Repairing the original iMac: Replacing the Flyback Transformer

The original iMacs, released in 1998 (and often credited as the computer that “saved” Apple) have a tendency to suffer from video failure after a few years of use. Most commonly, this is the result of the analog and video boards going bad. More specifically, it is the analog board and even more specifically it is the flyback transformer (FBT) on the analog board that goes bad. The FBT is responsible for stepping up the voltage required to make the cathode ray tube (CRT) monitor work and contains very fine, tightly wound wires inside to accomplish this. The design of the Rev. A-D iMacs does not allow for efficient ventilation around the FBT so this component tends to get hot, especially if the computer gets a lot of use in an area with poor air circulation. If the FBT gets too hot, the very fine layer of insulation on the very fine wires in the FBT melts and you get an internal short. The result is an iMac that won’t start up (note: no sound in movie file). If you press the power button, the light turns orange, the computer chimes and begins to boot After about ten seconds, the power button light turns to green and immediately the computer turns off (probably accompanied by a “zap” sound from within). Typically, the computer will not even begin to power up again unless you have let it sit for a while, at which point the same thing happens when you try to start it up again. Furthermore, if you open up the underside and disconnect the video cable, the computer will boot and operate normally (assuming nothing else it wrong with it) and you can actually use the iMac with an external monitor.

If you have an iMac (Rev A-D) that has these symptoms you have a few options to get your computer up and running again.

1. Use the iMac with an external monitor. You need to have a monitor with the old Mac connector (15 pins in 2 rows) or an adapter to get it to work with a VGA monitor. The advantage here is that you could get higher resolution out of your iMac if you use a better monitor than the built in one. The disadvantage is you need to have or buy another monitor and then you have a big half-dead iMac sitting on or under your desk.

2. Move the guts of the iMac to another computer case. This would include lots of modding and is interesting fix but you would still need an external monitor and you would have to invest in a case and possibly a new power supply.

3. Take it to an authorized repair center. I wouldn’t recommend this. It will be costly ($150-250 in parts plus labor from what I’ve heard) because they will want to swap out the analog and video boards rather than repair the analog board (we will see later that the analog and video boards are connected via some wires on the FBT so they come as a pair usually). I have also heard stories where the repair person thought the problem was with the logic board and replaced that instead. Of course this does not fix the problem.

4. Replace the video/analog boards yourself. If you can find the replacement parts (be sure you order the correct revision) on eBay or through MacResq, then you could replace them yourself. This would cost about $150-250 depending on where you get the boards.

5. Repair the analog board. This article will outline the procedure. Parts would cost you $50-100 depending on which revision of iMac you have and where you order the parts.

It cost me $69.19 including tax and shipping to get the replacement FBT for the fix I used to write this procedure. The original iMacs are the most upgradeable models and as of this writing, there are CPU upgrades available up to a G4 500 MHz. Most people likely do not want to shell out hundreds of dollars to fix a 5 year-old computer. What follows is the procedure to dismantle the iMac, replace and adjust the FBT and reassemble the computer.


During an attempt to repair of your iMac, you may inadvertently do more damage to the computer even if you very carefully follow the directions provided. If you are not willing to take this risk, do not attempt to fix the iMac yourself, pursue one of the other options I have listed in the previous section. No one but yourself can be held responsible for any damage to the computer or harm to you if you choose to follow this procedure.

The repair procedure involves disassembly of delicate parts of the computer, desoldering and soldering of electronic components. WARNING: There is potential for exposure to high-voltage during parts of this procedure. Death or serious personal injury could occur if proper precautions are not taken. When applicable, the procedure will describe proper safety measures. Furthermore, the CRT contains a vacuum and if shattered could also result in injury. Do not operate around the CRT without safety glasses on.

That having been said, I have fixed several iMacs without incident following this procedure because I take precautions to prevent injury/death. DO NOT ATTEMPT THIS PROCEDURE UNLESS YOU UNDERSTAND THE POTENTIAL DANGER AND WILL TAKE THE REQUIRED MEASUREMENTS TO PREVENT INJURY OR DEATH TO YOURSELF AND/OR ANY OTHERS WORKING ON THE COMPUTER.

- #2 Phillips head screwdriver w/ insulated handle
- #0 Phillips head screwdriver w/ insulated handle
- 3/32 slotted screw driver OR plastic blade (either can be used for prying apart pieces)
- wire cutters/strippers
- soldering iron
- safety glasses (TO BE WORN AT ALL TIMES)
- electricians gloves

- towel or blanket to set the computer on
- replacement FBT (part numbers and ordering information can be found in the procedure)
- heavy gauge wire
- solder w/ flux (rosin) core
- solder flux (rosin)
- #2 or #3 solder wick
- one zip-tie
- electrical tape
- scotch tape