Tektronix 3A74 Instruction Manual page 41

NOTE
As a general rule, do not use the RX1 and RX10
ranges. Use the higher ranges where the current
is limited to about 2 ma. You can quickly check
the current by inserting a milliammeter between
the ohmmeter leads and then noting the current
for each range you intend to use.
When you know which ohmmeter ranges will not harm the
transistor, then measure the resistance with the ohmmeter
leads connected first one way and then the opposite way
while using the information provided in Table 5-2.
TABLE 5-2
Transistor Resistance Checks
Ohmmeter
Connections *
Resistance Readings that can be
expected using the RX1K Range
Emitter-Collector High readings (but unequal) both
ways (about 60k to around 500k.)
High reading one way (about 200k
or more). Low reading the other
way {about 4000 to 2.5k}.
Emitter-Base
Base-Collector
*Ohmmeter test prods are first connected one way to the tran-
sistor leads and then the test prods are reversed (connected the
other way). Thus, the effects of the polarity reversal of the volt-
age applied from the ohmmeter to the transistor can be observed.
If there is doubt about whether the transistor is good
or not, substitute the transistor in the Type 3A74; but first
Maintenance — Type 3A74
be certain the supply voltages and the circuit are correct
before making the substitution.
Diode Checks
Diode defects are similar to transistor defects when the
diode opens or shorts. Therefore, checks similar to that
described for transistors can be applied to diodes.
NOTE
If you use an ohmmeter, observe the same pre-
cautions as described for checking transistors.
In-circuit checks of diodes (except if D424, D502, D468,
D469 or D595 open) can be made quite easily by using a
voltmeter to find out if the diode is functioning properly in
the circuit. Use Table 5-3 to determine whether a particular
diode (excluding Zener D494} should be forward biased or
not, then measure the voltage across it. By noting the
difference between voltages on each side of the diode as
given on the schematic diagrams, you can determine whether
the diode is normal or not. Refer to Fig. 5-9 if you need
to locate any of the diodes quickly.
If you are in doubt whether a diode is defective or not,
unsolder one end and check the forward-and-back resistances.
Use this method particularly for checking D424, D468, D469,
and D595 because voltage checks may not be very conclusive
if the diode is open. If the ohmmeter check proves inconclu-
sive, check the diode on a good tester or use the substitution
method.
TABLE 5-3
DIODE BIAS CONDITIONS
Chan. 1 MODE switch set to All MODE switches set to
Diode NORM.; Chan. 2-3-4 MODE NORM. or INV., CHOP.-ALT.
switches set to OFF. switch set as follows:
Ch. 1 Diodes! | Ch. 2-3-4 Diodes? CHOP.? ALT,! 3
D424 Reverse Reverse
D440 Forward Reverse
D44] Reverse Forward
D442 Reverse Forward
D443 Forward Reverse Refer to page 5-10 for
D468! Unbiased Unbiased sontinvation of these
columns
D469! Unbiased Unbiased
D502 Reverse None used.
D512 Forward None used,
* Exception—D468 and D469 are Output Amplifier diodes. The unbiased or zero volts reading
is obtained by setting POSITION (control(s) at or near midrange) so that a zero volts reading is
obtained on the VOM.
* With no signal applied to the Type 3A74 or the AC-GND-DC switches set to GND. Last two columns
show apparent bias conditions since VOM follows average pulse level.
* Time-base set for a 1 mSec/Div. free-running state.