The slot loading iMac G3s are convection cooled and have no internal fans for cooling. Over time the PAV (Power, Analogue, Video) board gets baked which can cause a multitude of problems. Tray loading iMacs have one fan but most of the heat dissipation is still reliant on convection.

If an iMac has had a productive and well used life it will now be in a much more delicate state. With the heat issues and aging of internal components, the following problems are known to occur.
 

• Flyback transformer failure (common)
  Can either prevent the iMac from powering on, or cause varying levels of electrical arcing. Typical symptoms of arcing are a static popping sound accompanied with a horizontal line briefly appearing across the CRT screen. The arcing may even be visible within the case. Electrical arcing can become a dangerous fire hazard. It is not recommended to continue using a CRT with an arcing flyback transformer.
  
  The flyback transformer can be replaced, however they are no longer produced and it's almost impossible to find a supplier that has "new old stock". This is a specialist job! The PAV board is a high voltage area that contains dangerous charges that can kill even when the machine is turned off.
  
   
• Brittle internal plastics (common)
  Brittle internal plastics are often a result of UV/sun damage or heat stress. The inner bezel supporting the CRT is the most fragile of all plastics within the iMac. Even a small amount of pressure, or a gentle bump can cause the bezel to crack and fall apart. Be extremely careful moving iMacs, and be mindful of how you handle them while working inside them. When fastening screws make sure you don't over-tighten them. Over-tightening the screws can shatter the inserts.
  
  At this point there is no reliable way to fix the bezel breaking, apart from careful handling to prevent cracking and breakages. A complete replacement/new production of the inner bezel is the only solution.
  
   
• PAV (Power, Analogue, Video) board failure (common)
  PAV board failure is usually caused by heat stress. It will often prevent the iMac from powering on and the iMac may show no signs of life when the power button is pressed.
  
  This can be fixed by close inspection/replacement of troublesome components. This is a specialist job! The PAV board is a high voltage area that contains dangerous charges that can kill even when the machine is turned off.
   
• GPU failure (common)
  GPU failures are typically caused by overheating. This problem is more prevalent in the slot loading iMacs.
  
  This can only be fixed by re-flowing the GPU, or logic board replacement.
  
   
• Clock battery leakage/corrosion (common)
  Damage to the logic board can be avoided by removing or replacing old batteries. Never leave an old clock battery in a vintage Mac!
  
  Repairs can be made if the corrosion is minimal, but it's more than likely that a logic board replacement will be required.
   
• Rotting speakers (common)
  The speaker foam around the internal speakers commonly rots/disentegrates over time. The speakers will no longer function correctly and the cones may fall out.
  
  The following replacements can be purchased:
  - Almost exact replacements ~$5
  - Brighter toned, paper cone replacements ~$5
  - Best quality replacements ~$36
  Thanks to Reddit user 2748seiceps for sourcing these.
  
  Replacement tutorial
  
   
• Cracked/dry solder joints (less common)
  Dry/cracked solder joints are usually caused by heat stress on the PAV board.
  
  This can be fixed by re-soldering the troublesome joints. This is a specialist job! The PAV board is a high voltage area that contains dangerous charges that can kill even when the machine is turned off.
   
• Capacitors no longer running at their rated values, or leaking (less common)
  Although this is a less common issue, the capacitors are getting to the age where they may need replacing. Old capacitors can start to operate outside of their rated values, and can eventually leak causing corrosion.
  
  Capacitors can be replaced with newer replacements. Be sure to get components with the same values and ESR ratings.

Both iMac G3 models suffer from flyback transformer failure due to heat stress. The flyback transformer is located on the analogue board (PAV), and amplifies the voltage necessary for the CRT monitor to function. Both iMac models have insufficient ventilation/cooling of the analogue board. A flyback transformer has tightly wound wires which short when the insulation breaks down due to overheating. This results in the flyback transformer failing and causing the following behaviour:

Flyback Transformer Failure Symptoms

• Arcing:
  When arcing occurs from the flyback transformer you may hear a static popping sound, accompanied with a quick display distortion or a horizontal line briefly appearing across the CRT screen. The arcing may even be visible within the transparent case. Electrical arcing can become a dangerous fire hazard. It is not recommended to continue using a CRT with an arcing flyback transformer.
   
• Boot failure
  When pressing the power button the light may turn orange, and the boot chime may be heard. Shortly afterwards the light may change to green, and the iMac may shutdown accompanied with a static popping sound. The iMac may not boot again for some time, but if it does the symptoms will reoccur.
  
  Alternatively, the iMac may not boot at all or show any signs of life.
   
• Unexpected Shutdowns
  While using the iMac it may shut down unexpectedly, and it won't power on again for some time. The shutdown may also be preceded with a static popping sound and display distortion (arcing).

Flyback Transformer Replacement

It has become incredibly difficult to purchase new old stock flyback transformers (especially for the slot loaders). These flyback transformers are no longer produced, and the only available replacements are old stock or have been pulled from other iMacs.

WARNING: Replacing a flyback transformer is a specialist's job! The flyback and PAV board contain high voltage components which are capable of killing you even when the machine is completely powered off.

Slot Loading iMacs

All slot loading iMac G3s I have worked on have the LG CRT, and all have the same model flyback transformer. There may be different models used across the slot loaders so you should check your model before ordering a replacement.

The flyback model I have seen in slot loading iMacs is the Goldstar (LG) 6174Z-1017H.
In 2022 I was able to purchase an equivalent replacement (the HR Diemen HR46177 or HR 46177) from Dönberg Electronics. As of writing they no longer have any stock available. When I enquired to HR Diemen, and Efitier, their response was that they have no remaining stock and are no longer producing them.

As of writing Electronica USA have two HR46177 flyback transformers available.
Electronica Castro (Brazilian website) also seem to have some original models available - (I'm unsure on the validity of this website).

Tray Loading iMacs

I haven't worked on tray loading iMacs to confirm this but the flyback transformer model used in these machines should be 6174Z-1003G for the Revision1 board, and 6174Z-1017D for the Revison 2 board.

• The 6174Z-1003G model is equivalent to the HR Diemen HR46176 or HR 46176.
• The 6174Z-1017D model is equivalent to the HR Diemen HR46455 or HR 46455.

The MacOpz website has an excellent guide for Flyback transformer replacement in tray loading iMacs.