iMac g3 (1998) Only boots if I hold the reset button!?

[Rikintosh]

I recently purchased an iMac 98, when trying to turn it on, it turns on the orange light on the button, the monitor degausses, and I hear the boot chimes that are interrupted earlier than it should. The light remains orange and nothing happens.

I found that if I press the reset button (that little hole on the side, that we use a paper clip) it will start up. But with a peculiarity: I need to keep it pressed for more than 30 seconds. If I hold it down for, say, 10 seconds, I will only be able to hear the chimes for a few thousandths of a second longer than before. The time I hold the reset button is proportional to the "necessary for the machine to start up as it should"

This is so crazy that I myself cannot find much logic on how to solve this problem.

I have a hunch, some capacitor is damaged, and needing more time than usual to charge, maybe?

 

I disassembled a part of the imac, checked the motherboard, and the reset button is apparently ok. I also changed the battery and the CUDA button procedure, tried to connect the machine without HD and cdrom drive, all in vain.

Strangely, the machine will not turn on (not even turn on the orange light) if it has no battery, and I remember having other imacs that turned on normally even with a dead battery, they just got the wrong date.

Can someone give me a tip where to start?

 

[Alex]

From the service manual which only shows steps that you may or may not have done.

I would be inclined to suspect the caps, they may not be obviously leaking but they should be replaced in my opinion only because they are old.

Measure the battery voltage. I strongly believe it is not required to boot the machine, my other iMacs of this vintage don't require a battery.

Resetting the PMU on the Logic Board
 
Resetting the PMU (Power Management Unit) on the logic board
 
can resolve many system problems. Whenever you have a unit that fails to power up, you should follow this procedure before replacing any modules.
 
1. Disconnect the power cord and check the battery in the battery
 
holder (BT1). The battery should read 3.3 to 3.7 volts.
 
If the battery is bad, replace the battery, wait ten seconds, and then reset the PMU (refer to the next step). If the battery is good, go to the next step.
 
2. Press the PMU reset switch (S1) once on the logic board and then proceed to step 3. Do
 
NOT press the PMU reset switch a
 
second time because it could crash the PMU chip.
 
3. WAIT ten seconds before connecting the power cord and powering the computer on. If the computer powers on, go to the next step. If the computer does not power on, there is something else wrong with the computer, refer to the symptom/cure chart, “No Power” in this chapter.

Chime back if you continue to need help.

 

[Rikintosh]

Yes I did it. I have a brand new battery, I did this procedure, and I also tried to reset the pram, without success. I think I'll have to change the caps

 

[Rikintosh]

I found a huge capacitor in the power supply, it seems to be 400v or something. He's stuffed, I've been reading about him, in some cases the vinyl cover produces the same effect, but in my case, he's really fat. I'll look for a replacement

 

[IlikeTech]

Did swapping that cap fix this issue?

 

[Rikintosh]

Did swapping that cap fix this issue?

Unfortunately I couldn't find the problem. The capacitor I showed in the photo, which looked bulging, was just a plastic cover, the capacitor was perfect. In other forums many say is problem in flyback, and this is not true at all, because I BOUGHT ANOTHER IMAC, and changed PLATES, and found that the problem is indeed the motherboard.

I believe it is a problem with pmu. I haven't measured the voltages yet, but if you want to venture out, a user from another forum, teaches you how to measure the voltages on the board.

oldturkey03: https://pt.ifixit.com/Answers/View/173281/its+not+powering+on