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I own a more than 20 years old Sharp microwave convection oven model R-9H10. It has a very large cavity and has given good service.
In late September 2011 it finally failed and after doing a bit of checking I concluded that the front panel touch membrane selector buttons/ switches/keypad had failed. Pressing any selection failed to produce an operation.
I called in a Sharp technician and he duly confirmed that the membrane switch pad was gone forever and that spares were no longer available. The technician is very experienced with microwave ovens made by Sharp and he did me a favour by checking over the rest of the machine which was diagnosed to be in good working condition. Finally he helped me dismantle the entire key pad into its various parts and this included separating the membrane keypad from the main plastic piece housing all the various components.
I was determined not to waste money and throw this magnificent machine on the scrap heap and so I have made up a replica key pad to perform the same functions.
This is how I did it.
I occasionally use Express PCB to design PCB’s with top and bottom circuits so I am familiar with the operation of the software.
I measured the length and breadth of the membrane keypad and used those dimensions to create a new board with the software. This is shown as a yellow line – in this case a rectangle. Next I copied exactly all the key configurations shown on the front panel – Compu cook, 140 C, Start and so on.
Eventually I was able to print out an exact copy of the keypad outline and check that I had named everything correctly.
Next was the insertion of the shape representing a micro switch taken from the software library – all 28 of them – and located in the centre of each switch.
At this point I was ready to follow the traces on the upper and lower plastic sheets that are glued together to make the membrane keypad. The circuit can be sketched out with a pencil and paper but I prefer to lay it out directly in the software as I go.
I discovered that 4 tracks ran under and 7 ran on top. There is actually a 12th track that runs around the entire circuit board and does not touch anything. I have no idea what this is for. Going back to the 4 and 7 numbers means that they produce 28 switch positions when multiplied. This matches the same number on the keypad.
I eventually worked out how the alignments were configured and the only difficult parts were lines that occasionally crossed.
One interesting thing about this keypad is that none of the lines on either side cross over each other on the same side. There are no ‘through holes’ or ‘via’s’ because of the nature of the plastic sheet.
So far so good and the next step was to take close up digital photographs of the tracks on both sides of the plastic membranes.
The next step was to separate the two plastic sheets and I found the best thing for this was a hair dryer. I tried acetone and paint thinner but this was not very effective and merely diluted the glue. A good heat build up and slow pressure applied to the separating sheets did the trick eventually but I did wreck the circuit in the middle of the membrane area. There was still some cleaning to be done on the plastic housing and the best item I found for this was methylated spirits, a kitchen paper towel and some
Fortunately all was not lost. The next step was Microsoft Word and a multimeter set on resistance setting and starting at Pin 1 (or whatever you decide to name it) I then followed the tracks I had already made and wrote down the function as I came across them.
Pins 1 to 4 on the underside on the sheet each contain seven function names. Pins 5 to 11 on the topside sheet contain 4 function names. Remember the mathematics:- 4 x 7 = 28.
Pin 1 - Compu cook, 40C, 150C, Med high, 130C, 70C, 160C = 7 items
Pin 2 - Sensor cook, 200C, 220C, 230C, 180C, 250C, Med = 7 items
Pin 3 - One touch reheat, Convec, Grill, Slow cook, Low mix bake, High mix roast, Med low defrost = 7 items
Pin 4 - Easy defrost, High, Stop clear, Instant cook start, Memory, Low, Auto start clock = 7 items
Pin 5 - Compu cook, Sensor cook, One-touch reheat, Memory = 4 items
Pin 6 - Low, Med low defrost, Med, Med high = 4 items
Pin 7 - Convec, 40C, High, 180C, = 4 items
Pin 8 - Low mix bake, 70C, 200C, Auto start clock, = 4 items
Pin 9 - Easy defrost, Grill, 130C, 220C = 4 items
Pin 10 - Slow cook, 230C, Stop clear, 150C = 4 items
Pin 11 - High mix roast, 250C, 160C, Instant cook start = 4 items
The next step was to convert the above details into a matrix which produced this:-
Pin 1 Pin 2 Pin 3 Pin 4
Pin 5 Compu cook Sensor cook One touch reheat Memory
Pin 6 Medium high Med Med low defrost Low
Pin 7 40C 180C Convec High
Pin 8 70C 200C Low mix bake Auto start clock
Pin 9 130C 220C Grill Easy defrost
Pin 10 150C 230C Slow cook Stop clear
Pin 11 160C 250C High mix roast Instant cook start
From the matrix it is easy to use a multi meter to select two pins and observe the needle move so that the function of that switch can be checked on the matrix table.
By now I had the circuit set out with nice thick lines, the silk screen showed an exact replica of the key functions so I wanted a do another cross check. This was done by using an AutoCAD programme and inserting a BMP photograph of the key pad. The lines were traced out again – green for bottom and red for top - and after tidying up this was printed for a like for like comparison with the Express PCB printouts. They matched.
I subsequently placed an order for two key pads (minimum order) and should be able to solder in the 28 micro switches on Saturday 29th October 2011. I already have the new connector wires and I have also made a plastic shape to fit over all the micro switches to absorb their thickness – around 3mm not including the push button itself.
Everything has been meticulously cleaned including the machine itself.