Philips chassis H image problem

[delaitt]

Hello

I have a Philips TV with a 'Chassis H'

The top of the image gets distorted towards the right hand side of the screen when the RF signal gets stronger. I've used a pattern generator to show the problem. The same happens when using a modulator with a broadcast video signal.

I've included the schematic for the line and frame circuits.

Do you know where i should search for troubleshooting?

Thanks

Thierry

[G6ONEDave]

Looks to me like the 'agc' circuit is being overloaded by the strong RF signal. There might be a pre-set resistor for it, which is likely in the tuner/i.f. stages. Otherwise fit an attenuator to the aerial input socket, starting with 3db and increase until the image returns to normal.
Dave

[delaitt]

here is the tuner. i dont have the schematic for it. it seems to have 3 adjustable potentiometers. I've used the opportunity to replace the old electrolytics.

I've added a -20db and it partially helps with the pattern generator

However, the vertical line linearity dynamically changes when using a modulator on a video broadcast when the video level of black/white changes especially at the top where the image dynamically shift left and right a bit.

Thanks

Thierry

[delaitt]

Hello

Here is a link to a youtube video that shows the picture issue https://youtu.be/Fve4YTssJMI

As shown on the video, I also seem to have some instability of the frame sync when the picture quickly changes from dark to white and vice versa.

I've replaced C3552 on the line sync module. that cap is meant to be 6.8uF according to the schematic but the actual one was marked as 1.5uF on the module. I've replaced it with 2.2uF. is that ok?

Do you think the video quality issue comes from the tuner/IF stage?

Thanks

Thierry

[peterpixel]

I don't know the chassis but from the pictures and the description this looks like the sync pulses are being crushed on high video levels during change of shot etc.I would suggest checking the video amp/sync separator circuit.Peter.