(3) Before installing a power transistor (Q507-Q508 on the DAX60, Q5Ol-Q504 on
the DAX120), 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.
Heat sink
with socket
attached 7
M 3
silicone grease or equivalent.
case
transistor
I L 0 2 5 3
After installing a power transistor, check for a short circuit:
Step 1. On the DAX120, isolate the circuit common from the chassis by discon-
necting 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 DAX60.
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.
Step
3.
After obtaining a high-resistance reading (at least 100 kilohms) be-
tween each transistor case and the heat sink, re-establish the connection
between the negative side of C503 and the star connector (DAX120 only).
Testing Transistors
The best way to test a transistor is to use
a
transistor tester. However,
if one is not available, use an ohmmeter. Most failures result in a short or
an open circuit between the collector and the emitter.
Connect the ohmmeter's leads to the collector and the emitter, then use
the low ohm range
to
read the resistance. If the reading in this range remains
the same when the leads are reversed, the transistor is shorted. If the read-
ings are "infinite" for both connections, the transistor is open.
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