HITACHI CST258PC
Service Manual

CIRCUIT DESCRIPTION A8/D8 SUB PSU CONSOLE {A527590}.
Speaker Box & Chassis removal Remove glass door by unscrewing hinges with a Phillips screw driver. Unscrew two screws on underside of speaker box, and slide speaker box out of console. Unhook 4-way extension block and put to one side. Remove 13 screws from rear of speaker box and two more from underside of speaker box. Remove chassis from speaker box. Unscrew two black screw from PHONO sockets, and one yellow screw fixing main PCB to plastic frame 38V rail is rectified via D17 & smoothed by C99 the voltage is then stepped down from R35 & R25 divider network. The 38V rail also provides the regulation via IC8 providing feedback via IC4 to pin 1 of IC10.

Feedback operation Increase in load demand by secondary rails results in higher voltage feedback on cathode of IC8 in turn rising the voltage on pin 1 of IC10, this voltage is compared to the sawtooth generated on pin 4 via R76 & C95, this produces the gate drive square pulse with larger off time therefore the larger the load demand the larger the duty cycle and vice versa.

Circuit Description The mains is switched on via the relay R54 , the relay is on when the 18v audio signal is up {TV on }. At this point the A.C mains is rectified by D8,D10,11,12 which produces approximately 340v D.C across C58, this is then applied to the drain of Q4 via the primary winding of T1 . Initially the voltage on pin 7 of IC10 will rise to approximately 16v via R15 , D24, and C54 this allows the internal circuitry of IC10 to generate a sawtooth waveform at pin 4 from which a square wave output is obtained at pin 6 . This output is applied to the gate of Q4 turning the mosfet on and off , generating the outputs in the secondary windings of T1. After initial power up pin 7 of IC10 is supplied from the bias winding pin 4 of T1 via D31 for continued operation hence R15 unable to provide running current demand for IC10 [20mA]. A current sense circuit consisting of R74, R75 & 79 feeds back a voltage to pin 3 of IC10,this voltage is compared with an internal reference voltage , of approximately 0.8V should the applied voltage exceed the reference, the pulse width is limited from output pin 6 [chip in over current mode] .In this way Q4 is offered protection from changes in primary current. Secondary overvoltage protection is offered also by R77 , should the feedback be disabled then the secondary voltage will be limited to safe levels, the output from pin 6 will be limited by the internal error amplifier and internal reference . R21 and C31 act as a soft start circuit , this limits the pulse width output from pin 6 during the initial start up period , allowing a gradual rise to full power . ZD3 & 4 will go short if Q4 should go short, protecting IC10. The secondary voltage induced in T1 winding 12/16 is rectified by D15 producing 38V which is smoothed by C40 . Winding 13/16 produces 32V when rectified by D29 & smoothed by C87. The 38V rail is also used to provide a 5V low current rail for muting of the audio amplifiers , in this case the