A2 with weird Display

[bibilit]

I have an Apple II at home, working (most of) but with a weird display problem.

Image is on the bottom of the screen, not on top, even games are displayed in the bottom part of the screen (screen is not the problem, tested with another, same problem)

Any idea ?

 

[bibilit]

The Logic board has a modification on the upper right part, related with video, connecting it to the screen i get no image, any clue ?

 

[Gorgonops]

The Logic board has a modification on the upper right part

If the "modification" you're referring to is the little silver box that's probably an RF modulator. It would be expected behavior that you wouldn't get anything on your monitor when you connect to it, the output from it is meant for a television tuned to the proper channel. (In the US that would most likely be VHF 2-4, although UHF modulators did exist as well. Your location says you're in France, is it an NTSC Apple II or a PAL modified model?) The monitor is designed to accept plain composite signals, which is what the jack on the board itself produces.
 

Regarding the misplaced video, my first guess would be that there's something wrong with the vertical timing counter chain. The circuitry/schematic of the Apple II is copiously documented on the web, here's an example of a contemporary text which talks about its operation in fine detail. Mike Willegal has a page about Apple II troubleshooting here that might be of some use getting you started as well. On that page he's linked a document of "Apple Service Notes" here. Possibly of interest: on page 4 there's an entry for

Video pushed down/to right:  C13-74LS02

Checking C13 against the Apple II Circuit Description book it says that IC is what produces the video sync based on the outputs from the vertical and horizontal timing chains. If you want to try just shotgunning it I might start with replacing that IC (or reseating it looks like everything is socketed on your board.) and working backwards from there. (The chain is described on page 35 of the book above.)

 

[waynestewart]

I'd probably start by reseating all the chips. That's solved 99% of the II+ problems I've had

 

[bibilit]

Thanks,

Problem solved, reseating all the chips worked for me, i touched some weldings as well, fully functional again.

Yes, found information concerning also the RF modulator, did not try it, have not the proper cable.