of the power relay K1 located in the machine base. The microprocessor
switches power through the relay to the refrigeration compressor at cut-out
and cut-in temperatures of 38°F (3.3°C) and 42.5°F (5.8°C). These
temperatures are sensed by the cold water reservoir probe RTl located in the
water reservoir. (See figure 12, p. 71.) Control of the cold water reservoir
temperature takes place whenever the machine is on.
The power supply board includes the
REMOVE FROM USE NOW / MACHINE
SHUTDOWN
circuitry, which includes U3, U4, U5, U6, U9, U10, D2, D3, Q1,
Q2, their interconnected components, and fixed, nonadjustable thermostats
S2, S3, and S1 located in the base. Under normal circumstances, Q1 and Q2
are kept turned on by the action of R7, C10, and D4 and R5, C15, and D5 to
complete the conduction path for the heater and pump. If the blanket water
falls into the low temperature limit range, S1 will open. If the blanket
temperature rises into the high temperature limits ranges, S2 and/or S3 will
open. (See section 4.2 Thermal Specification table, page 5, for the correct high
or low temperature limits with corresponding model number of your
machine.) When any one of these thermostats opens, it directly interrupts the
circuit and shuts off the pump and heater; at the same time, full line voltage
will appear between J1-2 and J1-3. In this case, U5 and U10 will be turned on
by the action of R4, D3, and associated parts while U3 and U9 will be turned
on by the action of R6, D2, and associated parts. U5 prevents Q2 from
turning on and U3 prevents Q1 from turning on even if the open
thermostat(s) closes again. The output of either U9 or U10, through buffer
Q10, signals the microprocessor that a thermostat has tripped and that a
REMOVE FROM USE NOW / MACHINE SHUTDOWN
condition has resulted.
Thus, should any thermostat (S1, S2, or S3) trip, the heater and pump are
shut off and the microprocessor is notified.
On the control/display board, a high signal from Q10 of the power supply
board feeds NOR gate U40 to drive the audible alarm, feeds driver U52 to
light the
REMOVE FROM USE NOW / MACHINE SHUTDOWN
LED, and feeds
driver U11 to light the
ALERT
LED. All this is done independent of the
microprocessor. This same signal is sent to port pin P3.2 of the
microprocessor through C1, R1, and driver U11.
If the microprocessor is operational at the event of a high signal from Q10 of
the power supply board, the signal at P3.2 causes the microprocessor to shut
off the 7 segment displays, flash the
ALERT
LED and light the
REMOVE FROM
USE NOW / MACHINE SHUTDOWN
LED, toggle the audible alarm, store the
appropriate RFU* code indicating the reason for the shutdown, turn off the
heater and pump triacs Q3 and Q4, turn off the solenoid triacs Q8 and Q9,
and turn off the compressor transistor Q12. Anytime the microprocessor
6.4.6 COLD WATER
RESERVOIR
TEMPERATURE
CONTROL
6.4.7 BACK-UP WATER
TEMPERATURE
LIMITING
* RFU =
REMOVE FROM USE NOW / MACHINE SHUTDOWN
16
