CHAPTER 3 MAINTENANCE PROCEDURES
Ohaus Corporation www.ohaus.com
3-4
MB23 and MB25 Service Manual
3.3 PRELIMINARY CHECKS OF THE TOP HOUSING COMPONENTS
Some problems can be identified by testing resistance on cable connector plugs, immediately
after opening the Housing: the Temperature Sensor, Heater, Fan, and Ground circuit (on the
MB23). It is also possible at this stage to test the Power PCB and Main PCB.
Figure 3-8. Cables for Temperature Sensor, Fan, Ground, and Heater, disconnected.
(
Note:
MB23 shown; MB25 does not require ground cable for heating element.)
3.3.1 Test the Heater Circuit
1. Measure resistance on Heater connector plug: insert probes into plug. (See Fig. 3-8.)
If there is no continuity, Thermal Overload, Cables, or Heater may be defective.
2. Open and close Top Housing while checking for an intermittent cable problem.
Readings should not fluctuate during this test.
3. Perform tests on Power PCB
– see Section 3.4 (next page).
3.3.2 Test the Temperature Sensor and Cable
1. Connect probes to the Te
mperature Sensor Cable’s plug. (See Figure 3-8.)
2. Measure resistance: it
should be approximately 1,000 Ω.
3. Open and close the Top Housing while checking for an intermittent cable problem.
Readings should not fluctuate during this test.
4. If resistance and flex tests (2 & 3 above) are OK, the Temperature Sensor is good.
If not, replace the cable or sensor. (See Section 3.9.7.)
CAUTION:
WHEN CONDUCTING TESTS WITH POWER ON, BE EXTREMELY CAREFUL!
TOUCHING PCB COMPONENTS WITH POWER ON MAY CAUSE SERIOUS INJURY.
3.3.3 Test the Fan
1. Connect AC cord to the analyzer and apply power.
2. Connect pins 18 and 13 on Power PCB Connector CN3. (See Section 3.4.)
Fan should run.
Disconnect power before proceeding.
3. If fan does not run, apply external power to fan. If fan runs, replace the Power PCB.
3.3.4 Test the Ground Circuit on the MB23
1. Connect one probe to the Ground Cable plug, and the other to the Heater.
2. Measure resistance. It should be approximately zero.
