Voltage Measurements - Codan X-2 Service Manual

X-2 Technical Service Manual
Transistor tests (dynamic)
Some transistor faults can be diagnosed by measuring voltages within the
circuit. One of the most significant voltage measurements is the base-emitter
voltage. The polarity of this will depend on the type of the transistor (PNP or
NPN). A base emitter voltage between 0.6 and 0.8 V should be measured on
a forward-biased base-emitter junction (double this voltage for a darlington
transistor).
With its base-emitter junction forward biased the transistor should conduct.
Some indication of satisfactory operation of the transistor can be obtained by
measuring the voltage drop across its collector or emitter resistor and short
circuiting its base to the emitter. The short circuit removes the forward bias
cutting off the transistor, so that the voltage across the resistor is reduced
considerably.
Integrated circuits
Before replacing an integrated circuit which has no output, make sure the
fault is due to the IC or its load. As a general rule, if changes in input cause
absolutely no changes in the corresponding output the IC should be
suspected. If, however, even a very small change in output can be detected
the load is more likely to be the cause. Depending upon the circuit further
tests should be made by disconnecting resistors, capacitors, etc to verify this
diagnosis before removing the IC.
Reading the Technical description and understanding how the Transceiver
functions, will assist in diagnosing any possible faults that may occur.
5.3.2

Voltage measurements

The circuit diagrams and relevant circuit notes give voltages at various points
under the various conditions to enable the faulty section of the Transceiver to
be located. The parameters listed below should always be checked first:
The supply voltages used on the Rx/Exciter & Control PCB are:
'A' rail unregulated supply 13.6 V nominal.
'B' rail regulated supply (IC401) +10 V ± 0.2 V.
+5VA regulated supply (IC3) +5 V ± 0.4 V.
+5VB regulated supply (IC402) +5 V ± 0.4 V.
TP6 switched supply (IC7/C), B rail voltage in receive, 0 V in transmit.
TP7 switched supply (IC7/D), B rail voltage in transmit, 0 V in receive.
The supply voltages used on the PA PCB are:
'A' rail unregulated supply 13.6 V nominal.
+5V regulated supply selected in transmit only (IC2) +5 V ± 0.4 V.
Zener controlled supply to IC1 (V1) +5 V ± 0.25 V.
Publication No: 15-02047
Maintenance 5-5