iMac G3 video problem

[mac2geezer]

My G3 iMac 400 DV has a video problem; the video is squashed in on the left, colors are wrong and can't be corrected, the icons are all washed out, etc. Plug another monitor into the VGA port and it's video is normal.

So, thinking, aha, I replaced the PAV board; nothing changes. Scratched my head and said it's got to be the tube or the high voltage circuits. There's another G3 iMac here that works fine, so I swapped motherboards, and get the same problem with the 400 MB in the second G3.

I can only conclude that the problem is in the 400 MB. Anyone else here have any other ideas; am I all wet, or is that a correct conclusion?

 

[tomlee59]

I can only conclude that the problem is in the 400 MB. Anyone else here have any other ideas; am I all wet, or is that a correct conclusion?

Well, if the problem moves with the board, that seems like a reasonable tentative conclusion. As additional confirmation, does the other imac now display properly? If so, it eliminates the possibility that there's a problem with the deflection yoke that kills some circuitry on the logic board. But if you now have two broken imacs...well, you get the idea.

 

[mac2geezer]

Thanks for the reply.

After switching the MB back to the 400 DV the other iMac displays and functions normally. So it almost has to be the MB.

My choices, as I see it, are to get another MB off eBay or just add a cheap VGA monitor and live with the weird built in display.

 

[iMac600]

Out of curiosity, did you swap the entire board assembly, as in both logic board and down converter, or just the logic board?

I can't be 100% certain, but if you moved the entire motherboard over and got the same results, i'd be guessing the down converter board, pictured here:

http://www.thebookyard.com/images/im3dwnconv.jpg

It clips in to the side of the motherboard via that black connector and drives the electrical circuitry, so if you've transplanted the board and the down converter attachment across systems, that could be the problem. If you've already tried transplanting the down converter from the working G3 on to the 400 board and got the same result, then chances are your theory is correct and the board itself is toast.

I will add I am having a blank stab in the dark, I haven't worked on iMac G3 video circuitry for a long time but it seems like a worthy consideration. Just remember "your mileage may vary", it caused my display to go weird in my old iMac, as in it wouldn't start up unless the system was "charged" and even then it would just flash on rapidly then cut out, but I have yet to see another iMac with exactly the same issue.

 

[mac2geezer]

Yeah I did swap the entire MB assembly, including the down converter. When I get some time I may take both machines apart again and try swapping just the MB. On the other hand there are a couple of 400 DV MB's including the down converter on eBay, so may just go that route if they're cheap enough.

Thanks for the tip!