I want to use a 17" iMac G4 with a 20" logic board

[ZNDK]

I have a question about my old Mac "iMac G4".
Can I use a 20-inch logic board on my 17-inch iMac G4?

The logic board on my iMac G4 (17inch) broke and I looked for a logic board. I was only able to find a logic board for 20". I also bought a power supply for 20" at the same time.
I tried to boot up my iMac G4. And it makes a start-up sound, but the screen is white and won't go on. I also tried to boot the DVD, but it won't start. I have also run PRAM clearing but it won't start. I can't even get into single mode. I boot up my iMac G4 and after some time has passed I touch the keyboard and I hear an error sound.

Thank you,

 

[CooperBox]

First things first. There are 2 models of 17" iMac G4, different bus speeds with the superior model being 1.25GHz with USB2.
The 20" is also 1.25GHz with USB2. Both the 17" & 20" USB2 models have physically similar ports around the baseplate, but I believe the logic boards are not compatible.
The 17" & 20" USB2 models do not share the same power supply.
I have both models and for a time preferred the 20" probably because it took me so long to find one at a reasonable price, but now definitely prefer the 17", imho it just looks and feels the right size.

 

[ZNDK]

First things first. There are 2 models of 17" iMac G4, different bus speeds with the superior model being 1.25GHz with USB2.

The original iMac G4 17" was a 1.25GHz model. I bought and installed a logic board and power supply for the 20".

I boot up my iMac G4 and let it sit for a while. And then the keyboard brightness and volume controls and such seem to work. I imagine the startup itself is successful, just the display is wrong.

 

[CooperBox]

The original iMac G4 17" was a 1.25GHz model. I bought and installed a logic board and power supply for the 20".

I boot up my iMac G4 and let it sit for a while. And then the keyboard brightness and volume controls and such seem to work. I imagine the startup itself is successful, just the display is wrong.

You state, "The original iMac G4 17" was a 1.25GHz model." I take this to read that it was an iMac G4 17" 1.25GHz model that you purchased.
To avoid any confusion and for record purposes, the original 17" G4 iMac/PowerMac4,5 (launched in mid 2002 alongside the 15") came with 700 MHz or 800 MHz clock speed.
Then early in 2003 came the 1GHz 17" model.
The 17" 1.25GHz model with X3 USB2 ports launched in Sept 2003.

 

[ZNDK]

I bought a logic board and power supply for the 20 inch.
The 17" logic board is broken. I've been using a 17" iMac G4(1.25GHz) for many years, but the logic board has broken.
[automerge]1595401233[/automerge]

You state, "The original iMac G4 17" was a 1.25GHz model." I take this to read that it was an iMac G4 17" 1.25GHz model that you purchased.

I bought a logic board and power supply for the 20 inch.
The 17" logic board is broken. I've been using a 17" iMac G4(1.25GHz) for many years, but the logic board has broken.

 

[DearthnVader]

I have a question about my old Mac "iMac G4".
Can I use a 20-inch logic board on my 17-inch iMac G4?

The logic board on my iMac G4 (17inch) broke and I looked for a logic board. I was only able to find a logic board for 20". I also bought a power supply for 20" at the same time.
I tried to boot up my iMac G4. And it makes a start-up sound, but the screen is white and won't go on. I also tried to boot the DVD, but it won't start. I have also run PRAM clearing but it won't start. I can't even get into single mode. I boot up my iMac G4 and after some time has passed I touch the keyboard and I hear an error sound.

Thank you,

Likely an LCD driver board issue, you'll be luck if you didn't fry you LCD.

There really isn't a way to fix this, don't use a 20" logic board with a 17" LCD.

 

[Windreader]

I can confirm that the panel isn't compatible with your model. The main thing about the 20" models to note is that there are a few extra pins for its internal connector that the 17" and 15" models don't have, so if your were to even try to cram the parts together, there is a serious chance of damaging parts due to the differences in the overall layout of the connector. I learned this all when my 20" iMac's connector on the motherboard had a pin for the display broken off, and I had tried to see if my 17" 1Ghz model would work with the display (Interestingly enough the 1Ghz model has the same general board layout as the USB 2 models despite being USB 1.1). Luckily I didn't power anything up with me trying to shoehorn the parts together, but I realized then that in regards to boards and screens, the 20" and smaller sized screens just don't work together nicely. Now, I have a nice little 20" in the other room that I have hooked up to an external display until I can find a good board to swap it out with. Hopefully you can find an alternative to get this working as I know it can be frustrating!

 

[ZNDK]

Here's the information I learned about the differences between the 17" and 20" models.

- The pin assignments of the display connector are different.
- The number of pins on the display connector is different.

Which information is correct?

 

[yakult121]

Here's the information I learned about the differences between the 17" and 20" models.

- The pin assignments of the display connector are different.
- The number of pins on the display connector is different.

Which information is correct?

Both are right.

There are two models of 17". The older 17" 700/800 Mhz models have different display connector compared to the newer 17"/20" models (1 Ghz/1.25 Ghz).

 

[ZNDK]

Thank you all. I've given up using the 20" logic board and am looking for a 17" model. I found the following three logic boards. "820-1501-A" "820-1550-A" "820-1599-A".
These are both stated to be PowerPC G4 1.25GHz, 17-inch models. And both support USB 2.0.
What is the difference between these?

