Philips ah 673 schematic

Page 2

CIRCUIT DESCRIPTION

Power Supply Assembly

The Power Supply Assembly provides the various voltages
for the operation of the receiver. There are six source
voltages available from the board; +15V, ~15V, +12V Aux,
+12V Switched, +12V Relay and 6 VAC.

The +12V Aux source voltage is developed whenever the
Master Power switch is On. All other sources are developed
only when both the Master Power switch and the Power
On-Off touch-control are ON. The +12V Aux source
voltage supplies power for the On/Off relay, K501, and its
associated switching circuitry. This source also supplies
voltage to the flip-flops and the Darlington switches on the
Touch Control assembly, thus maintaining the state of the
touch-control circuits when the Power Touch-Control is
turned off.

The +12V Relay source voltage supplies power for the
relays located on the Selector Switch board assembly.

The 6 VAC source voltage supplies power for the Function
lamps.

The +12V source voltage supplies power to lC1, a Quad
2-input Nand Schmitt Trigger, on the Touch Control
Assembly, and through R504 to forward bias the Power
Switch light emitting diode, D1, whenever the unit is
switched ON.

The +15V and -15V source voltages apply power to the IF
and selector switch circuits.

The following is a description of how these source voltages
are obtained. When the unit is connected to an AC source
and the Master Power switch is in the OFF position,
120 VAC is applied only to the unswitched AC receptacle.
Once the Master Power switch, 8501, is turned on, 120
VAC is applied to the primary of T502, where it is stepped
down to a low AC voltage at the secondary. This low AC
voltage is rectified by D506 and filtered by C510. The DC
level at the positive side of C510 is regulated at +12V by
zener diode 2501, forming the +12V Aux source. This
+12V is applied to the emitter of 0502, through R503 to
the base of 0502, and to the collector of 0501 through
K501. Since the Power Touch Control is not activated, base
current in 0502 will not flow, thus 0502 is cut-off. With
0502 cut-off, 0501 has no forward bias and is also cut-off.
Since 0501 is cut-off it will not develop collector current,
therefore, no current will flow through the coil of K501 and
it will remain de-energized. The 0n/Off relay, K501, is a
bistable relay employing a mechanical latch which opens or
closes the contacts each time the relay is energized. There-
fore, when the relay is energized (contacts initialiy open)
the latch will close the contacts and keep closed even after
the relay again becomes de-energized. To re-open the
contacts the relay must be energized a second time.

When a finger is placed on the Power Touch Control, 81,
on the Front Panel Assembly, 0502 is biased on through
the network of R503, R515 and the resistance of the finger
across the Touch Control. When 0502 conducts, its
collector voltage becomes positive causing 0501 to conduct.
Since the collector current of 0501 must flow through the
coil of K501 to reach the +12V Aux source the relay
becomes energized, reversing the state of the contacts.

With the contacts of K501 closed, 120 VAC is applied
through F503 and F501 to the primary of the Power
Transformer, T501. The main secondary winding of T501,
with its center-tap grounded, is connected to a full-wave

bridge rectifier. The positive output voltage of the bridge
rectifier is applied to the input of the Positive Regulator,
lCl, with its output of a regulated +15V. The negative
output voltage of the rectifier is dropped through R507,
R508 and is held constant by zener, 2504 (15V), which
develops the regulated ~15V source.

The voltage developed by the other secondary winding of
T501 is applied through R514 to D505 where it is half~
wave rectified. The rectified voltage is then filtered by
C507, R505 and C515 to form the +12V switched source.

Touch Control Assembly

The Touch Control Assembly functions as a switching
network for all the Front Panel Assembly touch-controls
except the Power On/Off touch-control. Since the switching
circuits on this board assembly are nearly identical only the
circuit which activates the Mute Switch, 84, will be
explained.

In this circuit 05 represents the switch used to activate
the Mute Schmitt-Trigger circuitry. 05 is either cut-off
or saturated. The purpose of the Touch Control logic
circuits is to make it possible to change the conduction of
the switching transistor, 05, by means of a touch-control.
The main components used to perform this function are:
04, which acts as a momentary switch, lC1, a quad 2-input
Nand Schmitt Trigger, and IC2, a dual flip-flop. Separate
voltage sources, +12V Aux source and the +12V Switched
source, are used by the components in the circuit to
maintain the state of the logic when the unit is switched
off. The inputs to I01 are connected together so that when
they receive a low potential, the output will go high. The
flip-flop, IC2, is connected so that the state of its output
(high or low) is reversed each time it receives a high input.

When the Mute switch circuitry is not activated, 04 is cut-
off because of the open touch-control in its base circuit.
The input to IC1, pins 5 and 6, is approximately +12V
volts, through R22, R18, and R1 to the +12V Aux source.
The output of IC1, pin 4, and the input of IC2, pin 11, are
low. The output of I02, pin 13 is low and keeps 05 at
cut-off. With 05 not conducting there is no current path
for the Mute switch LED, D4, to light, thus, indicating that
the Mute switch is not activated. Also with no current
flowing through 05, its collector voltage remains +12V,
keeping the Mute Schmitt-Trigger circuitry on the FM
MPX board assembly in the "Mute Off" state.

When a finger is placed on the Mute touch-control the
voltage drop across the divider R10, R6 and finger resistance
is sufficient enough to bias 04 into conduction. With 04
conducting the emitter voltage will go towards ground.
This low voltage is coupled through the de-bounce circuit,
R22, D4 and C6, to the input of lC1, pins 5 and 6. The
de-bounce circuit is designed to keep the switching circuit
from being activated more than once when firm contact
with the touchcontrol is not made. By brushing over the
touch-control, 04 may be rapidly turned On and Off
several times. The de-bounce circuit changes the rapid
changes in the 04 emitter voltage caused by this condition
to a more constant potential at the input of 101, causing it
to change states only once. When the inputs at pins 5 and
6 of lC1 go low, the output, pin 4, goes high. This high
voltage is direct coupled to the input of 102, pin 11,
causing its output, pin 13, to change states.

Since the output at pin 13, was low, it now becomes high
and remains high until lCZ receives another high input. This