Avon Induction Cooker VQ16 : No Heating Solved !!!!

This induction cooker was brought to me by my friend with the complaint that it was getting on, but did not heat. I opened the cover, did a thorough cleaning. This is a simple task. Remove the screws from bottom, remove the mains cord from its sockets, remove the coil, disconnect the sensor and disconnect it from main board, disconnect the fan and front control panel, and unscrew the main board from the bottom cover.

First thing in Induction Cookers is to look for black, large sized box fixed capacitors located at the output where the coil is connected, as these are found to be common failures. I removed these two capacitors for checking. See what a 0.3uF/400V capacitor was reading, first on Blue ESR meter and then on Capacitor Meter:

The other one, 5 mfd/400V though reading ok, had the bottom slightly bulged. This is an indication that it might fail any time. So, replaced these two and and checked the IGBT for any shorts, which might not be there, as the fuse was in tact and the unit was getting on. Reconnected the panel, coil, mains cord, fan etc. And switched it on, placed a stainless steel round vessel with water on it and touched the function button. No heat, though the fan was running.

So, I knew that there were more failures on the board. So, switched it off, dismantled everything once again, and removed the board for thorough investigation. I noticed that there were many rusted components on the board; a few IN4148 diodes, S8050/8550 transistors, 78L05 regulator which looks like a transistor etc. While removing, the legs broke as it had reached that condition:

After replacing these and doing a thorough dry solder patch up, and checking important spots where high watt resistors are used, I assembled the unit again and switched it on by placing a vessel like before. It started working, and slowly heating up. But all of a sudden, I heard a loud noise and spark from underneath and the unit became dead!  Smoke and burning smell came out!

So, I switched it off and dismantled the unit once again.Noticed that the fuse was in tact (!!!), but IGBT was short, PWM IC Viper12A had cracked, a 10 Ohm 1W resistor was charred.  As is common in Induction Cooker Main Boards, some values of components have gone high or low and this needs more thorough probing. So, I removed one leg of all the resistors and diodes on the board.

Checked and found the value of a few resistors had gone high. Then replaced all the electrolytic capacitors and suspected fixed capacitors as well. Checked the diodes and transistors and found these were ok. Looked for any track cuts and checked thin tracks using ESR Meter, as suggested by Jestine Yong in his article on more use of ESR Meter (This meter was bought from him). Found everything ok. Replaced burnt out components like IGBT, PWM IC, and defective resistors and fixed capacitors etc. Following are the components totally replaced in this ‘failure-monger’ Induction Cooker:

Cleaned the board thoroughly using Denatured Laboratory Spirit. Disconnected the jumper that goes to IGBT and connected only the control board and fan and switched it on. It worked fine. So, soldered the jumper, and assembled the unit back again. Switched it on! I heated two vessels of water on it and it worked very well:

Before concluding, let me share this for the sake of novices on Induction Cooker (Perhaps Jestine Yong can come out with a book on servicing Induction Cooker and Microwave Ovens)  that Induction Cooker works on two or three types of Voltages. Primary B+ voltage is derived directly from the 230V AC rectified by high Amp rectifier modules and smoothed by the high voltage fixed capacitors. For the PWM controller, series of high watt resistors are connected from AC line and rectified by diodes and smoothed by a suitable electrolytic capacitor, to reach the required DC level for its function. When it starts working, it is induced to the primary of a small SMPS transformer, from the secondary of which power supply to the micro controller and control panel etc. are given. On switch on, switching of the IGBT is achieved to produce high frequency electromagnetic field around the coil, which is transferred to the vessel and sets up a circulating electric current, that heats up the vessel.

The main difference between microwave oven and induction cooker is that the oven heats the contents whereas the IC heats the vessel. That is why in microwave ovens, only suitable plastic containers are to be used. For more details, please visit this site: http://theinductionsite.com/how-induction-works.php. There are replacement boards available for induction cookers and I am giving below the snaps of these along with a spare coil I have, for your information. I tried this on the IC under repair and it was working fine. When any board is so stubborn and cannot be repaired, the ideal way is to replace the board with these.Unfortunately, only press to on switch type control boards are available and not touch type.