Philips FW2017 Service Manual page 17

FRONT SUPPLY PART
A relatively costly short-circuit-proof stabilising circuit
is used here.
Starting point: STAND BY
The transistors 7464 and 7461 are functioning as a
"starting" circuit. The STBY line is high. Transistor
7464 is conducting, so TS7461 is blocking. The elco
2461 is discharged via R3469 and R3471.
Switching on
If the unit is now put into operation, the STBY line
goes to low. Transistor 7464 biocks, causing TS7461
to switch through (emitter becomes high). Elco 2461
is charged via R3471. During the charging process a
voltage drop arises at R3471. Consequently, the emit-
ter of TS7462 becomes high. D6461 prevents reverse
current via the +14V. Via potential divider
R3486/R3485 + R3484 a base voltage arises which is.
more negative than the emitter voltage. TS7462 con-
ducts, causing TS7468 and TS7460 to also conduct.
At the collector of TS7460 a positive voltage builds
up. This reaches the emitter of TS7462 via D6461 and
R3484 and is limited by D6462 to 4.7V. The base
voltage is derived via the potential divider
R3486/R3485. The circuit remains active if C2461 has
been charged.
Regulation
If the +14V drops, then the voltage at the base of
TS7462 will also drop. As the emitter has been stabili-
sed at 4.7V by D6462, the base-emitter voltage Uge
increases. As a result, a larger collector current flows
through TS7462; the voltage at the base of TS7468
increases. Consequently, the collector current of
TS7468 and thus the base current of TS7460 increa-
se. As a result, the voltage +14V increases again.
23 Ga)
Short-circuit protection
If the +14V are short-circuited, TS7468 receives no
base current and blocks. As a result, TS7460 also
blocks and switches the +14V voltage off. This volta-
ge can only be reactivated by switching on/off.
Switching off
If the unit is switched to Stand By (STBY line beco-
mes high), the base of TS7469 becomes high via
R3467. As a result TS7469 conducts and returns the
base of TS7468 to earth. This causes TS7468 to
block and therefore TS7461 as well.
At the same time, TS7464 also switches through via
R3466 and thus blocks TS7461. There is now no
voltage at the emitter. As a result, C2461 discharges
again via R3469 and R3471.
CD power supply
Transistors TS7470 and 7471 operate only as swit-
ches. The voltage of D6463 is stabilised.
SCI signal
The SCI line has a dual function. While pressing
DSP') Presets or Cancel a square-wave signal (clock)
is transmitted for approx. 200ms. Otherwise, the line
is used as Mute for the Source Selector and the out-
put stage or as Headphone Mute.
1) Digital Sound Processing
CS 57 068