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TDA8174A lowers imput impedance suddenly ?

 
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TVrepairamateur



Joined: 22 Jan 2006
Posts: 2

PostPosted: Sun Jan 22, 2006 10:27 pm    Post subject: TDA8174A lowers imput impedance suddenly ? Reply with quote

Hi,
I have a TDA8174A as vertical amplifier. The vertical trigger pulse from the TV processor is in good shape - but sometimes the output 1 of the TDA8174A looses signal and goes to Ground. Interestingly the normally 5.5 Volt trigger signal on the trigger input (3) falls to a 1.5 Volt p-p signal althrough the video processor output is still 5.5 Vpp. The 4.7K resistor between Video processor and TDA8174A now causes the drop by 4 Volts. Before the same resistor has a drop by about 1 volt only. There is very little info on the TDA8174A - what causes the impedance of input 3 to become suddenly so low ? This is already the second TDA8174A I try - with the same effect. Who knows why this happens?

Many thanks !
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R_ay
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PostPosted: Mon Jan 23, 2006 11:14 am    Post subject: Reply with quote

If nothing is wrong with the IC, based on your description, sounds like the IC is triggered in to shuttdown/protect mode. Provided there is no problem with it's DC supply &/or peripherals bad conn/etc, you may have problem in the final(yoke) stage or even in the feedback line. (pin 9)

http://www.st.com/stonline/products/literature/ds/9776.pdf
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PostPosted: Mon Jan 30, 2006 12:57 am    Post subject: Reply with quote

Thanks R-Ay,
Indeed, my yoke had ittermiting contacts - resoldered the thing and now it works again...
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