Philips Magnavox TP2785 C221 TV No Picture

[wb6nvh]

I have a Philips Magnavox TP2785 C221 27" conventional TV on the bench, chassis type is 27K800, and unfortunately no service data. The sound is fine and the channels change correctly, but the screen has no picture, it's dark just as if the TV were turned off. When you shut the TV off, the screen briefly flashes white/colors for an instant. The status LED on the front stays a steady red when the set is on, but I have never seen the set in working condition so I don't know if this is normal or not.

I checked all the electrolytics for high ESR (but not shorts) and they are fine, there is no evidence of burned components. I checked the diodes and most of the discrete semiconductors on the main board, and they appear to be good. The resistors on the CRT socket board are all good and I re-flowed the solder joints on the CRT socket board.

The CRT has good high voltage (32 KV approx) and the CRT filaments are all lit. I measured the voltages on the cathodes of the CRT and they are at 180 volts, which is too high and apparently are what's cutting off the CRT so there's no picture.

These cathodes are driven from a Philips TDA6107Q video amp IC on the CRT socket board. I took a look with a scope and the R/G/B inputs to that IC show about 1-2 volts of a video waveform going in to them, which I would guess is normal. But the video outputs from that IC to the cathodes are all way too high. The datasheet says 129V is typical and 142 is maximum.

I guessed that this IC was bad and replaced it, as they are cheap. However, the problem remains as before. Something is cutting off the CRT, perhaps insufficient video level to the video amp IC?

I'm wondering if anybody else has run across this fault already and whether I am on the right track!