The basic Beckman multimeter design that seems to apply to many of their 3½ digit models from the late 1970s onward include an LCD/processor chip assembly held together by various plastic parts that simply clip together. The later Wavetek-branded models in the 1990s changed the design slightly to use self-tapping screws to hold the stack together.
The main processor chip (part #270-100) and the LCD are both connected to the main circuit board via elastomeric connectors (aka “zebra strips”). These parts make up a stack that must be held together very tightly in order for the connectors to function correctly. When it fails, the meter may having missing or faded LCD segments, it may display nonsense or bad readings, or simply fail to power up at all.
Over time, these plastic bits become brittle and develop cracks. Attempting to disassemble the stack for cleaning of the contact surfaces can easily end up causing more damage. Repair parts are not easily available. But if you’re reasonably handy with tin snips and rotary tools, your Beckman DMM can be salvaged with 2 #8 screws and some scrap aluminum sheet.
This unit, an HD110, has both a broken pressure clip and broken posts on the LCD bracket. To start the repair, the old plastic posts extending from the LCD bracket were removed, to be replaced with two screws. I used a pair of #8 pan head screws, with the heads modified as shown in the photos. This size fits the holes in the PCB quite well and will help keep the arrangement aligned, which is essential.
It is also important that the heads of the screws are thin enough not to contact the LCD glass. This might break the LCD when pressure is applied. I modified the heads of these screws with a rotary tool to fit down into the slots of the plastic bracket. This has the added benefit of holding the screws from turning when the nuts are tightened.
You Say al-yoo-MIN-ee-um, I Say ah-LOO-min-um
The new pressure clip was fashioned from a scrap of 20 gauge (0.81mm) aluminum sheet. See this diagram for dimensions. The aluminum is fairly easy to bend by hand when fastened in a small vise. Tapping a hammer along the crease results in a good sharp bend. The shape I decided on requires that the bends be a little more acute than 90 degrees, so a pair of pliers helped to finish the job. I used that shape to lessen the possibility that the clip might spread outwards when tightened.
[In the comments below: Harvey suggests using a half-section of plastic pipe for the clip. An excellent alternative to this metal one I fashioned.]
Small strips of self-adhesive foam cushioning material finish out the new clip.
Re-assembly begins after two holes for the screw heads are made in the rubber spacer that goes behind the LCD. To avoid breakage, I did not actually install the LCD and snap the bezel on until ready to test.
Using screws instead of the old perfectly-sized plastic posts allows the arrangement to wobble around more than before. Care has to be taken to get it positioned so that the LCD and the processor chip will be directly aligned over their respective PCB contact pads.
Eep, Opp, Ork
The first power-on test resulted in meaningless garbage on the display, but I noted that the LCD bracket had moved off-center a bit, so after loosening the nuts and straightening the bracket, the meter started to show a normal display. The nuts were then given a final tightening. Note that the metal clip and screws can provide more pressure than with the original parts, so over-tightening should be avoided.
Another old Beckman saved from the scrap heap!
Update Oct-2016 Reader Wayne C. writes in to describe an alternative procedure using the original pressure clip, which in his case was not broken:
Instead of carving out the claw-posts as you did, I left them intact and drilled through their centers with a hole big enough to pass a #2-56 screw (on the drill press this can be done quite precisely). I was able to use the original “W” shaped pressure clamp which isn’t cracked or broken. Two small washers and hex nuts holding it all together completed the repair. I didn’t have to do the machining work you did on the screws nor did I have to worry about centering them since I drilled through the centre of the existing alignment posts. Also, you can put a lot of torque on the #8-32 screws you used with catastrophic results, my #2-56 screws are a lot safer because you really can’t tighten them that much.