If you are a do-it-yourselfer with electronics expereince, read on.
If not, look at just the last couple of paragraphs.
What I would suggest first is to replace all of the electrolytic capacitors on the smart board. They may be black, green, blue, etc.,
and look like those seen in the photo here: http://www.sparkfun.com/shop/images/163.jpg
What happens is that the electrolytic (liquid) wears out, changes pH level, or dries out over time, and they stop filtering the rectified DC
voltage from the power supply. The fluctuations (noise) that get through will drive ICs crazy! New caps may fix this for you.
To get the replacements needed, you can visit Tritronics at http://www.wehaveparts.com/index.cgi?page=electrolytic.htm
Be sure the value of the caps you order is the same. Voltage can be a step higher. These are all 105-degree caps, and very good, too. Be sure the plus-minus polarity is the same when you install the new ones or they can actually pop and take off like little rockets! Safety first!
Also, I would recommend resoldering and if possible actually testing any parts which are burned or which have darkened spots under them on the circuit board. Big transistors and resistors are your first suspects. They can desolder themselves over time as the solder experiences metal fatigue from the heat.
Lastly, look for brown glue (goo) used to hold components down before they were machine soldered during manufacturing. This glue can become conductive over the years, causing odd and aggrivating problems.
Still need help? I fix microwave control panels for only $32.50 in most cases. See http://www.MicrowaveDisplay.com for details.
Good luck, and observe safety rules (such as those at http://www.gallawa.com/microtech/safety.html)!
William Miller
July 2005
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix microwaves.