NKI1480B DAX60 and DAX120 Amplifiers—Installation
Page 13 of 18 (Jul-98)
Removing and Replacing Transistors
Transistors are inherently long-lived devices that normally should not need
replacement. If, however, systematic troubleshooting indicates a problem, observe
these precautions when removing and replacing transistors:
(1)
Transistors can be damaged by excessive heat, so use a small soldering iron
when removing or replacing a transistor with solder connections.
(2)
Transistors come with a wide variety of cases and leads. To avoid a costly
mistake, make a careful sketch of the lead connections before removing a transistor
from a printed circuit board or tie points.
(3)
Before installing a power transistor (Q507-Q508 on the DAX6O, Q501-
Q504 on the DAX12O), obtain an appropriate Silpad
®
insulator. If a mica insulator
is used instead, coat both sides of it with silicone grease (Dow-Corning DC4 or an
equivalent). Fit the insulator between the transistor and the heat sink, as shown
below.
Figure 2
After installing a power transistor, check for a short circuit:
Step 1. On the DAX12O, isolate the circuit common from the chassis by
disconnecting the wire between the negative side of C503 (the large electrolytic
capacitor nearest the auxiliary 120-VAC outlets) and the star connector at the right
end of the amplifier (as viewed from the front). This capacitor is not grounded to
the chassis in the DAX6O.
Step 2. Use an ohmmeter to check for a short between the transistor case and the
heat sink. If there is a low-resistance reading, check the insulator, the screws, etc.,
as possible causes.
