Wonder if you folks can help... I'm trying to restore a friend's 800, which is giving... well, just awful video. Using the RF mod, when powering on the 800, the screen definitely CHANGES, but the screen is close to static (you can almost see a distorted frame as the display cycles around). We attempted composite video from the 5-pin DIN, but didn't get much of a picture, although trying the O-scope on the composite port yielded the below picture; it's trying, but not giving a picture.
We found the repair manual and followed the flowchart for grey/black screen, tracing all the clock pins (which all seemed to look OK). Pin 25 of the GTIA gives a nice clean (composite sync?) signal.
Since the flowchart recommended replacing the GTIA, just in case, I bought a "known tested" CPU board from eBay, which gives the same results. So I don't think it's the GTIA, but maybe something in between it and the RF mod/DIN port. But I'm not sure what to check next. I found this schematic https://atariage.com...-1338873000.jpg, but it'll be mildly painful to trace the whole thing through and I'm not 100% sure what I'm looking for.
Any suggestions for the next step? Thanks--it'd be quite a thrill to get this working again.
Oh! One more clue (maybe?)--I haven't owned an 800, so I don't know; the speaker never seems to make any sound, either when powered on or when trying control-2 (which I hear buzzes). Is it supposed to? Not sure if that helps.