Microwave oven keypad repair

My trusty KIC microwave oven is no longer functioning, because of a keypad failure. I think it's repairable.

Public Chat
Similar projects worth following
How to make a new keypad, or repair the existing one.

I bought this microwave oven in the year 2000  from a friend who was leaving South Africa to go live with her husband, who got a plush job at Microsoft. It's been a steady companion, moving with me to th UK and back. I like it because it is simple and robust, but mostly because it is a powerful machine with a 900 W magnetron. 

I think I've had the magnetron replaced twice, and I am happy that we have a spares and repair shop who do a good job. But as far as keypads go, we're out of luck. The model KIC MWST 145 is no longer supported, so I can't buy a replacement keypad. (I was pleasantly surprised that LVB do stock keypads: not everybody is forced to ditch their microwave ovens because of a failing switch.) 

I was lucky that I was able to diagnose the problem fairly easily. The symptom was that a "0" was displayed on the screen, and pressing buttons had no effect. 

I think the most fortunate observation was about what happened when the power was switched on. From experience I knew that on power-on all the elements of the display was lit. When I had the cover off and the keypad unplugged, powering on showed the test pattern. From this I deduced that the microprocessor 'saw' a button being pressed, which prevented proper operation. 

I cleaned up what I thought might be a short circuit, and then the keyboard worked. But the next morning it started beeping even when no-one was touching it, and the condition returned. This indicated that it was a dreaded intermittent problem: but I had solved worse intermittent and erratic problems. 

My next step was to Google how keyboards worked. I knew what to look for, because earlier I had researched how many simultaneous key presses a computer keyboard could handle. <link to animation here>

My next step was to pull out my multimeter and rip the face off the keypad. 

I mapped the traces on the circuit. There are 5 incoming and 4 outgoing conductors, giving a possible 20 keys, but only 15 keys. The layout is nicely consistent, with the incoming conductors grouped together and the outgoing conductors grouped together. In the switches themselves the little interleaved "combs" are similarly consistently oriented: downward for incoming and upward for outgoing. 

Next I made a nice table of the buttons and their conductors. This is similarly well-organised. 

Then I measured the voltages on all the terminals in the switch. It was immediately apparent that one of the outgoing traces had an anomalous voltage on it. 

When I consulted the microwave oven's operation manual, I saw that the apparent button press was for 'Time Cook'. This button is on the trace with the anomalous voltage. 

My diagnosis so far is that there is voltage leaking from one of the incoming traces on to one of the outgoing traces. Finding the location of the leak and fixing it i should fix the problem. 

Conductor Notes.pdf

These are the notes I took for figuring out the keyboard matrix.

Adobe Portable Document Format - 522.56 kB - 11/29/2018 at 19:18



This is a diagram of keypad buttons. I might use this just to help me with design, or I might use it as a draft for printing some kind of replacement.

svg+xml - 11.50 MB - 11/29/2018 at 14:34


  • Cut/Uncut

    Niel Malan12/31/2018 at 15:55 0 comments

    My preferred way of doing a repair is to diagnose the problem with pin-point accuracy. This often means eliciting the undesired behaviour repeatedly while varying conditions. 

    In this case I need to make and break connections, and on a printed circuit that is not too easy. But I have a little bottle of silver conductive paint, and brought it out. 

    My hypothesis is that there is an unintended leakage of current between a probe line and a sense line in the matrix. This happens where the one passes over the other in a bridge. 

    So the next step was to identify the bridge(s) that gives the problem. 

    But first I had to undo my previous cut, which removed all power from the line. I will admit that I was trepidatious, because I did not know if my silver paint would be compatible with the printed conductor, or work at all. I had left the power on, and a few seconds after I'd painted the machine beeped and the fault condition was established again. 

    The matrix lines I suspect of involvement in the malfunction are Line 5 and Line 8. Line 5 is the probe line, and Line 8 is the sense line. They cross at two places, at bridges I call Arnhem and Nijmegen, shown on the picture below.

    Cutting the track east of Arnhem bridge made the malfunction disappear, but also disabled the same number of buttons as did cutting the track west of Arnhem bridge.

    This showed that the current leakage did not take place between Line 5 and Line 8 at Arnhem bridge. 

    Next I cut Nijmegen bridge itself, just before it crossed the first track. This removed the malfunction, and it only disabled Button H. To confirm that it was the crossing that cause the problem and not another leak somewhere else, I repaired the cut and made a new cut in Nijmegen bridge east of the tracks. This did not make the malfunction go away. 

    I made a few other cut/repair combinations, and I am now certain that the source of the leak is the crossing of Line 5 and Line 8 at Nijmegen bridge. 

    I can now start considering my options for repair. It might be as simple as nail varnish and silver conductive paint. 

