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iMac G4 20": HDMI pinout and power supply problem

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I recently recovered my brother's old iMac G4 lamp 20". The LCD panel should be good, but it's either the mobo or the psu who falied.
Since this thing has been sitting in a garage for a few years, I don't want to spend money on it before I test that the monitor actually works. I got an inexpensive, yet easy to use, hdmi plug with screw terminals, but after I connected all of the video signals, I realized I am not understanding how to manage power supply, in particular for a test. I don't want to buy a notebook PSU + a picoPSU + a 24v step-up converter just to test if it works: is there a way to power on the lcd panel using one or more variable dc PSUs, and how?
In the meantime, I post my own imac to hdmi conversion table, derived from dremeljunkie's dvi table, specific to the 20" model.
Hope it can be useful to someone.

upload_2018-4-21_23-34-9.png
 

pgh

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You can test the psu if it is working in various ways , make sure you test it with diferent components than the original imac parts which should be disconnected

(remember this psu is different then an Atx psu and doesnt need to be jumped...its is always on) .. Next to that i remember to have read somewhere that the 20inch imac mobo does the 24v stepup proces ... So the psu itself can not provide the 24 v needed

You can try the inverter with a cheap laptop psu which has 24v ... But make sure that the watts and the amperage meet the imac g4 display requirements

Whatever you do...be extrme carefull with the inverters wires an Dont blow up the inverter... It almost imposible to find another one...

Good luck!
 
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Thanks for the reply! Let's assume the original PSU is gone, since it probably is and I don't want to take the risks of putting my hands and connecting equipment to it.
I already have a variable notebook PSU, I can probably get the the 24v from that, and if I also need a 12v line, I can get it from another variable PSU (I have many).

The question is where do I connect what. Despite the pinout scheme, I still don't understand anything. From what I read 22, 23 and 24 should be connected to a 12v line. But then again, what about + and -? And the 24v line? Where does it go? It's pretty confusing for me.
 

pgh

Joined
Dec 28, 2012
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Motherboard
HP probook 4740s
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4740s i7-3630QM ivybridge@ 2.4 GHz
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Intel HD 4000
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The 20 inch conversion apears to be less strait forward than the 17 or 15 inch version, but if you follow the guide at the end it should be posible

http://www.dremeljunkie.com/2011/11/all-in-one-20-imac-g4-genuine-tmds-to.html

Heres his wire sheme and pinout formthe inverter:

http://4.bp.blogspot.com/--f2oDQP28y8/TtcKEiHNl9I/AAAAAAAABFY/znznsRGSkD8/s1600/Inverter+20%22.png

Ps For testing purpose it is posible to use two psu's (just to test if the inverter is still working) but once again tripple check the wires and make sure the wires you use are thick enough so that you create a stable connection

Please Let the forum know what the results are

Good luck!
 
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Thanks again! I already checked the dremeljunkie guide, but I was totally misunderstanding the inverter cable thing as I thought it was one of the two cables tied together (grey and black). Cleaning the inside of the imac I found the inverter cable, now it will be easier. I'll post my progresses.
 
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Everything is ready for the test:
I even found the bottom of the iMac and already stripped that. I have a 24v power brick (unknown wattage, 4.1A) and several 12v variable power supplies that can do the job. I still need the resistors (1Kohm, 6.8 Kohm), however, this is where I'm afraid to fry something:
upload_2018-4-27_18-34-18.png


So as you see:
- I already connected the pins from the black monitor black cable
- I connected the pins I could connect from the monitor grey cable, and twisted together the blue, green and grey (which go to ground) and the orange, yellow and purple (which go to 12v). You can see the 18 pin on the HDMI is still empty, i need the resistor
- On the inverter black cable, I separated the green and black (->ground) from the blue and red (->24v) from the orange and purple (->nowhere)

Now the question is: how do I make sure I don't fry everything? Even If finding the inverter cable made everything clearer, I still wonder how do I connect the + and - of my power supplies? I do I make sure that I'm not inverting polarity and making a mess?

I guess this is more a language barrier than a truly electrical problem (I'm Italian), nonetheless it leaves me scratching my head:
my guess is what dremeljunkie calls 'ground' is what I call '-' and what he calls 'line' is what I call '+'. But I've learn not to connect anything basing on a guess. I'm used to scenarios where +, - and GND are three distinct things, that's why I'm so confused. If someone can confirm, or correct my guesses, please any help would be very welcome.
 

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pgh

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HP probook 4740s
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Yes language could be confusing ... But if you understand the concept of electricity ,the rules are always the same
(make sure you dubbelcheck on wikipedia so you are confident)

Since this is your first time, Make sure you test the voltage out of the lines to use from the psu's with a multimeter first to make sure you connect them to the right imac components

I am not an expert but Your discription About positive + and negative - sounds clean and ready to go for me..

Maybe you can contact this person to dubbel check in your own language... I think he is italian to

https://www.tonymacx86.com/threads/15-imac-g4-mac-mini.137124/

Anyways Before you hook up al the wires (inverter and hdmi) together ... Do a test first with the inverter wires to the psu's only to see if it turns on , ...if it does then hook up everything all together... Once again: take your time and tripple check if there are no shorts!!

Fingers crossed

Good luck!
 
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Ok. Took me a while to find some spare time to devote to this project. And it didn't go well. Now, I just need to share this with someone. Take it as a cautionary tale on the importance of fact checking.
I wanted to do everything clean, possibly just once, so I ordered the 24v upconverter, bought the resistors, and put my hands on everything only today when I had all the components ready.
I soldered, connected, insulated with hot glue, made a very clean job. So proud. Then I realized I had everything connected (except the orange and purple wires from the inverter cable) and the 6.8 KOhm resistor was left unsoldered.
I double and triple checked the guide. Yellow wire from the inverter cable. There's no yellow wire in my inverter cable. Scratched my head for about a min, and said to myself "Oh, well, that must be a slightly different model". Plugged in, no sign of life. No smoke, no explosion, no nasty click or any other sound luckily, but it doesn't turn on. I hastily unplugged to avoid potential damages.
"Think again - I said to myself- if there's no yellow wire, this must be another model. Let's check if the 17" has no yellow wire."
And of course the 17" has no yellow wire. Grab a ruler, check the screen from corner to corner, and to my horror it's 43cm. Yes, that's about 17 inches. Why on earth was I so sure I had a 20", and why didn't I check before???
Because I have so little spare time on my hands, because I have a thousand more important things to take care of, because I'm stressed... (yeah, all excuses) but in the end, if I'm going to put time and (albeit little) money in a diy project I should double and triple check before doing anything. I checked back and forth less important details, and left this macroscopic information totally out.
If you read up to this point and you are thinking I am an idiot, you have an idea of how I feel.
I briefly fed 24v to a line that required 12v/5v, there are good chances I fried a perfectly good inverter lost a couple of good hours on it, and will have to remove the hot glue and unsolder to check.

"Chi è causa del suo mal pianga sé stesso." (tr. "He who is the cause to his own damage should cry on/against/by himself")
 

pgh

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HP probook 4740s
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Intel HD 4000
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iOS
Woops... That could have been prevented indeed...but at the end there are worse things in the world ;)

If you had connected the inverter colours in the sheme as above in post 1 (on the orange and purple wires), then there might be a chance that your 17" inverter survived the 24v Adventure

Check it by following dremel junkies 17" guide and first test the inverter and display only if it works .... You can use a power consumption Measure device to read how much watt the inverter consumes

https://www.tonymacx86.com/threads/imac-g4-display-mod-alternative-psu-questions-for-mactester57-ersterhernd-or-rick26.214280/
 
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