Philips Magnaovox TP2785 C221 No Picture

geo727December 5, 2006

I have a Philips/Magnavox 27" CRT TV on the bench which has no picture. Chassis number is 27K800 7591, Model is TP2785 C221. This is a 2001 vintage TV set.

The anode high voltage is present and normal, the CRT filaments are lit, there's sound and apparently normal tuner activity. The screen in a darkened room shows a very faint gray raster with horizontal lines across it and there's a brief normal-brightness full screen flash of white when it's powered off.

I am an electronics technician (but not a TV tech.) I checked all the electrolytics for ESR issues and they were all fine. The 10 Ohm resistor on the CRT board which goes open and which is a common fault on this model, is good. There are no obvious burned or defective parts on the main board. I curve traced all the diodes and the discrete semiconductors and they are all good.

I don't have any schematic or documentation on this set but I am suspecting the processor IC or whatever IC controls the video has gone bad. There are two larger IC's on the board, I assume one is a CPU. My question is whether anyone else has run across this problem before and what their solution was. I hate to waste money replacing the IC's (there are only three) presuming they are even available if it may be something else. I do have vacuum desoldering stations so removing a 40 pin IC is no problem. Also, does anyone have a favorite source of these IC's? I have never dealt with Philips before and was wondering if there are alternate sources. I don't think ECG carries the proprietary chips...

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This set has a TDA6107Q triple video amp IC on the socket board of the CRT, driven by a TDA8841 NTSC/PAL video processor down on the main board. I measured the voltage at the cathodes of the CRT and they are 165, 188 and 188 Volts, all too high which is probably cutting the CRT off and explains why there's a brief white raster when the power is shut off. I think the video amp chip is bad and is putting the 190V Vdd rail onto the cathode outputs as the nominal output voltage of the TDA6107 to the cathodes is 129V, NOT 165-188V, according to the chip's datasheet.

My plan is to put a scope on the inputs to the video amp to see if there's video there. Presuming there is, that doesn't seem to leave much except a bad video amp IC. Fortunately that chip is $ 5.00 or less...

    Bookmark   December 7, 2006 at 4:44AM
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