Thermo Scientific 815 Operating And Maintenance Manual page 25

Section 4
Service
Possible Solutions
(continued)
4-4 Refrigerated Incubator
There is another device known as an Optoisolator which serves as the
high/low voltage isolator between the triac and the microprocessor.
When the incubator starts experiencing temperature problems and/or
variations, due to known controller malfunctions, these two components
become prime suspects. The reason being they are under higher operating
stress than other components.
If the incubator starts experiencing temperature problems such as no heat,
constant heat, or "creeping" upward heat, then follow the troubleshooting
instructions below. These instructions require the use of a voltmeter being
able to measure DC and AC voltages and preferably a digital voltmeter.
Warning The following troubleshooting instructions require that power be
on. only qualified service personnel should perform these procedures.
A) Remove the control cover and familiarize yourself with the power
supply assembly #3176814. Locate the Triac (Q2), the Opto-isolator
(U1), resistor (R4). Also locate the test point #2 (TP2). The first
measurements will be DC voltage measurement, so a DC scale of at
least 10 volts should be selected. Connect the negative lead to TP2.
This is DC ground. Also locate the terminal block (TB1).
B) This first section will be to verify the heater command is correct from
the microprocessor, through the optoisolator, and through the Triac
when the incubator is NOT requesting heat. The steps in this section
must be followed in succession.
1. Select a setpoint temperature at least 10° below what the actual
temperature is. The incubator should not be requesting heat as
indicated by the "Heater On" indicator lamp on the front panel. It
should not be on continuously or even flashing.
2. Measure the voltage at U1-Pin 2 with respect to TP2. It should be
no less than 4 volts DC.
3. Measure the DC voltage across R4, since the incubator is not
requesting heat, there should be no current flowing through this
resistor, making the voltage drop equal to zero volts.
4. If the last two steps are not as stated, then most likely the CPU
board is bad and should be replaced.
5. Switch the voltmeter to an "AC Volts" scale capable of reading 120
volts.
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