Service manual

STP 11-25R13-SM-TG
F - 1
APPENDIX F – MULTIVIBRATOR CIRCUITS TROUBLESHOOTING GUIDELINES
F-1. Basic Approach to Troubleshooting Multivibrator (MV) Circuits.
In troubleshooting MV circuits, as in troubleshooting any other circuit, try to isolate the stage before
looking at specific components. The test equipment and tools used for troubleshooting can vary from
common to highly specialized equipment. The instruments considered essential for multivibrator circuits
are a multifunction meter and an oscilloscope.
a. Voltage measurements are often the fastest way to isolate troubles in any electronic circuit.
Resistance measurements can be used to detect changes in resistance values and can quickly locate
shorts or opens. DC current measurements can also be helpful in tracking down troubles.
b. The oscilloscope is useful to determine whether or not the circuit under test has an output
signal. In multivibrators that require an input trigger in order to produce an output, two methods can be
used to determine if the circuit is operating properly.
(1) One basic test method is the dynamic test. It consists of observing how the circuit functions
with an input signal.
(2) A check of circuit conditions with no signal applied. Examples of static tests are voltage,
current, and resistance measurements.
c. In addition to the dynamic and static tests, there are a few other testing methods that apply to
all MVs. In general, they take the form of observing the results of opening circuits and replacing or
shorting various components. Do not attempt these tests unless absolutely necessary.
d. With no signal applied (triggered MVs), or where there is no output signal (free running MVs),
voltages can be measured at various points in the circuit. One method to determine the proper voltage
readings is to compare some readings from an operational unit with one that has a fault.
e. When current measurements are taken and the indications are larger than normal, it is probable
that a short exists somewhere in the circuit, or that a transistor is drawing too much current. If the current
readings are low zero, it is probable that an open exists somewhere in the circuit.
f. Voltage tests isolate the trouble to a particular stage in the faulty circuit and sometimes indicate
a certain component as the cause of the trouble. However, a resistance test is often the only positive
check of the suspected component.
g. Although voltage tests have indicated that a particular component is defective, it is best to
confirm your suspicions by making a resistance test before replacing the item. When you suspect that a
component is shorted or open or has changed value radically, cheek the resistance of that component.
h. Remember, all resistance checks are made with the power OFF and possibly the component
should be isolated from the rest of the circuit. If isolation is not possible, then a comparison with
resistance measurements of an operational circuit will be helpful.
i. Continuity testing is a variation of resistance measurements. This is checking for a continuous
or unbroken path for DC. A circuit has continuity if it will conduct direct current. Normal resistors,
connecting leads, or soldered joints should have continuity. An ohmmeter is normally used for continuity
checks. If the portion of the circuit under test cannot be isolated, remember to take into account alternate
paths for current when making your check.