Question about Toshiba TN50X81 50" Rear Projection Television

1 Answer

TN50X81 - one convergence IC runs hot...TV powers off after 5min

I bought this set with a convergence issue...I replaced the convergence ICs (used the Sanyo parts, and the uprated 150 ones, too) and the picture came up fine. I could adjust the convergence and all seemed well.

But after running for 5min or so, the TV would shut off. I noticed that 1 of the convergence ICs was running HOTTER than the other. I've rechecked my soldering (looks OK...I'm an EE, so I'm not inexperienced in these kinds of repairs). I also checked the low-ohm resistors to the convergence ICs...all of these measured fine and showed no signs of overheating. I did use fresh thermal grease when fastening down the new convergence ICs, too.

Again, the TV is a 50' rear projection Toshiba TN50X81 made in March 2000.

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  • Karl Nitz Nov 27, 2008

    They were STK392-110 - I went up to STK392-150's. I'm wondering if one of the 150's (the one that is running hot) is just bad? I did by extras this time, so I have a spare to swap in instead.

  • Karl Nitz Nov 28, 2008

    So, I replaced that hot-running convergence IC with another...the same thing is happening - it still runs very hot, compared to the other. I don't have a schematic for this TV set, yet, but the PCB is marked V (for vertical?) by the hot-running convergence IC - the H (horizontal?) IC is running much cooler.



    I've left the TV running to see what happens. I do have an oscilloscope and current probe available (in addition to a multimeter) to look deeper into this. Any guidance would be appreciated. I want to get this one running fine so my kids can have a nice-sized TV set in their play room.

  • Karl Nitz Nov 28, 2008

    The TV shut-off again after 15minutes or so. Again, the vertical convergence IC is running very hot - you can only barely touch the heat sink. The horizontal convergence IC's heatsink runs warm, but not uncomfortably so.



    I've read else where about a problem with bad solder on the low-voltage regulators on the deflection board of Toshiba rear projection sets. These are the boards at the end of the RGB CRT tubes, right?

  • Karl Nitz Nov 29, 2008

    OK, I'll give all that a go in the morning and will report back on how it goes. Thanks for the help so far!

  • Karl Nitz Dec 01, 2008

    As you recommended, I resoldered all the connections on the convergence PCB. I paid particular attention to the joints if Q782, Q755 & their heatsinks, as well as connector 726B. I reconnected everything and let the TV play for almost 4 hours straight...it never shut off !



    Still, the vertical convergence IC is running MUCH hotter than the horizontal convergence IC. Is this normal? As described before, the V IC heatsink is almost too hot to touch, whereas the H IC heatsink is warm, but bearable.



    Thanks for all your help, ladywinn!

  • Karl Nitz Jan 19, 2009

    The unit powered off again. I'm going to have to go back in and look at the power supply next, I think.

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  • Toshiba Master
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What was the STKs that was in set ? If 110s always upgrade to STK392-180s. If they were 392-560s go back with the same. they worked differently then the 110s 150s 180s

Posted on Nov 27, 2008

  • 1 more comment 
  • ladywinn
    ladywinn Nov 28, 2008

    Let me know how everything turns out or if you need anymore help.

  • ladywinn
    ladywinn Nov 29, 2008

    Solder loose connections on Reulator ICs Q782 and Q755 located on convergence board also clean plug 726b resolder everything on convergence board and all jumper wires. this should correct your problem.

  • ladywinn
    ladywinn Dec 01, 2008

    It very well could be normal for vert ic side to run hotter because of different circuits but just in case ill do some checking , i have see as to where a leaky capacitor can also do the same thing,anyway as long as set plays it must be OK.

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The convergence ICs are mounted on a large heat sink and are marked with codes starting with the letters STK and there are two of them. You need to replace both.

Look for the heat sink with two ICs mounted on it and check the markings.

Once found, cut the IC legs with a small side cutter and unscrew it from the heat sink. Remove the legs left soldered on the board.

Take note of the IC markings for ordering the right part and how it was installed. Don't forget to buy along with the IC a thermal compound.

Check the pico fuses on the same board. Most likely you will find a blown one. These fuses look very much like a resistor, not like the usual transparent fuse, and usually yellow.

Once you have the replacement ICs, put it on the board in the same way as it was installed before. Make sure the screw holes are aligned before soldering the legs into the board.

Put a reasonable amount of thermal grease/compound between the ICs and the heat sink and fix them with the screws.

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