 

[yakult121]

I've got these "820-1501-A". I have neither come across "820-1550-A" nor "820-1599-A". The latter two should be newer revisions of the PowerPC G4 17-inch. I think "820-1501-A" is a 1Ghz.

 

[ZNDK]

The 17" logic board I was using had "820-1599-A" printed on it. I contacted one of the shops and they said that you can't generally say that this is for a 17" based on this model number alone. It was sold in that shop as a 20" logic board.

 

[yakult121]

You could try removing the heat sink from the logic board and check the chip model... after removing layers of dried thermal paste.

 

[ZNDK]

I got a genuine 17 inch logic board. But it won't start. I can hear the startup sound, but the screen is blank all the time. This is the same symptom as when a 20" logic board is installed. I boot up my iMac and wait for a while. The screen is still blank, but I can use the keyboard to adjust the brightness and volume level. The screen will also sleep.
I have done a PRAM clear and the situation remains the same. What's happening this time?

 

[yakult121]

I got a genuine 17 inch logic board. But it won't start. I can hear the startup sound, but the screen is blank all the time. This is the same symptom as when a 20" logic board is installed. I boot up my iMac and wait for a while. The screen is still blank, but I can use the keyboard to adjust the brightness and volume level. The screen will also sleep.
I have done a PRAM clear and the situation remains the same. What's happening this time?

1. Try reseating the video cable on the logic board. Try straightening those video header pins on the logic board, if they are bent.
2. Are the screen's LCD backlights dead? Try shining a flash light onto the black screen. Observe for any (very dark) display changes. You may need another replacement screen to counter check.

 

[r6mile]

I watched a @dosdude1 video where he says he used a 20' logic board in a 17' g4 imac, and fried the LCD because they run at different voltages. Are you sure that hasn't happened to you?

 

[ZNDK]

I watched a @dosdude1 video where he says he used a 20' logic board in a 17' g4 imac, and fried the LCD because they run at different voltages. Are you sure that hasn't happened to you?

Did I damage the display by overvoltage? I can't rule out the possibility. Too bad. Looks like I need to replace the LCD panel next.

 

[r6mile]

Did I damage the display by overvoltage? I can't rule out the possibility. Too bad. Looks like I need to replace the LCD panel next.

Do you have an external display to plug the imac into? That will tell you

 

[yakult121]

Did I damage the display by overvoltage? I can't rule out the possibility. Too bad. Looks like I need to replace the LCD panel next.

Usually the inverters in the screen are the ones which die. The LCD panels are usually fine.

 

[ZNDK]

Do you have an external display to plug the imac into? That will tell you

I don't have an external display I can connect to.

I just got a set of iMac G4's in very poor exterior condition. Its display has numerous large scratches and LCD blotches. However, the display itself is possible. So this logic board itself is not broken. I ported this logic board.

 

[ZNDK]

Possibly the display/LCD is damaged or there is a variation in the logic board and the display connectors (pins) are slightly different.

 

[Rikintosh]

If you want my humble opinion, I think you should try to repair your original card. There are some common problems that are easy to discover, and you would avoid spending money on other motherboards. One of the main defects of this model is that its owners do not change the thermal paste, so they overheat and the video chip stops working

 

[CooperBox]

If you want my humble opinion, I think you should try to repair your original card. There are some common problems that are easy to discover, and you would avoid spending money on other motherboards. One of the main defects of this model is that its owners do not change the thermal paste, so they overheat and the video chip stops working

Interesting comment about the thermal paste.
There are a number of on-line videos that show tear-down of G4 iMacs, but very few which make mention or indicate precisely where to apply fresh thermal paste to the heat-sink locations. And I recall seeing just one video which showed removal of the heat-sink assembly itself, cleaning and repaste of the CPU and heat-sink reinstallation. Ref my photo HERE.
My link HERE shows the heat-sink end blocks where thermal paste should be applied - just prior to refitment of the lower housing.

 

[Rikintosh]

Interesting comment about the thermal paste.
There are a number of on-line videos that show tear-down of G4 iMacs, but very few which make mention or indicate precisely where to apply fresh thermal paste to the heat-sink locations. And I recall seeing just one video which showed removal of the heat-sink assembly itself, cleaning and repaste of the CPU and heat-sink reinstallation. Ref my photo HERE.
My link HERE shows the heat-sink end blocks where thermal paste should be applied - just prior to refitment of the lower housing.

I also take the opportunity to suggest that you lap/polish the heatsink, apple has always had problems with the quality (poor quality) of its heatsinks. In fact the G4 powerpc are low thermal production chips, but we know that they all heat up like hell. The reason for this is the poor cooling system. You can polish your heatsink using sandpaper on a perfectly flat surface, with continuous "eight" shapes. In the end, you will have a sink that looks like a mirror. The reason for doing this is that the more imperfect and rough the heatsink is, the smaller the contact area it will have, and the more difficult it will be to change the temperature, of course I am speaking at the microscopic level. When you polish, the surface becomes so smooth and perfect that it is able to reflect perfectly, so it looks like a mirror. A level surface is able to have more contact with the processor die, so it can exchange heat much better.

I did this on a 1.33ghz powerbook, and lowered the temperature by 10 degrees.

At microscopic level of view*

Mirror like heatsink*

 

[ZNDK]

Thank you everyone. I asked a repairman to replace the display panel. It displayed without any problems.

I learned a lot of things. Thank you.