(a) When you depress switch "B", a positive pulse is placed to the base
of Q4. Its collector goes low and is applied to the base of Q5. Q5 turns on; its collector
goes high. This high turns on Q13, energizing K4. The high from the collector of Q5 is
also applied to the base of Q8. Q8 turns on, turning Q9 off. Q9's collector goes high.
This high is applied back to the base of Q4, latching it on.
(b) When K4 energizes, its N.O. (normally open) points close and
power is applied to the windings of the motors bringing the chair back to the exit
position. When the chair reaches the exit position, the limit switches will open the
current paths to the motors.
(c) If switch "A" is depressed before the chair has completed its
movement, a high will be placed at the base of Q6. Its collector will go low to the base
of Q13, turning it off and de-energizing K4. Power from the motors to CR7, CR8, and
CR9 will be removed, turning off Q7 and Q4. Q4 and Q5 will keep Q13 off and K4 de-
ISOLATE MALFUNCTIONS TO THE COMPONENT LEVEL
Troubleshooting is a systematic method of isolating malfunctions by means of
tests based on symptoms. It is intended to reduce the amount of time normally required
to locate maladjustments and defective components. It is also intended to restore
equipment to a serviceable condition.
When you isolate dental chair malfunctions you
use a troubleshooting guide. Refer to the Dental Chair Troubleshooting Guide in the
appendix. Based on the observed problem, the guide tells you a probable cause, how
to test for that cause, and if the test is positive, the corrective action to take. If the test
shown is negative, check for other chair problems and troubleshoot them. The
troubleshooting guide supplements your experience as the repairer. As you gain
experience isolating malfunctions in the chair, you will rely less and less on it. To
perform some of the appropriate tests you need a digital multimeter.
a. If the chair will not operate (lift), the troubleshooting guide shows 16 probable
causes from a list of all possible causes. This lesson covers the first eight of those
probable causes. See the guide, "Probable Cause" column.
"Will not operate" means that the chair does not operate at all. "Fails to
function properly" means that it operates at less than manufacturer's
(1) The power cord is not plugged in to the supply receptacle or that the
circuit breaker is off. The testing procedure for this probable cause is to visually inspect
the power cord and the circuit breaker. See the guide, "Testing Procedure" column.