Shure VA 300 - Manual (Page 15)

Figure 13
Figure 14
Small Signal and Predriver Transistors:
Transistors Q1 through Q30, Figure 14, Page 15 and Q31 and
Q32, Figure 12, Page 14 are mechanically supported by their
leads. When replacing these transistors, it is imperative that
proper lead configuration be followed. A minimum of sold-
ering heat should be used to avoid damage to the transistor.
Refer to the lower right corner of the circuit diagram, Fig-
ure 18, Page 21, for lead code.
Diodes:
Diodes D6 and D7, see Figure 12, Page 14 are located on the
black finned heat sink with the output transistors. Special care
is required to insulate these diodes from the heat sink while
providing good thermal transfer from sink to diode. Heat
shrinkable tubing or “spaghetti” should be placed over the
diode and connecting leads; the diode should be securely
clamped to the heat sink with the clamp provided.
Check Transistors and Diodes:
Defective transistors and diodes may be located by use of an
ohmmeter. Polarity of the ohmmeter must be verified before
these checks are made.
With a known diode orientation, measure the diode resist-
ance in the forward and reverse directions. The lowest meter
reading will establish the probe at the cathode end (schematic
symbol arrow points to cathode) as the “minus” probe while
the other probe will be “plus”. Some ohmmeters are not polar-
ized in this manner with relation to “volts plus probe” and
“volts minus probe.”
To check transistors, the ohmmeter should be set to the 100
ohm or 1,000 ohm scale. Small signal transistors (Q1 through
Q32) must be removed from the circuit before testing. Transis-
tors mounted with screws (Q33 through 038) may be tested in
place; however, the leads to these transistors must be removed.
I f a l l c o n d i t i o n s i n t h e f o l l o w i n g t a b l e a r e m e t , t h e
transistor may be considered free of any gross defect; if any of
the following conditions are not met, the transistor should be
replaced. See lower right corner of circuit diagram, Figure 18,
Page 21, for transistor terminal code.
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