Service manual

CB-1
Power Supply
Introduction
The CB chassis is a cold chassis designed to support 27, 32, and 36 receivers. Its power supply
features a controlled turn-on MOSFET gate driver circuit, direct B+ regulation, thermal protection circuitry,
over-current protection (OCP), over-voltage protection (OVP), over-load protection, provision for future
universal power input (85VAC  265VAC), and software controlled degaussing.
As with many SMPS circuits, this one activates as soon as it is connected to AC line current via plug
connector 3R8. Depending on the screen size, the CB can draw anywhere from 125 to 140 watts from the
line supply. AC operating tolerances range between 90 Vrms and 135 Vrms. Note that while most of the
functional portion of the CB chassis is considered to have a cold ground, the rectifier and primary side of the
switch-mode power supply have a hot ground.
Switch-Mode Power Supply Primary Side
FX3401, a 4 amp/250 volt fast-blow fuse, protects the CB chassis from AC line surges. EX3401, the
spark gap, also provides some protection for the set. Incoming AC passes the fuse and is filtered by
LX3401, CX3401, and CX3402. The switch-mode power supply (SMPS) uses a fairly standard bridge
rectifier arrangement to develop raw B+, the voltage that runs the SMPS and ultimately the entire television
set. DX3400 is the packaged bridge used in 32 and 36 sets. The 27 sets use discrete diodes DX3401,
3402, 3403, and 3404. Rectifier output (raw B+) will be somewhere between 127.3 and 190.9VDC.
This VDC is smoothed by and can be measured across CX3407.
DX3402
DX3403
DX3401
DX3404
RX3401
CX3406
C3403
CX3407
CX3404
CX3405
CX3402
B+ VDC
LX3401
CX3401
EX3401
RX3400
CX3400
FX3401
3R8
To SM Regulator
DX3400
To Degaussing
Circuit
<<Figure CB-1>>
ICX3412 is the switch-mode regulator IC used to develop the numerous individual power supplies off the
secondary side of TX3401, the chopper transformer. The same chopper is used in all three of the CB sizes.
This transformer isolates the hot portion of the chassis from the cold portion. Raw B+ feeds into pin 18 of
the chopper. The other end of this coil, pin 11, connects with pin 3 of ICX3412. Pin 3 is the drain of the
internal MOSFET that drives the chopper transformer, while pin 2 serves as the MOSFET source.
ICX3412 functions when the voltage on pin 4 reaches +16 volts DC. This voltage is developed on
CX3410. As it starts, the regulator pulls nearly 20mA of current from CX3410, causing its voltage to