Service manual

STP 11-25R13-SM-TG
F - 3
(5) If the two outputs are not symmetrical, then an adjustment of R6 should correct this problem.
(6) Any change in the values of resistors and capacitors in the base circuits of the two transistors
would have an effect on the output frequency. If Dl or D3 became shorted, the collectors of the two
transistors would no longer be isolated from the charge and discharge paths of C3 and C4. This would
affect the output frequency and cause the output pulses to be more rounded on their respective corners.
b. Bistable MV.
(1) Troubleshooting the bistable MV will be slightly different than the astable because the bistable
requires an input signal to operate. However, once an input signal has been applied, the principles
involved are basically the same.
(2) Refer to Figure F-2. If D4 or D5 were to open, there would be no switching of states and no
output pulses would be produced. Shorting D4 or D5 would have an effect on the switching time between
the states and cause the output pulses to be more rounded on the corners.
Figure F-2. Bistable Multivibrator
(3) If there is no output pulse at TP16, the first thing to check for is a trigger pulse at the input and
that the components in the input circuit are good. Check diodes D4 and D5 (front-to-back resistance
ratios). Isolate if possible; otherwise, make comparative resistance checks.
(4) In Q3 or Q4, if the base-emitter or collector-base junction were to short or open, there would be
no output pulses. As in-circuit comparison of front-to-back resistance ratios, compared to known good
readings, should confirm which transistor is bad.
(5) A short or open in any part of the MV would in most cases eliminate the output pulse, change
the pulse width, or change the output frequency; a check of the output at TP16 should confirm this type of
malfunction.
(6) An open in either transistor circuit between the collector of one transistor and the base of the
other would not allow the switching process to take place and there would be no pulses in the output.