Troubleshooting - Hitachi HMA-8300 Servicehandbuch

CHECKING THE OPERATION OF THE PROTECTION CIRCUIT
When the output circuit is repaired by replacing the
power transistors, etc., perform an operation check on
the ASO (Area of Safe Operation) detection circuit and
the speaker protection circuit.
1. Operation check of the ASO detection circuit for the
output transistors
Connect the audio oscillator to the INPUT terminals
with the speaker terminals unloaded (speaker: dis­
connect). Set the frequency of the audio oscillator at
1kHz and adjust the level of the input signal so that the
voltage at the speaker terminals is approx. 5V rms.
Under these conditions, short-circuit the speaker termi­
nals of the channel to which the input signal is applied
using a lead wire, etc. If this short-circuit makes the
ASO detection circuit operate, no output appears at
the speaker terminais even if the lead wire used for
short-circuiting is removed.
Next, turn off the power switch and, after approx.
30 sec., turn the power switch on again. When output
comes out of the speaker terminals, this indicates that
the ASO detection circuit is operating normally.
AC voltmeter
Audio oscillator

TROUBLESHOOTING

Carry out the troubleshooting using the following pro­
cedures.
1. Appearance check
(1) Check the primary fuses and the fuse in the printed
wiring board.
(2) Check the insertion of the connectors.
(3) Check the appearance of the parts.
2. Operation check
(1) Make sure that the relay operates approx. 7 sec. after
the power is supplied with no signal and no load.
When the relay does not operate, check whether or
not the DC voltage comes out at the neutral point of
Lch and Rch output in the audio printed wiring
board(when the neutral
±150mV, it is OK).
Oscilloscope
Fig. 15
point voltage is within
2.0peration check of the speaker protection circuit
Make sure that the relay operates (a click sound is
heard) approx. 7 seconds after the power switch is
turned on with the
(speaker: disconnect).
Next, when a resistor of approx. 10 kohms and 2 dry
cells (1.5V) are connected in series to the earth pattern
on the audio printed wiring board and the + side of
C501, the relay turns off within 1 sec. When the dry
cells are taken away, the relay operates again. Next,
change the polarities of the dry cell and carry out the
above-mentioned operation to check the operation of
the relay. If this operation makes the relay activate,
this indicates that the speaker protection circuit is op­
erating normally. Moreover, pay attention during this
operation check so that nearby parts are not short-
circuited.
Earth pattern
(2) Apply a signal into the input, and check whether or
not the output waveform appears at the speaker
terminals. When no signal comes out, check whether
or not the ASO detection circuit is operating. (Check
whether or not the anode voltage of SCR701 is 20V.
It is approx. 0V when the ASO detection circuit is
operating).
(3) Determine the channel that is malfunctioning and
check all the semi-conductors in that channel.
Even when repair is made by replacing the defective
parts, other parts may fail; therefore, check all the
semi-conductors in that channel.
speaker terminals unloaded
Fig. 16