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TV Grundig CUC7851. Problem with TEA5101 and higher Voltages

 
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kis62



Joined: 12 Dec 2007
Posts: 2
Location: Plovdiv

PostPosted: Wed Dec 12, 2007 4:36 pm    Post subject: TV Grundig CUC7851. Problem with TEA5101 and higher Voltages Reply with quote

Hi,
i have one TV Grundig M72-795 TOP, CUC7851. TEA5101A is dead also the resistor R793 (330Ohm).

If i replace the items with new one, they blows immediately after i switch the TV on. The voltage must to be 200V but in my case is 260V (without TEA). I do 1 Zener Diode BZX200 but this dont helps. I checked every possible point from a to z but i can,t find the problem. All voltages in the TV are higher and i reduce them artificially (R654), so i can do mesuriments and tests. All capacitors are OK (also in power unit) and i have not more ideas.....Sad(

Can somebody help me?

Thanks in advance!
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torbjorn



Joined: 07 Jun 2007
Posts: 370
Location: Sweden

PostPosted: Fri Dec 14, 2007 3:28 pm    Post subject: Reply with quote

Does the 200 V voltage originate from the power supply or from the line output stage?

If all voltages from the power supply are 30 % above normal, then there is probably something in the feedback circuitry that is not working. For example, a resistor in a voltage divider chain to the voltage reference (probably a TL431 or similar) can bee too high resistance.
Try supplying the tv from a variac and adjust the mains voltage back and forth to see if the power supply regulates at all.

If the voltages from the power supply are normal but those from the line output are high, a possibility is an open-circuited flyback capacitor.
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kis62



Joined: 12 Dec 2007
Posts: 2
Location: Plovdiv

PostPosted: Wed Dec 19, 2007 4:28 pm    Post subject: Reply with quote

Thanks for the answer!
I solve the problem. The CRT is the reason of all.
After i chk all and can not find any thing i do the next steps:
1. I connect a resistor 15k betw. 5 pin of TEA5101 /where is the 200v power suplay/ and GND. /I immitiate ~15mA a normal working current of TEA./ Then the 260v are 216 and i thing this is not so bad.
2. connect Osciloscope to pin 7/red cathote/
3. Turn the TV on.
4. I saw big voltage pics comming from cathode! This is each time when i turn the TV on and this is also comming while the CRT is warm, after 3-4 minutes.
5. I reduce the heating voltage of the CRT with a 4.5 Ohm and i solve the pics in working state, but the picture quality is no good and poor.
So, I thing its happening : The pics comming from cathode kills the TEA in working state. After many replaces of TEA and many blows
dead is also the TDA4685 /RGB decoder/. When i replace only the TEA with new one, she start work without input signal and cannot become a normal current of 15 ma and the 200v are 260! If i replace and the TDA, all is ok, only picture quality is no good. If i connect the heating normaly /without 4.5 Ohm/ every think happend again! So i think the CRT is guilty Twisted Evil Sorry for my bad english!
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PostPosted: Sat Feb 02, 2008 12:34 am    Post subject: Reply with quote

there solution:
disconnect heater source on crt panel
make new heater source (not grounded to chassis or connected
anywhere except directly to CRT tube)
be careful on heater voltage that you make
sorry for bad eanglish
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