-10-
Note the schematic diagram on the back cover of
this manual. The test transistor in this circuit is a
NPN. Adjusting the variable resistor will cause the
NPN LED to light, indicating that base current is
flowing. The output of the test transistor is fed to
amplifier Q2 and Q3. The output of Q2 is fed back in
phase to the base of the test transistor causing the
circuit to oscillate. Part of the oscillations are fed to a
power rectifier Q5 which switches on the OK LED
indicator.
The design configuration is such that in-circuit
transistors can be measured provided that the base
and collector resistors are greater than 100 ohms.
When measuring PNP transistors, the power
supplied to the test transistor is reversed via the
NPN/PNP switch, therefore the PNP LED will light.
Varying the base current control will reduce the base
current. The lower the base current, the higher the
gain of the transistor under test. Comparative tests
of two transistors gain (beta) can be made by
observing the dial setting or the intensity of the base
LED diode. The lower the setting with the PNP LED
lit, the higher the beta of that transistor.
On diode operation, power is applied to IC1. This
causes the circuit to oscillate at about a 1Hz rate.
Placing a diode in series with the LED indicators will
cause a current to flow depending on the direction of
the diode. Thus, the red test lead will identify the
cathode or anode of the diode via the LED readout.
Transistor Q4 reverses the current flow in this circuit.
All types of diodes may be tested: Silicon,
germanium, LEDs or zeners over 6 volts. Zener
diodes under 6V causes the second LED to glow at
lower intensity, indicating that zener breakdown has
occurred.
THEORY OF OPERATION
If any problems occur, first check your wiring and
soldering. Keep in mind that most problems are due
to poor soldering or wiring errors. Be sure that there
are no solder shorts, poor connections or wiring
errors. Check that the battery is connected. Be sure
that the transistors are in correctly. Check the LEDs.
The LED leads are hard to identify and can easily be
wired backwards. The following is a guide to help
solve most problems.
Diode Position:
Red and black leads shorted
together. LEDs should alternately blink at 1Hz rate.
A. LEDs do not blink.
Check that IC1 is in
correctly. Check wiring around IC1.
B. Only one LED blinks.
Check for open LED.
Check transistor Q4.
C. Both LEDs blink together.
One LED is in
backwards.
Transistor Position Using Leads:
B&E test leads
shorted together. Rotate base current from minimum
to maximum. NPN LED should light up when
switched to NPN and PNP LED should light when in
PNP position. OK LED should not light.
A. No LEDs light.
Check that the battery is
connected properly. Check for open circuit at
R15, B or E leads.
B. Only one LED lights.
Check wiring and
soldering. Check for open LED.
C. Both LEDs light at the same time.
LED is in
backwards.
Transistor Position Using Socket:
Put the good
NPN transistor into the test socket. Switch SW1 to
NPN position. Rotate base current control. The OK
lamp should light. If not, check the following:
A.
No test leads are shorting together.
B.
The NPN LED should be lit. If not, then refer to I.
C.
Components around transistors Q1 to Q5 are in
properly. Be sure no soldering errors exist.
D.
Transistors are not in backwards.
E.
Touch the collector of Q1 with the black (E) test
leads. The OK
LED
should light. If not, then the
LED is either open or in backwards.
F.
Look for a possible defective transistor or other
component.
TROUBLESHOOTING GUIDE
