TM 9-6625-1753-14
C1
5-9.1. Troubleshooting Dual Trace Plug-In 79-02A
a. General. Dual trace plug-in 79-02A, a dc coupled amplifier, contains two preamplifiers, a common amplifier, a
gating and control section, and a trigger generation circuit (fig. 4-4.1); front panel controls, available at key points in these
circuits, may be used to isolate malfunctions. Using these controls, initial isolation of a malfunction to a suspected group
of circuits is relatively easy. The verification that the selected group of circuits is the cause of fault, and the further
isolation of the malfunction to a part, should be performed by normal troubleshooting procedures, including signal tracing
techniques and voltage and resistance measurements. Methods of using controls in the isolation of malfunctions is
discussed in b through d below.
b. Weak or missing signals. Although weak amplification of signals may be noted during use of the instrument, this
condition is normally detected or verified when checking the gain of the instrument, using the CAL position of the VOLTS
DIV switch. If either channel does not respond properly to the CAL signal, the other channel should be checked. If one
channel is weak and the other normal, the channel preamplifier should be suspected as a cause of the malfunction. Since
the POSITION control, electrically located in the center of the preamplifier, causes an action very similar to a signal,
adjustment of this control simulates a signal applied to the center of the preamplifier. If the preamplifier responds normally
to movement of the position control, the malfunction is likely to be electrically located before the control. If both channels
are weak, the amplifiers common to both channels, or the switching circuits should be suspected (the calibration voltage is
a possible, but unlikely cause of trouble). The switching circuits can be eliminated by measuring the bias on the switching
amplifiers.
c. Improper Switching. The MODE switch may be used effectively to determine the cause of a switching
malfunction. If switching does not occur in the ALT or CHOP modes, the MODE switch should be set to CH1 and CH2
alternately. If the preamplifiers can be switched with manual control, the switching multivibrator and switching amplifiers
should not be suspected; instead, the basic timing oscillator should be suspected. If the unit fails to switch with manual
control, the switching multivibrator B1 and switching amplifiers should be suspected.
d. Trigger generation failure. If the internal trigger will not operate the oscilloscope, the trigger amplitude should be
tested to assure that the dual trace plug-in 79-02A is at fault. If the unit is at fault, the faulty circuit may be a normal
trigger amplifier, a channel 2 trigger amplifier, or a common section of amplifiers. By switching the TRIGGER switch
between NORM and CH2 ONLY, the area of the fault can be localized.
e. Poor frequency response. Only the instrument calibration personnel are capable of correcting problems of poor
frequency response in this instrument. The instruments issued to support personnel are not suitable for detecting these
problems, or for alining the associated circuits.
Figure 5-13.2. Bottom view of dual trace plug-in 79-02A
Figure 5-13.1. Bottom view of dual trace plug-in 79-02A
showing transistors.
showing controls.
5-10.1