    There is a danger that I might not complete this project, because the microwave oven is now usable, except for power level control. It reduces the urgency to complete the job. 

  • A source

    Niel Malan12/03/2018 at 15:09 0 comments

    I hope that it would be possible to somehow repair the printed touchpad of the microwave oven, but there's a possibility that it might prove unfeasible. 

    In that case, I might replace the membrane keypad with pushbuttons. Here I have a source of them: the front panel printed circuit of a fax machine that's on its way to e-heaven. 

    I only need 15, and here's 35, so ample margin for a learning curve and any accidents.

  • I've made a cut

    Niel Malan11/29/2018 at 19:59 0 comments

    From a set of resistance measurements it seemed that there was a short circuit at button D (Time Cook). This is quite possible because there are two bridges where Conductor 5 (energizing, or incoming) crosses over Conductor 8 (detecting, or outgoing). I'll call these places 'bridges'. If this was true, then cutting the energizing conductor would de-energize the outgoing trace that is wrongly giving the signal. 

    I therefore cut the trace at the spot marked by the blue circle. The problem disappeared.

    Unfortunately it all the buttons on Conductor 5 is now disabled. This includes the original ghosting button "Time Cook", and "Power Level" and "Stop Reset". This basically means that the microwave oven cannot be used for programmed functions any longer. But the "Min +" button still works fine, so I can still heat things at full power for a minute or longer. I can stop the cooking by opening the door, and I can restart it by pressing the "Start" button, so there is a measure of control. 

    But this is a hack, and I'm still aiming for a repair. 

  • Worse than I feared.

    Niel Malan11/29/2018 at 08:07 2 comments

    I though that the problem might be simply that some deposition had formed between an incoming and outgoing conductor, causing a short circuit or a leak. In that case all that might be needed would be a simple cleaning. 

    I don't think it's going to be that simple. As far as I can tell all the traces are printed on, which means, I think, that it is the interleaved layer of insulation that is failing. 

View all 4 project logs

Enjoy this project?



K.C. Lee wrote 11/29/2018 at 12:14 point

The worse case scenario is to replace the whole keypad.  Something like this:

Traces on the PCB could be cut etc to match the key matrix if needed.  There are other membranes ones, but they can't be modified as easily.

I used a 4x4 (from digkey) with snap on key caps (you can make printed labels) a long time ago and it was ~$15 US which is a large percentage of a new microwave hese days. 

A custom PCB for dome switch that matches the overlay is possible, but can get expensive due to size.

  Are you sure? yes | no

Niel Malan wrote 11/29/2018 at 14:31 point

Yep, that can be done. Here's one such hack:

I have a bunch of pushbutton switches I harvested from dead devices which I have in mind, should the day come. Perhaps with a 3D printed mount? We'll see. 

  Are you sure? yes | no

Mike Szczys wrote 11/28/2018 at 17:44 point

I'm interested in where this one goes. I installed a venting microwave over the stove about 8 years ago. It still works great but you can see the face of the keypad is showing wear and I fear a failure on the horizon. I'd rather not replace the microwave if I can hack together a fix!

  Are you sure? yes | no

K.C. Lee wrote 11/29/2018 at 12:27 point

My 20+ years old microwave back when they made solid appliances is still working.  A few years ago, the (shipping) protective film starts to winkle up.  Fingers crossed as I use it a lot.

  Are you sure? yes | no

Mike Szczys wrote 11/29/2018 at 15:19 point

Yeah, I have to think the microwave itself will be just fine. I suppose there could be materials in there that would degrade over time (do microwaves have belts?) but all in all it's used for like 4 minutes a day so runtime is likely about 140 hours at this point. My washer and dryer are likely over 700 hours at this point so hopefully have I another 30 years before the microwave wear even catches up to the clothes washer wear.

  Are you sure? yes | no

Niel Malan wrote 11/30/2018 at 07:24 point

My parents have a National Microwave that's been in the house since at least 1981. That's what, 37 years old? Not less than 35, for certain. It's had two magnetrons replaced. I think the last repair I had to do was to rework a rivet that had worked loose in a relay in a hold circuit. (For a while we operated it by jamming the start button with a toothpick.) 

I think that was in 2005.

It is now starting to show an age problem. If it runs for long and gets hot, it sometimes trips the earth leakage protection. I suspect that this might be caused by deteriorating insulation. 

  Are you sure? yes | no

Clara Hobbs wrote 11/28/2018 at 13:18 point

Wow, a microwave oven that features only the time entry buttons we need, rather than a whole 10-digit keypad for no real reason.  Good for you, keepin' the thing workin'!

  Are you sure? yes | no

Similar Projects

Does this project spark your interest?

Become a member to follow this project and never miss any updates