Philips 22 ga 212 service manual

Page 2

OPERATION

a. miligstion oi the supply volm

The supply voltage has been adjusted to -9 V. Assume that
this voltage drops. when. ior instance. the load of the circuit
is increased. Via D488 and R472 the base oi 18441 becomes
less negative in regard to the emitter. This emittor has been
adjusted to a constant voltage owing to diode D457 being ad-
Justed to its knee voltage.

As a result of the voltage drop. 18441 will conduct more.
Consequently, the collector current oi 18441 will also rise.
Thus, 18402 will become more conductive.

The circuit is so dimensioned that owing to the iact that 18405
is more conductive. the emitter collector voltage decreases
so much that the drop in supply voltage is just compensated.

b. m circuit;

1. Switching on the mains

The circuit is so desiyied that. when the mains voltage is
switched on with 8D. the transistors 18426 and 18431 conduct
and the transistors 18428 and 18429 do not conduct. Conse-
quently, no voltage is applied. Transistor 18435 is conducting
so that M414 (stop) is burning.

Note:
The transistors 18426 and 18428 as well as 18431 and 18429
are bistable multivibrators.

2. start 38 r.p.m. or 45 r.p.m.

ii the touch control (88 r.p.m.) is touched by a finger. the base
oi 18432 is connected to earth through the finger resistance

oi some M9. As a result. 18432 starts conducting. The collector
current flows via R880 to the base oi 18428 so that 18428 be-
comes conductive and 18426 does not conduct any more.
(Bistable multivibrator). Lamp LA410 (33 r.p.m.) is burning.

A positive voltage is applied to the collectors oi 18438 and
18489 via D447. Consequently. the motor is then running.

18485 then conducts no longer and LA414 ("stop") goes out.

As 18426 becomes conductive, 18436 also starts conducting

via 3865. As a result. the speed control 83 r.p.m. is switched
and the motor ms at a speed oi as r.p.m.

The start 45 r.p.ln. happens in the same way as the start

38 r.p.m. from the position "stop".

Switching over irom 33 r.p.m. to 45 r.p.m. and vice versa
always has to take place via "Stop".
Switching over directly from 83 r.p.
as follows:

. to 45 r.p.m. is prevented

Example:

e apparatus is running at a speed of 33 r.p.m. The collector
oi 18428 is connected to earth. The emitter oi18433 is con-
nected to the collector via R553. This implies that 18433 can
never become conductive as long as 18428 is conducting. li
touch control 45 r.p.m. is new switched on. nothing happens.

To prevent 18428 and 18429 irom becoming conductive at the
lame time. they have been cut oii as iollows:

Exam ls:

Ii nth cmducts. 18426 does not conduct (bistable multivi-
brator). Via D446 a current flows to the base oi 18431. Conse-
quently, 18431 is being kept conductive so that 18429 cannot
become conductive.

c. stop ciguitg

1. "Stop" with touch control

Assume that the apparatus is running in position 38 r.p.m. 80
18428 is conductive.

1! the "stop touch control is operated by a finger, the base oi
18434 is connected to earth owing to the finger resistance. and
18434 starts conducting. The collector current oi 15434 flows

to the base oi 18426 via R557 and R530. 18426 then starts con-
ducting. whereas 15428 does not conduct (bistable multivibrator).
No current flows to the motor. and therefore it will not run.

2. "Stop" with LDR. the stylus moving into lead-
out groove oi a record

Assume the apparatus is running at a speed of 38 r.p.m.

When the stylus oi the P.U. head is moving inwards and is

about 65 mm irom the middle of the turntable. the film on
bracket 98 exposes R404 less strongly. Consequently, the
resistance oi the LDR is increased and the voltage also rises.
Alter one revolution oi the turntable the stylus has moved one
groove inwards; the LDR is then exposed less strongly. which
results in a voltage increase oi A E volts per revolution oi

the nirutable. This increase oi A E volts is also available across
the series circuit C726 - R532 - R466 (apparatus in position

88 r.p.m.). The RC time oi these components has been so chosen
that the increase in voltage per revolution oi the turntable can
Just ilow oii. So nothing happens.

However. when the stylus oi the P.U. bead moves into the lead-
in groove oi the record, the increase in voltage across the

wit (R404) per revolution oi the turntable is much larger than
All volts.

(Note: The lead oi the lead-out groove is much larger than that
om music groove.) This larger increase in voltage cannot
ilow oii completely within the same time so that part oi this
voltage will become available betwecnths base and the emitter
oi 18427. This transistor is driven into conduction. and. con-
sequently. the base oi 18426 becomes more negative through
3530. 18426 starts conducting. 18428 does not conduct any more
(bistable multivibrstor). The motor then stops. because than

no voltage is applied to it. 18485 now also starts conducting.
M414 ("Stop") is burning again.

d. Control oi the fled

The apparatus are equipped with a tacho-controllsd motor.
Thoreiore, an AC voltage generator has been coupled to the
shsit oi the DC motor; this generator delivers a speed-dependent
voltage.

Operation

When the speed of the motor decreases owing to an increasing
load. the voltage delivered to the tacho generator also decreases.
This voltage is rectified and causes the base voltage of 18438
to decrease also. As a result. the transistor becomes less
conductive. The collector current also becomes less conductive.
and the voltage across R579 has become less high. Owing to
this voltage drop the base of 18439 becomes more positive.
causing the collector current to increase.

The emitter current oi 18489 also increases; this current is
also the base current of 18440. The collector current of 18440
also increases and is then also the current ior the motor.

The speed increases.

Note:
The transistors 18438 - 18439 - 18440 form a DC amplifier.

e. Explanation oi some camancnts

- C728 and C729 are anti-interference capacitors.

- D458. D454 and D455 are used lor stabilising the temperature.
- C733 serves to smooth the ripple on the tacho voltage.

- C780 and R569 serve to start the motor quickly.

- C731 is used to prevent generating.

- D447 and D448 cut oii 18433 in position 83 1/3 or 45 r.p.m.