Macintosh SE video problem

[Ildefonso]

Thanks to all you guys in advance for helping me to solve my problem.

I recently bought a mac SE with the tipical scanlines video problem. Here an image of  what I’m talking about

As you can see in the image the mac starts to the desktop, the start bong can be heard, but the image displayed has this scanlines patern.

I’ve followed all the Larry Pinas steps recomended in his Dead Mac Scrolls book, changed resistors 20 and 21, tried to tune up the cut-off resistor, but nothing has changed. Furthmore, I’ve swaped a known good working analog and video crt boards with the same result, which make me think that the fault is in the logic board, but I’ve not been able to find any clue regarding this video problem in relation with the logic board.

Any help would be greatly appreciate.

 

[Ildefonso]

As an update to the request for help I made, I’ve tried an other analog board and now the mac is flawlesly working.

That should mean that the fault is in the original analog board.

As said, I’ve followed all the Larry Pinas advices and changed resistors 20, 21 and 22, the U1 chip and the variable resistor 24 (the one that regulates the cut-off).

Any aditional clue?

 

[LaPorta]

This may sound silly, but what does the normal brightness knob do?

 

[codepoet]

Replace the chip shown in this picture.

Also see my similar, but worse, situation which was resolved thanks to the above advice from a kind stranger on Facebook:

 

[trag]

https://www.digikey.com/product-detail/en/texas-instruments/SN74LS38N/296-1664-5-ND/277310