Bottom Removal:
To remove the chassis bottom plate, turn the amplifier
upside down and remove the ten screws located at the
edges of the chassis.
Cover Removal:
For servicing of the components on the top of the chas-
sis, remove the two screws on the handle end of the cover,
and the two screws within the plastic feet nearest the bot-
tom on the other end of the cover. Uncoil the A.C. line
cord from the storage spindles and slide off the cover.
Small Signal and Predriver Transistors:
Transistors Q1 through Q3, Figure 6, Page 7, are me-
chanically supported by their leads. When replacing these
transistors, it is imperative that proper lead configuration
be followed. A minimum of soldering heat should be used
to avoid damage to the transistor. Refer to the lower right
corner of the circuit diagram, Figure 7, Page 9 for lead
code.
Driver Transistors:
The driver transistors Q4 and Q5, Figure 5, Page 6, are
located on the amplifier chassis. Before removing these
transistors, write down the lead color and location at each
transistor solder junction. If replacing transistors, apply
type 120 Wakefield thermal joint compound to each side
of the insulating wafer to provide good thermal transfer
from transistor to chassis. After replacement and before
connecting transistor leads, check transistors with an ohm-
meter between case and chassis; there should be no con-
tinuity. Be sure that these transistors are not inverted in
the circuit; they are not identical devices. Q4 is an NPN
transistor, while Q5 is a PNP transistor. Refer to the lower
right corner of the circuit diagram, Figure 7, Page 9 for
terminal code. NOTE: When replacing driver transistors,
perform the following modification (if not already per-
formed): place insulated tubing over the leads of a 3.3k,
½W resistor and solder it across the terminals to which
the white and black leads of transformer T2 are con-
nected. Solder a .01 µF, 100V disc capacitor from termi-
nal “A” (see Figure 6, Page 7) to the terminal strip
chassis mounting lug next to terminal “A”.
Output Transistors:
The output transistors Q6 through Q9, Figure 5, Page 6,
are located on the black finned heat sinks. Replacement
procedure is the same as that used for the driver tran-
sistors, Q4 and Q5.
NOTE: The output transistors, Q6 through Q9 must be
matched for current gain. When replacing output
transistors be sure to replace with devices which
have the same gain code
and part number
as the
original transistors. Shure transistors are coded
either by a color dot or a letter stamped on the top
of the transistor.
Blue Dot
= A
Brown Dot
= F
Red Dot
= B
Pink Dot
= G
Orange Dot
= C
Violet Dot
= H
Yellow Dot
= D
Black Dot
= J
Green Dot
= E
White Dot
= K
FIGURE 5.
6
