Hitachi HA-7700 Serviceanleitung Seite 8

HITACHI HA-7700
Parts installation method
Great care is taken in installing parts to improve the
safety of products. For example, parts are covered with
tubes, or they are kept away from the printed wiring
board surface. Be sure to maintain the previous con-
ditions after replacing parts for repair.
DIRECT COUPLING/CONDENSER COUPLING switch
Since this is a DC amplifier, when direct current leakage
occurs in equipment connected to the input, it will be
amplified and a DC bias is placed to the speaker causing
deterioration of tone quality and generation of dis-
tortion : this switch prevents this.
This also prevents the flow of a large DC component
because of trouble in the control amplifier which would
damage the speaker. If there if DC leakage in the input,
a noise ("'puu ...'') is heard when turning this switch, so
change to CONDENSER COUPLING at that time.
R737
The ground of the signal line and the chassis ground of
the unit are connected using R737 (12 ohms) as shown
in Fig. 6.
' a (|e
Chassis ground Chassis ground
Fig. 6
(1)
(2)
(3)
This is because the POWER MOS FET is in the source
case, and in order to maintain the capacity between FET
and the radiation fin, stability deteriorates by this, so
the FET is isolated from the chassis using a resistor of 12
ohms.
Accordingly, be careful not to cause an output by touch-
ing the Olead wire of the speaker to the chassis or dis-
charge the electrolytic capacitor during servicing, since
this may cut the lead wire of R737 and introduce noise.
Precautions on troubleshooting and replacing parts
When the stabilized power circuit and the peripheral
parts of Q701 and Q702 (D701, R703, R709 and
their peripheral parts) are replaced, the adjustment of
the input/output DC may drift, so be sure to check
the input/output DC voltage.
Since (F) grade of hpe is specially classified into )
— 3 for Q701 and Q702, replace them matching
the proper grade with the proper channel.
Since the socket for transistor use is used for the
socket of the POWER MOS FET, polarity is indicated
as B, C, E. Match it to the indications shown in Fig. 7
when replacing the POWER MOS FET and when
checking.
Gate iG) Drain «D!
Source(s)
Fig. 7
ADJUSTMENT
Adjusting the idle current
Adjust the idle current after removing the cover and
bottom plate, applying no signal and leaving the speaker
selector switch OFF. Adjust Lch, disconnecting lead
wire @) (red) shown in Fig. 8 from the terminal on the
main amplifier PWB. and connecting the positive @
lead of the DC ammeter to the terminal on the chemical
capacitor PWB. and the negative © to the terminal on
the main amplifier PWB. (same for Rch). Next, adjust
R719L or R719R on the main amp. PWB. s0 that the
ammeter reading is 200mA. Re-connect the leads wire
after adjustment.
Adjusting the input DC
Disconnect the junction plungs of the PRE 0UT-MAIN
IN terminal, insert the shorting pin to the MAIN IN
terminals set the DIRECT COUPLING/CONDENSER
COUPLING switch to CONDENSER COUPLING, turn
the speaker selector switch ON, and turn the power
switch ON with no load for more than about 8 minutes
before performing the adjustment.
To adjust Lch, connect the @ side of the DC voltmeter
to the L IN terminal of the main amp. PWB, and the ©
side to the G6 terminal.