Reciprocal Shaking Water Baths
10-5
Thermo Scientific
Troubleshooting
Section 10
*********** Problem A: No Heat (continued) **************
6. Check heater.
a. Disconnect the bath from its electrical supply.
b. Isolate the heater from any circuitry by disconnecting one of the heater
leads.
c.
Using an ohmmeter, check the heater resistance. Appropriate heater
resistance values are listed at
right:
If the resistance reads 0 or
infinity, then replace the
heater.
d. Check the resistance between the heater leads and ground (green wire). If
the resistance reads 0 ohms, replace the heater.
********* Problem B: Constant Heat **********
1. Bath is heating when it is not requesting heat.
a. Select a setpoint temperature at least 10° below the actual temperature.
The bath 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.
b. Measure the voltage at U1-PIN 2 with respect to TP2-COM. It should
be no less than 4 VDC.
c. Measure the DC voltage across R4. The voltage should be approximately
zero volts.
d. If the last two steps are NOT as stated, then most likely the CPU board is
bad and it will have to be replaced, otherwise go to the next step.
e. Switch the voltmeter to an AC volts scale capable of reading 120 volts.
f. Measure the voltage between U1-PIN 4 and U1-PIN 6. It should be line
voltage 110 VAC to 120 VAC.
g. If it is not, most likely the opto-coupler U1 is bad and the POWER
SUPPLY board will have to be replaced. If it is, continue on.
h. Measure the voltage directly across the heater. It should be approximately
0 VAC.
i. If it is not, them most likely the triac Q2 is bad,and the POWER SUPPLY
board will have to be replaced.
MODEL
120 Volt
240 Volt
2870/2871
15 OHMS
57 OHMS
2872/2873
9 OHMS
38 OHM
