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Normally I am pretty darn good at diagnosing stuff. This one has me baffled.
We have an over the stove microwave that I bought less than a year ago. Just prior to Thanksgiving, we had a small fire in the oven - no overheating or anything, just a small grease flare-up that I put out with baking soda. We used the vent fan on the microwave to clear the smoke over about an hour period.
Once the smoke had cleared out, the pad on the microwave only responded to the numbers 1, 7, and the clock button. I figured that the unit had failed and brought in a countertop unit for use instead.
That day and the next day, I had the OTS unit unplugged, and replugged it twice briefly, just to see if it would do a cold start. No dice.
Three weeks later, I plugged it in, and it sorta worked. The display kept having phantom readings on the unlit segments, and most, but not all of the pad keys worked. After a few days being plugged in, it is almost back to normal, even though I have a warranty repair person coming out to look at it.
I have never seen a circuit heal itself over time like this, and I have worked with electronics and computers for a long time. Does anyone have any ideas what happened to cause this? I'm about ready to believe in electron ghosts.
It almost sounds like the microwave keypad and EL display connectors got wet, causing a malfunction, but not frying the control chips. After a couple of weeks go by, the moisture evaporates and the signal connections become reliable again.
Maybe the oven fire timing was just a coincidence and not the true cause of the flaky behavior?
Not that I can explain how any moisture would get up into the keypad area....
I agree that it sounds like a wet electronics issue. Was water used to put out the fire? Like say, water from the sink sprayer? That could definitely cause symptoms like you describe, especially if water got into the vent that's above the control panel on many OTR microwaves.
For future reference, if any of your electronics ever get wet, pull the power cord and/or remove all batteries (even backup batteries) from the device immediately. If possible, disassemble the device for faster drying. Wait until everything is completely dry before reconnecting the power. If it still works after that, you should be good to go indefinitely. (I say indefinitely because the incident might or might not have shortened the lifespan of the product.)
I can't stress enough that it should be absolutely dry before power is reconnected, not "shake it off and see if it still works in an hour" dry. For the best possibility of saving your Let it dry for several days with a fan pointing at it if possible. Having power connected to wet electronics can severely accelerate corrosion and oxidation on the solder, components, circuit board(s), and connectors, and the moisture could cause short-circuiting, which can damage some of the more sensitive parts.
Unplug that microwave, take the front panel off if possible, and see if there's moisture in there. If so dry it out before plugging it back in.
Good luck.
Last edited by Thegonagle; 12-26-2009 at 12:20 AM..
I've had similiar things happen to a number of small (and larger) appliances/devices. The problem in nearly every case that I've bothered to really "tear" into has been contamination of one sort or another of the conections between the actual display (typically LCD but not always) and the controlling/driving circuit. The displays are made up of a sandwhich of glass layers with the actual digit segments "printed" on one ore more of the internal layers. One of the glass layers typically extends beyound the others and it's on this exposed layer that interconnect fingers are printed. The clock/timer circuts (microcontroller etc) connects to this row of fingers printed upon the glass layer through an elastomer strip that consists of a series on conductive and no conductive regions. Sometimes the mutilayer glass display plugs into a multi-conductor edge connector but more often than not the elastomer method is used. In both cases the design attempt is to allow for the difference in thermal expansion versus temperature between the glass display (extrememly low rate of expantion) and the other typically fiberglass pc-board supporting circuitry. It's at this interface area that the problems occur. The display panel is a very high impedance device so it takes very little contamination at these conection points to screw things up.
If you really get into it and disassemble the area you'll find the structure I'm trying to describe here. The fix is to really really clean all surfaces in the connection. The contamination often occurs across adjacent circuit traces and is more significant to the problem than the "in series" contamination. In effect the adjacent trace contamination provides a low impedance path across traces which in a uncontaminated device would normally be a high impedance path. I've always used alcohol...not the medicine cabinet variety as it contains a large percentage of water. Most home supply palces have small containers of propanol etc for thinning shellac. That and a handfull of clean cotton Q-tips will do the trick. Don't let the cleaning fluids travel as it's real easy to "wash" contamination into a previously clean area and make matters worse. Dip the Qtip into the cleaner and give it a quick flick to shake out excess liquid. During reassembly the realignment of the compliant elastomer strip is critical due to the small width of the circuit traces.. Make a good mental note of how it was placed before you take it apart. A pair of over the head magnifiers is good...especially if your eyesight isn't that of a teenagers anymore.
What happens over time is that the contamination is typically hydroscopic to some extent. Changes in the surronding humidity changes the impedance of the adjacent trace isolation. Winter is here, things tend to dry out a bit..and so things may apear to improve somewhat. Boil a big pot of water on the stove for an afternoon and I suspect the display will staring acting up again.
Good luck
btw...the same "contamination" problem occurs with elasotmer keybrds/keypads as well..same cleaning procedure will do the trick.It's the careful dis and reassembly that's the challenge.
JBrown, I think that makes sense. I was scratching my head over the water comments, because no water was used in putting out the oven fire (I'm dumb, but not that dumb.) However, it was a large turkey, and there was considerable steam venting out of the oven vent prior to the fire. The jostling of the connection from fan vibration, coupled with the condensed steam, makes your thought of a poor connection and contamination seem the most likely scenario. Thanks to all of you. When the repair person comes out (under warranty , I'll show him this thread.
I agree that it sounds like a wet electronics issue. Was water used to put out the fire? Like say, water from the sink sprayer? That could definitely cause symptoms like you describe, especially if water got into the vent that's above the control panel on many OTR microwaves.
If you use water to put out a grease fire a flaky keypad on the microwave would be the least of your troubles.
I didn't realize this thing was still under warranty.. I doubt a field service tec will get down to the real nitty gritty details I described.. Most likely they'll just replace things at the module/pc brd level. As long as it's covered..no matter to you.
The keypads are normally wired in colunms and rows, you mention that 1,2,3,4,5, and 6 worked OK leaving the third row 7, 8, and 9 off, this can be caused by an open or short circuit on your board, if the wathever that was causing the short evaporates or fall then the problem will go away.
Trivia, it was supposed to be fixed, then the part has been backordered, and it doesn't look like it will arrive in time for the Feb 15 backorder date, which means I'll have to raise hell with the company supposedly doing the warranty service. Ridiculous.
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