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.
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.