Service manual

CA-1
Power Supply
Introduction
The CA chassis is a cold ground chassis deriving its power from a switch-mode, flyback type, power
supply. It features indirect B+ regulation, over-current protection (OCP), software controlled degaussing,
necessities for Energy Star compliance, and provision for future universal power input (85VAC - 265VAC).
The switch-mode power supply, or SMPS, converts AC line power to the various DC voltages required by
the receiver. Note that there is no switch to activate or deactivate the SMPS. It operates as soon as AC
line power is present. Input power demands can reach a maximum of 130 watts on 25 and 27 inch sets and
145 and 150 watts on 32 and 36 inch sets respectively.
Switch-Mode Power Supply Primary Side
The receiver is protected from AC line surges by FX3401, a 4A, 250V fast-blow fuse. The AC operating
range is between 90Vrms and 135Vrms. AC is first filtered by LX3401 and CX3402. It is then rectified
using a diode bridge (package DX3400 on 32"/36", discrete components DX3401 through 3404 on 25"/
27"). CX3407 smoothes out the resulting DC voltage. It is from this voltage (called VDC or raw B+) that
the heart of the SMPS, the Switch-Mode Regulator, is run. Raw B+ can run anywhere from +127VDC to
+191VDC.
DX3402
DX3403
DX3401
DX3404
RX3401
CX3406
C3403
CX3407
CX3404
CX3405
CX3402
B+ VDC
LX3401
CX3401
EX3401
RX3400
CX3400
FX3401
3R8
DX3400
To Degaussing
Circuit
<<Figure CA-1>>
Primary components of the Switch-Mode Regulator circuit include the switching transistor, QX3401; the
regulator IC, ICX3400; and the chopper transformer, TX3401. The actual chopper used will depend on
the size of the set. This transformer isolates the rectifier and regulator sections, which use hot ground, from
the rest of the CA chassis, which uses cold ground.
When line power is introduced to the set, raw B+ is supplied to pin 18 of the chopper (switch-mode
transformer). At the same time a small amount of raw B+ is drawn through RX3404 into pin 2 of
ICX3400. This voltage triggers the switching of a transistor internal to the regulator IC. Once this happens,
current begins to alternate between flow and rest in the raw B+ winding (pins 18 and 11) of the switch-
mode transformer. Alternation will occur at a frequency of approximately 25 KHz when the set is in stand-
by, and near 50KHz when the set is powered up. Because of the transformer action, base drive to pin 2 of
ICX3400 is from this time forward maintained by another winding on the chopper (pins 13 and 15) through
CX3416.