Replacing Power Transistors
Verify the following conditions exist when replacing
power transistors .
1.
2.
3.
4.
Mica insulator is not damaged. If damaged, use new
insulator.
No grit or metal particles are between transistor and
heat sink.
Both sides of mica insulator are covered with silicone
grease or fluid.
Mounting screws are tight.
·Testing Transistors
Transistors should be checked with a transistor tester. If a
tester is not available, use the following procedure for test-
ing transistors with an ohmmeter.
Step 1. Remove suspected transistor from circuit (see
Replacing PCB Components).
~------NOTE-----------.
Step 2.
Step 3.
Step 4.
Transistors may be tested while still in the
circuit; however, the nominally accepted
10 : : 1 resistance ratio for 'front-to-back'
measurements may be lower.
Connect ohmmeter leads to base and emitter.
Read on lowest ohms scale. Reverse leads and
read again. Normal readings should be at least
10 times greater in one direction than in the
other.
Connect ohmmeter leads to base and collector.
Ohmmeter readings should be similar to those
obtained in Step 2.
If Steps 2 and 3 show normal function, connect
ohmmeter leads to collector and emitter. Read
on lowest ohms scale. Reverse leads and read
again. If reading is low and virtually unchanged
PIN 1 - BASE
PIN 2 - EMITTER
when ohmmeter leads are reversed, the transistor
has a short circuit between collector and emitter.
Replacing PCB Components
Before removing PCB components for testing or replacement,
read and perform the following instructions.
1.
2.
3.
4.
Solid-state components and PCB's may be damaged by
excessive heat. Use a small soldering iron with a
1/8-inch diameter chisel tip and use small-diameter
60/40 rosin-cored solder.
Remove components by placing soldering iron on
component lead on conductor side of PCB and pull
out lead. Avoid overheating the conductor.
~-----CAUTION----------.
The conductor on the PCB is a metal
surface plated with solder and laminated
to the board. Too much pressure or over-
heating may lift the conductor from the
board.
If component is faulty or damaged, clip leads close to
component and then unsolder leads from board . With-
draw leads from component side.
Clear solder from circuit board holes before inserting
leads of new component. Heat solder remaining in
hole, remove iron and quickly insert a pointed non-
metallic object, such as a toothpick, from conductor
side.
5. Shape new component leads and clip to proper
length. Lead shape should provide stress relief for
component. Insert leads in holes, observing same
polarity or orientation of removed component. Apply
heat and solder on conductor side.
Repairing Fractured or Damaged PCB Conductor
If a conductor is fractured, damaged or lifted from the cir-
cuit board, a recommended method of repair is to solder a
section of good conducting wire along the damaged area
and then seal with epoxy.
LEAD 1 - EMITTER
LEAD 2 - BASE
T0-3©2
& 0 01 0
TO-66
MTG FLANGE - COLLECTOR
LEAD 3 - COLLECTOR
LEAD 4 - CONNECTED TO CASE
TO-5
INDEX TAB
LEAD 1 - EMITTER
LEAD 2 - BASE
LEAD 3 - COLLECTOR & CASE*
*NOT ALL TYPES. SOME HAVE BASE-
TO-CASE INTERNALLY; OTHERS
HAVE NO CONNECTION TO CASE.
LEAD E - EMITTER
LEAD C - COLLECTOR
LEAD B - BASE
TO-
92
~ • • LEAD E - EMITTER
E C B LEAD B - BASE
LEAD C - COLLECTOR
Figure 7. Typical Solid-State Component Configurations
-9-
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