Power On/Degaussing; Power On; Degaussing - Sony WEGA KV27FV15 Training Manual

Power On/Degaussing

Overview
This section discusses how the converter B+ voltage is created when the
set is turned ON. When the set is turned ON, the degaussing circuit is
activated.

Power On

When the set is plugged in, the AC voltage is applied through F601, T601
and T602. Additional components C605, VDR601, C604 and VDR602
are across the AC line input. They are in place for protection from AC line
spikes and surges. This AC voltage is applied to the standby power sup-
ply and will be applied to D602 Bridge Rectifier when RY600 Power Relay
is closed. R623 and R664 will limit the current when the relay is initially
closed.
When the customer turns the set ON using either the remote or the power
switch, a 5-volt signal is output from IC001/62 Tuning Micon O-Relay.
This HIGH is sent to the G board via the wiring harness between CN1641
on A board and CN641 on the G board.
This signal is used to turn Q648 ON. When Q648 turns ON, it places a
LOW at its collector. This causes RY600 to energize, closing the contact
and allowing current to flow to D602 Bridge Rectifier. The LOW from
Q648/C is also applied to the Q646 Backup and Q647 Soft Start transis-
tors.
Voltage Doubler
D602 Bridge Rectifier is used with C606, C607, D629 and D630 to form a
voltage doubler. Therefore, the output of the voltage doubler is approxi-
mately 310 VDC. This voltage will be referred to as Converter B+. Con-
verter B+ is applied to the converter circuit through R607. R607 is a
fusible resistor. If the converter circuit draws too much current, it will
open. When RY600 Power Relay is closed, a click is heard. When the
Converter B+ is applied to the converter circuit, it will begin to operate and
supply power to the rest of the set.
13

Degaussing

When the set is turned ON, a click is heard from the Power Relay RY600.
About one second later another click is heard from RY601 Degauss Re-
lay. After this a loud hum can be heard. This indicates that the degauss-
ing circuit is operating. The degaussing circuit operates because every
time the set is turned ON, a HIGH is output from IC001/33 Tuning Micon
O-HP SEL. This HIGH is applied via the cable between CN1641 on the A
board and CN641 on the G board. This voltage is used to turn ON Q644.
When Q644 is turned ON, RY601 is energized, causing current to flow
from the AC line through THP601 and THP602. When current flows,
120VAC is applied to the degaussing coils. When this occurs, a hum can
be heard.
THP601 and THP602 are thermistors whose resistance increases as they
get warmer. The thermistors should read less than 10 ohms when cold.
The current flowing through them when voltage is applied to the degauss-
ing coils is great and causes the thermistors to open after a few seconds.
This stops the action of the degaussing coils since no more current is
flowing through them. The set will not degauss properly if the set is turned
OFF and ON again quickly. You typically need to wait at least a few
minutes until the thermistors have cooled to degauss again.
If the degaussing circuit is not operating, no hum will be heard when the
set is turned ON. It is also likely that purity problems will occur since the
tube could become magnetized.
Room vibrations may cause the contacts of RY601 to make contact when
they are not supposed to, causing the degauss coils to be activated while
the set is producing a picture. If this occurs, the customer may complain
about an intermittent rainbow pattern or intermittent loss of purity. If a
customer complains about this, try replacing RY601.