I have had an RH Designs ZoneMaster II enlarging meter for years that I use regularly. A few weeks ago I noticed that one of the buttons had become unreliable. I had to press it hard several times to get it to work. Since I am in the US and RH Designs is in the UK, I decided to try and fix it myself. As it turned out, that was not terribly difficult.
I should note that electronics has been both a hobby and career for me, so I am used to working on electronics circuits. If you are not comfortable doing simple electronics repair work, you might not want to attempt this. If you decide to go for it, you can reduce the risk of damage by using an anti-static wrist strap.
I can’t say precisely what button was used in the manufacture of the ZoneMaster II, but ALPS makes a close match. The ALPS part number is SKHHBWA010. I ordered from Mouser at $0.15 each plus a small shipping fee. To see an accurate picture of the switch you may have to click on the data sheet rather than relying on the representative illustration shown on the website.
The ZoneMaster can be disassembled by removing the four black Phillips head screws on the bottom and carefully separating the top and bottom of the clam shell box.. I recommend checking to make sure the power switch is turned off and then removing the battery.
The circuit board is held in place by three short Phillips Head screws, one in each corner. Be careful not to accidentally damage the push-button power switch. Jewelers screwdrivers come in handy at this point.
Carefully work the circuit board out of the plastic box and turn it over to see the buttons. To remove the buttons you will need a small soldering iron (25-50W or so) and either a “solder sucker” or some solder wick. Flip the circuit board back over and desolder the faulty button. The molten solder can be sucked off with the solder sucker or soaked up with the solder wick. Being too rough or using too much heat for too long can damage the fragile copper traces on the printed circuit. After removing the solder, use small needle nose pliers or tweezers to work the pins loose so the button can be removed.
The new button is a match for the old faulty one and seems to require very similar operating force. The “click” sound made by the new button is a little different, although certainly not enough to matter to me.
The hole pattern for the switch is rectangular, so it can be installed in two ways. Since the pins are symmetrical, it doesn’t matter which way you install it. Be sure to press it all the way into the holes on the circuit board and solder all four pins.
Reassembly is the reverse of disassembly. Be careful not to strip out the holes in the plastic box when reinstalling the screws and remember to reinstall the battery before buttoning everything up. It might not be a bad time to replace the battery while you have everything apart.
If everything went well, you didn’t ruin your very expensive enlarging meter.