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Discussion in 'Electronic Repair' started by Ricky Eck, May 8, 2004.

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  1. Ricky Eck

    Ricky Eck Guest

    Does anyone know if the Vert chip LA7838 (From a Sanyo 25") will work in
    place of a LA7837 (from a Sanyo 19") Upon inspection of my Second Sanyo TV
    (The one with the bad Tuner), the Vert chip had a hole burned in it.
    Apparently all the banging that my wife did on it, shorted it out or
    something. Well, out of the old Sanyo, the tuner worked on the smaller TV,
    but now I have to replace the vert chip. Then I will be back up to two TV's
    out of Three again.

    Thanks, Rick
     
  2. Ricky Eck

    Ricky Eck Guest

    I found some info on this, but I would like a second opinion It reads to me
    that there is no difference, but could someone look at make sure my thinking
    is correct?
    http://www.steveselectronics.com/files/LA7838x.pdf

    I would hate to pull the chip and put it in there, to only find out that it
    won't work, and burn something else up...

    Thanks, Rick
     
  3. Jason D.

    Jason D. Guest

    At our shop we stock most of ICs that is highest capacity to keep cost
    down and easier shopkeeping.

    Use LA7838 it is higher current capacity compared to '37.

    You will need to replace 100uF 35V cap that has + side of cap pointed
    by a diode. That cap also has both leads going to that vertical IC.
    The 26V source goes through this diode as well as vertical IC's pin.

    Also you will find a crispy flameproof resistor around area near
    flyback transformer. This is "fuse" for the 26V source for some
    circuits including vertical IC. Usually 4.7 to 1.8 ohms. Others will
    confirm the exact value and wattage for that.

    Cheers,

    Wizard
     
  4. Ricky Eck

    Ricky Eck Guest

    Ok, I am going to try this one last time, before nixing the unit. Now, keep
    this in mind. I have the knowledge of electronics, and far from an amature,
    however, since I stoped repairing, I no longer have the equipment (My
    stupidity) to test the componates. So I am like an ASE Mechanic, working on
    a car with a screwdrive, plyers, and duck tape (very frustrating).

    With that said, I have searched near and far for the values listed above,
    but can't find them. I replaced the chip, and the same problem continures.
    I only briefly turned on the unit, and turned it off, to prevent dammage to
    the replaced chip. The only remotely burnt resistor I have found was a 1
    ohm 5% and it wasn't toast, just a little hot spot on the bottom side. I
    used a pen flashlight and a magnifying glass to find it.

    When you say flame proof, I am assuming the Huge ceramic resistors located
    on the PCB. All of them are as white as can be. And I can't find any caps
    that have the values that were stated above. I found one near the vert.
    chip that was 1000uF at 16v and another that was 10uF at 25v The only one
    near the vert. chip was near a diode, like listed above, but it is 47uF at
    35v

    If this helps, the Chassis No. is : 19350-00 Sanyo

    I don't feel it is worth taking to a shop that has all the equipment to
    repair. Concidering, I bought my wife a 19" last Christmas for 75 bucks. I
    am just tring to see if it can be salvaged. If you think it is worthless,
    then I will just throw the unit out. If you think I can repair it, I will
    give it one more shot..

    Thanks in advance,
    Rick
     
  5. Jason D.

    Jason D. Guest

    This can be any kind of flameproof resistors design not just the big
    sand blocks. They actually looks like dull flat texture of a typical
    axial resistors in many different wattages. They're designed not to
    flame out when it overheats then opens, it simply chars into ashes or
    almost intact, have to check with meter. The markings can be typical
    resistor color codes or numbers printed on it.

    The vertical's diode's anode (arrow) leads back to the diode &
    flameproof resistor (but in some very cheap TV they omitted that
    "safety" flameproof resistors, just a diode) to the flyback
    transformer's one of many pins. The 26V source feeds both diode and
    the vertical IC power pin.
    The cathode (the bar that arrow is pointing to) of that diode near the
    vertical IC both goes to the positive side of 100uF capacitor and to
    the vertical IC. The minus side of said capacitor also goes to the
    another pin on vertical IC.

