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Dead Toshiba 42H81 repair

 
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Lateralus
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PostPosted: Fri Jan 18, 2008 7:26 am    Post subject: Dead Toshiba 42H81 repair Reply with quote

Hello -

I am attempting to repair my TV that would not even turn on. If anyone is familiar with this particular model (Toshiba 42H81), fuses F850, F851, and F852 were all blown. Those go to the convergence IC's, so I replaced those (upgraded to O.E.M. STK392-150's too) and the fuses. The TV worked for about 15 min. and then I smelled burning so I shut it off. None of the fuses blew, but I immediately saw a bad resistor (R7711).

I had checked all these resistors before beginning the repair and they all came up 2.2 ohm so I assumed they were all good. Now when I just checked the service manual it says they should all be 1.8 ohm. The bands (brown, silver, gold, gold) indicate 1.8 ohm as well. Is it possible all of them could be bad and by some coincidence all read 2.2 ohm? And that is what caused R7711 to burn up? OR do you think my digital multimeter is just off by 0.4 ohm?

I think my new STK's should still be fine since the TV still had display when I unplugged it, and none of the fuses blew. So now I need to know if I should replace just the burned resistor or all six of the ones that should be 1.8 ohm. And any ideas as to why the resistor burned up would be helpful as well. I checked the jumper wires and also didn't find any cold solder or bonded solder joints.

Thank you for any help you can offer.

-Shane
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minnie



Joined: 18 Aug 2005
Posts: 2877
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PostPosted: Fri Jan 18, 2008 11:01 pm    Post subject: Reply with quote

Convergence IC's in this case are dual amplifiers. On IC does red and half of green. The other IC does the other the half of green and blue. You have 6 resistors, one does red vertical and one does red horizontal, one does blue vertical and one does blue horizontal, one does green vertical one does green horizontal thus 6 resistors. Today I had a 43H72 with R7716 fried. R7716 is the blue horizontal resistor. So my blue was way off until I replaced it with yes a 1.8 ohm 1 watt resistor. When the convergence IC fails it taxes the resistors which supply the voltage to the IC or in your case R7711. Now go back and resolder jumper 010 and 050 so the pico fuse doesn't blow again. These are the + and - voltages to the convergence IC's.
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Lateralus



Joined: 18 Jan 2008
Posts: 2

PostPosted: Sat Jan 19, 2008 3:31 am    Post subject: Reply with quote

Thanks. I found a bad solder joint at jumper 010. Could that be what caused the TV to go out to begin with? Or was that a result of a component failing and putting to much current through the jumper?
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minnie



Joined: 18 Aug 2005
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PostPosted: Sat Jan 19, 2008 4:50 pm    Post subject: Reply with quote

The bad jumpers will cause 850 or 851 to blow but with finding a bad resistor your convergence ic's failed.
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Lateralus



Joined: 18 Jan 2008
Posts: 2

PostPosted: Mon Jan 21, 2008 1:26 am    Post subject: Reply with quote

Thanks for the help minnie. Just to summarize for anyone who should come across this thread:

I think what happened was jumper 010 had a bad solder joint, causing the fuses to blow. Unaware of the bad solder joint at the time, I replaced the fuses and the STK's, only to have the fuses blow again. I redid my soldering and replaced the fuses again and this time they blew along with resistor R7711, at which time the vertical STK went bad as well. Then I found the bad solder joint at jumper 010, fixed it, replaced the resistor, the vertical STK, and the fuses once more, and the TV has been working properly since.

By the way, as for the resistors all reading 2.2 Ohm instead of 1.8 Ohm, that was just my inaccurate multimeter.
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vtech



Joined: 08 May 2006
Posts: 1264
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PostPosted: Mon Jan 21, 2008 6:19 am    Post subject: Reply with quote

It is basically an inherent problem in some amplifier designs (as in a STK) where it is depending on seeing a balanced rail voltage.
if/when there is a mismatch on either voltage rails &/or load, upon powering up, it will quickly cause a no-control "latch up" condition where half of the circuit will fully conduct to destruction---hence causing the new device fail.
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