Wohlenberg Keypad Display Repair

Over the last week we have been working on a Wohlenberg 115 Guillotine (built in 1999). A large number of the keys were no longer registering key presses which made the whole machine difficult to operate. These large membrane keypads often end up being an issue as the operators need to adjust settings using them all day long and the keys themselves only have a limited lifespan.

The diagram below shows the typical construction of a membrane keypad. There is an outer and an under layer of conductive plastic which can be pressed together to make a connection. They are a very cost effective and flexible design however the springiness of the plastic and the conduciveness of the contacts both wear out over time. For this repair we have milled out the bottom contact material and fitted a proper mechanical button which should be more robust.

Typical construction of a membrane keypad

“Membrane keyboard diagram FULL SCALE” by Fourohfour – Originally uploaded to English Wikipedia by Fourohfour. Licensed under CC BY-SA 2.5 via Wikimedia Commons

Previously we have repaired this very display by fitting 3 new buttons through the back of the membrane keypad. This time though a lot of the keys were not working so we decided to replace all of the keys on the right hand section of the keypad. This involved removing and entire matrix section.

The first step was to produce electrical and mechanical drawings of the keypad. The mechanical drawings allowed for the physical positions of the keys to be mapped out whilst the electrical drawings made sure that our new keypad connected up to the computer system correctly.

Wohlenberg Keypad/Display panel with dimension overlay

Wohlenberg Keypad/Display panel with dimension overlay

Electrical schematic for Wohlenberg Keypad/Display

Part of the schematic for a Wohlenberg Keypad/Display module

Then the LCD display, backlight drive and interface PCBs were removed so that we could have full access to the aluminium panel that was the backing for the membrane keypad. A wooden mount was made so that the aluminum front panel could be mounted into the milling machine and the existing membrane switches were milled out through the back of the aluminium plate. The tactile domes were removed. M3 mounting holes were drilled and taped.

Milled out aluminium sheet

With all the cards removed we were able to mill through the aluminium backing to remove the faulty membrane keys

Now that the old membrane keys were removed and there were holes through the backing panel we could make up a new keypad. We used a set of PCB switches mounted to a hand milled PCB.

Custom made PCB Keypad

The new keypad.

Manually milled keypad PCB

Manually milled keypad PCB

The PCB was made in our milling machine using a pointed milling cutter. Being able to make simple circuit boards in the office greatly reduces the repair time for jobs like this. The push buttons were then mounted to the PCB, the PCB was mounted to the aluminium panel and connections were made from the buttons to the edge connector of the display card. The completed assembly is shown in the photo below. By fixing the failed keys we were able to get our customers guillotine back into production quickly without the substantial cost of purchasing an entire new display.

Repaired internals of a keypad display module

The finished product. A repaired keypad display ready to go back onto the guillotine