Shure PM300 SERIES Manual de Serviço descarregar pdf (Página 15)

FIGURE 12.

Driver Transistors:

The driver transistors

Q33

and

Q34,

Figure

12,

Page

15,

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 insulation wafer to provide good thermal transfer from

transistor to chassis. After replacement and before con-

necting 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.

Q33

is an NPN

transistor, while

Q34

is PNP transistor. Refer to the lower

right corner of the circuit diagram, Figure

17,

Page

for

terminal code. NOTE: when replacing driver transistors,

perform the following modification (if not already per-

formed): solder

3.3k,

Y2W

resistor across the terminals

to which the white and black leads of transformer T2 are

connected. Add a second ground wire from the terminal

nearest the front of the unit to which resistor R21 is con-

nected, through the chassis grommet, to the ground on

the Speaker Output jacks (same path as existing wire).

Output Transistors:

The output transistors

Q35

through

Q38,

Figure

12,

Page

15,

are located on the black finned heat sinks. Replace-

ment procedure is the same for the driver transistors,

Q33

and

Q34.

NOTE: The output transistors,

Q35

through

Q38,

must be

matched for

current gain

and part number. When replac-

ing output transistors be sure to replace with devices which

have the same gain code as the original transistors. Shure

transistors are coded either by the last letter in the part

number or a color-dot on the top of the transistor.

Blue

Dot

Red

Dot

Orange

Dot

Yellow

Dot

Green

Dot

Brown

Dot

Pink

Dot

Violet

Dot

Black

Dot

White

Dot

1 2 ... 10 11 12 13 14 15 16 17 18 19 20 21 22

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