Question about Hitachi 50SBX78B 50" Rear Projection Television

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Have a 50sbx78b had a blown e994. No lights red blinks. replaced e994 and q777 all lights came on the off b+ dim with red blinking.

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  • Donald Chatman Jul 17, 2009

    replaced c707 all lights come on for a few seconds set resets its self about 3 or 4 times with all lights coming on then all go out and red continues to blink. will reset make a sizzlng sound like high voltage ark. Put meter on b+ voltage comes up then drops to about 14v each time it resets then 0v when it finally shut down. I thinking flyback but don't want to condem it till i'm postive that's it.

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E994= fuse, C707= .01uF/2kV.replace that fuse to fix the problem.this will help.
http://www.riddledtv.com/repair/hitachi.html
click the link directly and check out your model number in the list for more help.
replace C707 (blue cap) and the 2A pico fuse (E994). this will clkear the problem.

Please do rate the solution. Thank you for using fixya.keep updated. thanks.


Posted on Jul 16, 2009

  • charlymonty Jul 17, 2009

    you think is right.but check out first.The current in the flyback primary and collector of the HOT are *not* equal. The horizontal deflection yoke, damper diode, HOT collector, snubber HV capacitor(s), and flyback primary all connect to the same point. We begin our adventure at the end of the scan - retrace - when the flyback period begins:


    1. At the end of scan, current is flowing through the flyback primary to the HOT, Q1. At the start of the flyback period, Q1 turns off. (This must be done in a controlled manner - not just a hard shutoff to minimize stresses on the HOT - but that is another story). Since current in an inductor (the primary of the flyback has inductance) cannot change instantaneously, the current is diverted into the snubber capacitor, C1. The inductance of the flyback primary (T2) and C1 forms a resonant circuit so that the voltage climbs on C1 as the current goes down. At its peak, this voltage will be 1000 V to 1500 V.

    2. C1 now begins to discharge in reverse through the primary of T2 (back into the B+ supply - the filter capacitor will stabilize the B+ output) until its voltage (also C-E of the HOT) reaches 0.

    3. If there were no damper diode (D1), this voltage would go negative and continue to oscillate as a damped sinusoid due to the resonant circuit formed by T2 and C1 (and the other components). However, D1 turns on as the voltage goes negative and diverts the current through it clamping the voltage near 0 (-Vf for the diode).Note that the damper diode D1 may have been built into the HOT T2 in the case of an inexpensive or small screen TV or even some monitors that don't have any circuitry for E/W correction.
      Steps (1) to (3) have accomplished the flyback function of quickly and cleanly reversing the current in T2 (and, as we will see, the deflection yoke as well). The full flyback (and yoke current) are now flowing through the forward biased damper diode, D1.


    4. At the beginning of scan, the damper diode (forward biased) carries the bulk of the current from the yoke and flyback. The nearly constant voltage of the B+ across T2 results in a linear ramp of current now through the damper diode since it is still negative and decreasing in magnitude.

    5. At approximately mid-scan, the current passes through zero and changes polarity from minus to plus. As it does so, the damper diode cuts off and the HOT picks up the current (with a voltage drop of +VCEsat). Current is now flowing out of the B+ supply.The base-drive to the HOT must have been switched on before this point! Timing is not very critical as long as it happens between the end of the flyback and the zero crossing of the summed current. The location of the zero crossing depends on the secondary load, notably the beam current. Larger beam current requires that the HOT be switched on earlier. The designer has to do some optimizing here...


    6. During the second half of the scan, the HOT current ramps up approximately linearly. This is again due to the nearly constant voltage of B+ across the inductance of the flyback primary.

    7. Near the end of scan, the HOT turns off and the cycle repeats.The HOT has a storage time between 3 us and 7 us, thus the base-drive is switched off earlier, in a controlled way to properly remove the charge carriers from the collector region in the HOT. The peak amplitude of the base current and the way it is decreased determine the ultimate dissipation in the HOT and are thus subject of heavy optimization. This is hampered by the fact that there is much spread in HOT parameters.
      http://www.repairfaq.org/sam/flytest.htm
      click this link and get more details
      thanks for updating.

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