Philips ga 212 service manual

Page 2

OPERATION

a. Stabilisation of the supply voltage

The supply voltage has been adjusted to -9 V. Assume that
this voltage drops. when. for instance. the load of the circuit
is increased. Via D458 and R472 the base of T8441 becomes
less negative in regard to the emitter. This emitter 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, T8441 will conduct more.
Consequently, the collector current of T8441 will also rise.
Thus. T8402 will become more conductive.

The circuit is so dimensioned that owing to the fact that T8405
is more conductive. the emitter collector voltage decreases
so much that the drop in supply voltage is just compensated.

b. Start circuits

1. Switching on the mains

The circuit is so designed that. when the mains voltage is
switched on with 810.. the transistors T8426 and T8431 conduct
and the transistors T8428 and T8429 do not conduct. Conse-
quently, no voltage is applied. Transistor T8435 is conducting
so that LA414 (stop) is burning. .

Note:
The transistors T8426 and T8428 as well as T8431 and T8429
are bistable multivibrators.

2. Start 33 r.p.m. or 45 r.p.m.

If the touch control (33 r.p.m.) is touched by a finger, the base
of T8432 is connected to earth through the finger resistance

of some Mil. As a result. T8432 starts conducting. The collector
current flows via R550 to the base of T8428 so that T8428 be-
comes conductive and T8426 does not conduct any more.
(Bistable multivibrator). Lamp LA410 (33 r.p.m.) is burning.

A positive voltage is applied to the collectors of T8438 and
T8439 via D447. Consequently, the motor is then rimning.

T8435 then conducts no longer and LA414 ("stop") goes out.

As T8428 becomes conductive. T8436 also starts conducting

via R565. As a result. the speed control 33 r.p.m. is switched
and the motor runs at a speed of 33 r.p.m.

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

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

Switching over from 33 r.p.m. to 45 r.p.m. and vice versa
always has to take place via "Stop".

Switching over directly from 33 r.p.m. to 45 r.p.m. is prevented
as follows:

Exam 1e:

The apparatus is running at a speed of 33 r.p.m. The collector
of T8428 is connected to earth. The emitter of T8433 is con-
nected to the collector via R553. This implies that T8433 can
never become conductive as long as T8428 is conducting. If
touch control 45 r.p.m. is new switched on, nothing happens.

To prevent T8428 and T8429 from becoming conductive at the
same time, they have been cut off as follows:

Exam ls:

l? Tޤ cmducts, T8426 does not conduct (bistable multivi-
brator). Via D446 a current flows to the base of T8431. Conse-
quently, T8431 is being kept conductive so that T8429 cannot
become conductive.

c. Stop circuits

1. "Stop" with touch control

Assume that the apparatus is running in position 33 r.p.m. 80
T8428 is conductive.

If the "stop" touch control is operated by a finger, the base of
T8434 is connected to earth owing to the finger resistance, and
T8434 starts conducting. The collector current of T8434 flows

to the base of T8426 via R557 and R530. T8426 then starts con-
ducting. whereas T8428 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 grpove of a record

Assume the apparatus is running at a speed of 33 r.p.m.

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

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

the turntable. This increase of A E volts is also available across

the series circuit C726 - R532 - R466 (apparatus in position

33 r.p.m.). The RC time of these components has been so chosen
that the increase in voltage per revolution of the turntable can
just flow off. So nothing happens.

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

LDR (R404) per revolution of the turntable is much larger than
AE volts.

(M: The lead of the lead-out groove is much larger than that
of the music groove.) This larger increase in voltage cannot
flow off completely within the same time so that part of this
voltage will become available between'the base and the emitter
of T8427. This transistor is driven into conduction, and. con-
sequently, the base of T8426 becomes more negative through
3530. T8426 starts conducting. T8428 does not conduct any more
(bistable multivibrator). The motor then stops. because than

no voltage is applied to it. T8435 now also starts conducting.
M414 ("Stop") is burning again.

d. Control of the med

The apparatus are equipped with a tacho-controlled motor.
Therefore, an AC voltage generator has been coupled to the
shaft of 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 T8438
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 T8439 becomes more positive,
causing the collector current to increase.

The emitter current of T8439 also increases; this current is
also the base current of T 8440. The collector current of T8440
also increases and is then also the current for the motor.

The speed increases.

Note:
The transistors T8438 - T8439 - T8440 form a DC amplifier.

e. Explanation of some commnents

- C728 and C729 are anti-interference capacitors.

- D453. D454 and D455 are used for stabilising the temperature.
- C733 serves to smooth the ripple on the tacho voltage.

- C730 and 3569 serve to start the motor quickly.

- C731 is used to prevent generating.

- D447 and D446 cut off T8435 in position 33 1/3 or 45 r.p.m.