NKI1480B DAX60 and DAX120 Amplifiers—Installation
Page 14 of 18 (Jul-98)
Step 3. After obtaining a high-resistance reading (at least 100 kilohms) between
each transistor case and the heat sink, re-establish the connection between the
negative side of C503 and the standard connector (DAXI2O 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 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 readings are “infinite" for both
connections, the transistor is open.
DAX6O Circuit Description
Refer to the attached schematic, KC1546, to trace the circuit descriptions that
follow.
Input and Preamplifier Circuits: Resistor Ri and Capacitors Cl and C507 reject
Radio Frequency Interference (RFI). RV1 is the input LEVEL control
(potentiometer), which controls the signal level that is applied to the DAX's
preamplifier circuits. Capacitors C2 and C8 govern the low-frequency response of
the input signal, to protect horn-type speakers. Transistor Qi is a common-emitter
amplifier that provides the preamplifier gain. It is also a common-base inverter for
the feedback entering its emitter.
Power-Amplifier Circuits: Transistor Q2 is a high-voltage-gain amplifier that drives
Transistors Q3 and Q4. The latter are phase splitters that form a unity-gain
configuration. Diode D503 is a negative-temperature-coefficient device mounted
on the heat sink. This diode controls the DC bias for the drive and the output
transistors. Transistors Q505 and Q506 are current amplifiers (drivers) that supply
high base current to the output transistors, Q507 and Q508. The latter are push-pull
power amplifiers that drive the output transformer. Capacitor C506 blocks DC from
the output transformer and limits the low-frequency output to protect horn-type
speakers.
Protective Circuits: There are several protections against excessive current.
Resistors R16 and R19 are current limiters; if a short occurs across the audio
output, each resistor will drop 10 VDC. An excessive current surge will cause the
fuse to open. In the 28-VDC supply, a short circuit on the DC output or a DC load
that draws significantly more current than 1.2 Amperes will cause thermal Breaker
CB2 to open within 70 seconds. It will try to reset within 15 seconds, then continue
opening and closing until the short or the excessive load is removed, at which time
it will remain closed for normal operation. A more serious problem will cause CB1
to open; it must be reset manually.
DAX12O Circuit Description
Refer to KC1547 (attached) to trace the circuit descriptions that follow.
