TM 11-6625-1633-45
and R2 and the positive lead is placed at ground, the
3-11. In-Circuit Transistor Tests
ohmmeter battery forward-biases the base-emitter junc-
The charts in a and b below contain voltage and resis-
tion to essentially cause a short circuit between the base
tance measurements taken with the transistors connected
and emitter. This places resistor R4 in parallel with re-
in the circuit. The measurements are made with Mul-
sistor R1. The ohmmeter reading in this case will be
timeter TS-352B/U.
slightly less than 1,000 ohms because of the parallel
NOTE
resistances. If the ohmmeter leads are then reversed, the
These readings will be valid only if the same
transistor will be reverse-biased, causing a very high
type of ohmmeter is used and the polarity and
resistance to appear between the base and the emitter. In
range scales are strictly adhered to.
this case, the reading would then be 10K or the value of
a. In-Circuit Transistor Voltage Chart .
resistor R1. If the reading is the same in both cases, the
transistor is probably an open circuit or a short circuit.
Transistor
Voltage (Volts)
Also consider that different values of resistance will be
(All measurements indicated are with
respect to ground.)
obtained with the ohmmeter on different ranges. For
Number and Type
Emitter
Base
Collector
Q1 2N174
-290.0
-290.0
-300.0
transistor junction is measured in the forward direction
Q2 2N600
-290.0
-290.0
-300.0
on the Rx10 range, the actual reading will be less than
Q5 26404
0.0
-0.16
-2.05
Q6 2N404
-0.25
-0.38
-11.2
that if it were taken with the ohmmeter on the Rx100
range.
b. In-Circuit Transistor Resistance Chart
Transistor
Measurements with negative ohmmeter lead connected to base
Measurements with positive ohmmeter lead connected to base
Number and Type
Emitter to base
Collector to base
Emitter to base
Collector to base
Resistance
Ohmmeter
Resistance
Ohmmeter
Resistance
Ohmmeter
Resistance
Ohmmeter
(Ohms)
Range
(Ohms)
Range
(Ohms)
Range
(Ohms)
Range
Q1 2N174
80
RX100
100
RX100
400
RX100
200
RX100
Q2 2N600
380
RX100
330
RX100
310
RX100
320
RX100
Q5 2N404
330
RX100
330
RX100
8,000
RX1000
10,500
RX1000
Q6 2N404
340
RX100
330
RX100
1,650
RX100
4,700
RX100
3-12. In-Circuit Transistor Resistance
Pin to Ground
Voltage
Checks of Module HR2
2
-250.0
4
-270.0
The measurements in the following chart are taken with
6
-270.0
8
-270.0
the module connected in the equipment. Module HR2
may be removed from the circuit by removing it from its
b. Resistance Checks at Chassis Socket of Module
socket. Out-of-circuit tests for HR2 are given in para-
HR2.
graph 3-13.
Pin to
Positive Ohmmeter
Positive Ohmmeter
ground
lead connected to pin
lead connected to pin
Module HR2
Measurement with positive
Measurement with positive
Resistance
Ohmmeter
Resistance
Ohmmeter
pin to
ohmmeter lead connected
ohmmeter lead connected
(K ohms)
Range
(K ohms)
Range
ground
to pin
to pin
2
250
X10,000
260
X10,000
Resistance
Ohmmeter
Resistance
Ohmmeter
4
250
X10,000
260
X10,000
(K ohms)
Range
(K ohms)
Range
6
250
X10,000
260
X10,000
2
250
X10,000
260
X10,000
4
250
X10,000
260
X10,000
c. Resistance Values of Module HR2
6
250
X10,000
260
X10,000
8
250
X10,000
260
X10,000
Pin to Pin
Positive ohmmeter lead
Negative Ohmmeter
connected to lower
lead connected to pin
3-13. Out-of-Circuit Tests of Module HR2
pin number
Resistance
Ohmmeter
Resistance
Ohmmeter
When trouble has been localized to module HR2, isolate
(ohms)
Range
(ohms)
Range
2 to 4
130 K
X10,000
1.4M
X10,000
the defective part by removing the module from its
2 to 6
1.9K
X100
10M
X10,000
socket and making the following voltage and resistance
2 to 8
1.7K
X100
9K
X10,000
measurements.
4 to 6
200
X100
10K
X10,000
4 to 8
1.5M
X10,000
260K
X10,000
a. Voltage Checks at Chassis Socket of Module
6 to 8
X10,000
12K
X10,000
HR2.
3-17