    Google the IC part number for example LA7838 (same as LA7837) turns
    up:

    http://www.steveselectronics.com/files/LA7838x.pdf

    This will show you example circuit of this typical vertical circuit
    design. This is just to help you understand the circuit better but
    not all of the is used, TV makers uses slightly different designs but
    basic vertical circuit is very similar based on LA7837/LA8738).

    The other cap across ground to power supply for the vertical is a
    filter, the other one like 100uF connected to pin 9 of vertical IC is
    the one you want to change.

    Hope this helps!

    Cheers,

    Wizard
     
  6. Ricky Eck

    Ricky Eck Guest

    You would not believe this. I think I traced it down. If I read it
    correctly, it should be the cap that comes off pin 9 according to the .pfd
    you sent me to. It shows it is 100uF, However, the one on the board, is a
    47uF @35v. I am confused..:) LOL The way it is on the PCB, The diode is
    between pin 8 to pin 13. The positive side of the cap is on the diode, and
    a jumper carries it over to pin nine (The Neg. of the cap), so the cap that
    should be 100uF on the .pfd, is 47uF on the PCB. I am stopping there for
    now, till I figure this out. Before attempting to find the resistor..:)
    LOL

    PLEASE HELP..:) LOL
    Rick
     
  7. Ricky Eck

    Ricky Eck Guest

    On top of what I wrote above, I looked at the original board that I took the
    LA7838 off of, and it had a 220uF @ 35v Cap.

    Hope that Helps,
    Rick
     
  8. Jason D.

    Jason D. Guest

    Bingo, that what I meant, That .pdf file is just a example but to get
    you going.

    If that TV use 47uF 35V, change it with same value. (47uF for 47uF
    but voltage can be same or higher than old like 50V for 35V to make
    finding parts easier) and backtrack to the supply to make sure you do
    have power for that vertical.

    Usually when I see a crater in the vertical IC, somewhere else is
    either blown resistor and or diode shorted because of overload when IC
    died. That is where checking with meter after you find them as I
    described. That resistor can be anywhere: near vertical IC or near
    flyback, this is why backtracing from that diode that is part of that
    47uF cap to make you got everything fixed.

    The primary cause for blown vertical ICs in any kind was that cap (in
    your case 47uF, it got dried out by the vertical's heat. I bet that
    old cap has marking saying 85 C. Get one that is marked 105C for
    better life or if you don't care, get 85C anyway bec this tv will only
    live for another few years before tube poops out.

    Cheers,

    Wizard
     
  9. Ricky Eck

    Ricky Eck Guest

    Well, I think I found the resistor. It was the one I suspected in a
    previous post. It is actually kinda small. 1 ohm. But I traced it down,
    from the 24v pin on the Vert chip, on back. It is sitting next to the
    Flyback, and it is the one that has the burn mark on the bottom. And the
    one that is on the old board, that I pulled the chip from, also has a 1 ohm
    resistor on it, in almost the same place.

    Now, if I can figure out the cap problem, then I may be on the way..:) LOL
     
  10. Jason D.

    Jason D. Guest

    Well done! Now this confirms that vertical IC took out that resistor,
    measured it? It should read high or open, in series to that resistor,
    there is another diode that comes from flyback pin. Check this also
    just in case creased to be diode because of overload caused by
    shorted vertical IC.

    Okay, on that 24V source to that vertical supply pin, there is another
    part that is hooked to same 24V source near vertical IC heatsink is an
    diode. In series after this diode is capacitor near vertical IC
    heatsink. Both legs of that capacitor also goes to vertical IC pins.
    Note the value on that capacitor and change it also.

    This capacitor is used to double voltage to appox 48V using capacitor
    acting as a temporary battery for kicking beam from end of scan at
    bottom to top for start of scan. When this capacitor go dry with age
    & heat, kick voltages becomes less with time, stressing the vertical
    IC to the point that vertical IC fails violetly.

    By the way, very common fault in so many kinds of TV usually.

    Cheers,

    Wizard
     
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