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<< Solved >> Help needed - iMac G4 swirly LCD screen of death

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Hi all - I got my hands on a pair of G4 cases with working screens this week, and have been trying to get them reading display inputs from DVI / HDMI following DremelJunkie's instructions plus the MacTester improvements (correct green 'dimmer' voltage and 3.3V to LCD supplies). The inverter fires up perfectly using the hotplug. BUT on both machines, testing with a Pi-3 gives me a swirly pattern of pretty LCD colours slowly fading to near black, instead of an actual display. I had originally assumed it was damage to one of the TDMS cables but exactly the same thing has happened on the second machine as well. The first was hooked up to the DVI pin connector, and the second has a screw-in HDMI breakout with all the pin conversions calculated. One is 1Gz, the other 1.25Gz, and they both had the expected LCD connector for that neck.

I'm stuck! So, just in case anyone has any suggestions:
  • Is it possible that there is another colour-coding scheme out there for these necks (eg European distribution) and the whole red-brown-green-blue schematic on DJ's website needs to be switched round for them? TBH my outer cables on the black LCD cable look more like coral pink - red -green - blue, although the 'pink' could be a very faded brown at a stretch of the imagination. (I've tried rewiring on the assumption that pink=faded red and red=faded brown, but that didn't get me anywhere).
  • In both macs, there is an extra shielded cable coming down from the neck tube (2xred 2xblack 1xgreen) which is mentioned in none of the forum posts I've read - should this be connected to something to enable the displays to work properly? If not, what on earth is it?
  • Does anyone have any idea at all how to troubleshoot this? I suppose one thing is to crack open the monitor surround and check the pinout on the other end, but a 1.5mm hex screwdriver is pretty much the only one I don't have by now...

Thank you!
h
 
Fixed now - but posting below for info in case anyone else is trying to navigate the long conversion threads for info. Especially important for the 'use 3.3v for everything' posts, which are potentially misleading. Here goes:
  • The red LCD cable must be wired up so it receives the VEDID 5v on connection - although the online guides don't mention it, display source needs to be told info about the screen to ensure correct resolution etc and it seems the only way the screen knows to provide this via the EDID 'data' cable is by being 'woken up' by VEDID. Wiring LCD red in parallel with the LCD 3.3v supplies (purple/yellow/orange) as hinted by various forum posts won't work, it seems.
  • The red inverter cable (at least on my iMac) won't wake the inverter on 3.3v but does wake it on 5v. So not everything on there is 3.3v!
  • For 1Ghz/1.5Ghz models, the 'brown' cable on DremelJunkie's pinouts is now a 'light red' colour - I guess the plastic has faded in the last decade.
  • If you're on Linux, it's not a foregone conclusion that every kernel will support the monitor itself. My PI 3B won't pick it up (or maybe I just fried the hdmi while messing around) but an old laptop running Linux Mint had no issues at all.
 
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