Recently someone asked how I get the old used multimeters featured in these blog posts to look shiny and clean. For the most part, the answer is surprisingly simple: disassemble, scrub parts with soap and water, dry, and re-assemble.
But of course there are many details. Results are rarely perfect with used equipment, but lots of improvement can often be achieved. The normal published photo size on this blog is 1024 x 768, which hides some imperfections. Here’s a couple of before and after photos with a little higher resolution. Neither of these are perfect, but the improvement is worth the effort.
This operation is key to the process. You need to be able to fully immerse the parts in water, so all case parts should be separated from circuit boards and sensitive electronic bits. I strive to disassemble the device as completely as possible without destroying it. Pulling a device apart like this requires a bit of confidence in your abilities to re-assemble the parts later. If you lack that confidence, perhaps a wipe-down with a damp rag is as far as you should go.
A digital camera is practically a necessity. Take many pictures from different angles as you disassemble the device. This creates a record that can help when you forget which way round this little clip goes, or where on the circuit board the blue and orange wires were soldered.
Keep track of the little bits. I use soap dishes, old plastic dinner plates, and little baggies (of the sort you get from Digi-Key) to hold screws, clips and small parts to keep them from disappearing. Some parts like knobs, buttons, keypads and so forth will need to be scrubbed along with the bigger parts, so keep these separate, and take measures not to let the really small bits go down any lavatory drains. For me this usually means placing a coarse rag over the drain when rinsing cleaned parts.
Cleaners and Solvents
There are no magic products that will work for every type of grime and stain. But I find that cleaning products you commonly find around the house will be sufficient in all but the most extreme cases. Just soap, water, a rag, an old toothbrush and some effort will usually produce dramatic results. Here is a table of some products that I have found useful, along with example brands available in my location. Exercise caution for products in the yellow area, the others are fairly safe.
|household cleaner/detergent||Mr. Clean Multi-Surfaces, Dawn||general cleaning of case parts, holsters||rinse and dry thoroughly|
|citrus solvent||Goo Gone||sticker residue||none|
|pumice hand cleaner||Fast Orange, Gojo||removal of grime from Fluke holsters||very slightly abrasive|
|plastic polish||PlastX, many others||light scratches and fogging of clear LCD lens||requires lots of work|
|metal polish||Brasso, Wright’s||cleaning/brightening probe tips||none|
|melamine sponge||Mr. Clean Magic Eraser||tough stains on hard plastic||slightly abrasive, will dull shiny surfaces and haze clear plastic|
|powdered cleanser||Bar Keeper’s Friend||deeper scratches and damage to clear LCD lens||clouds the lens and requires plastic polish to finish|
|denatured or isopropyl alcohol||any brand||nicotine, organic stains||can cloud plastics, smear labels, leave a haze|
|acetone solvent||any brand||tough adhesive residues||attacks some plastics|
|“Remover”||Goof Off||paint, ink||last resort; almost guaranteed to damage any plastic|
It’s Only Water, the Universal Solvent
Dunking plastic parts in soapy water is a fairly benign thing to do. I can only recall two times that soap & water contributed to unintended damage. 1) The markings on a Tektronix oscilloscope vertical attenuator knob came off as if they were decals. 2) The ink came off a bar-code/serial number label on a Fluke DMM. Both situations were unique in that I have immersed other knobs and other labels many times without negative consequences.
Allow time for thorough drying. Shaking/tapping the parts and blotting with paper towels will speed things up considerably. Place the items in front of a fan for at least an hour or so. Forced-air bathroom heaters on low setting work well, just beware of excessive heat that might melt or soften plastic.
Two special items to note with regard to multimeters, test leads and piezo elements. Don’t forget to dunk the test leads into the soap and water when cleaning the rest of the unit. Use a rag to clean the handles, wires, and plugs. Remember the black one is just as dirty as the red one, although you can’t see it. When finished, shake or tap the water out of the shrouded plugs and hang up to dry.
Piezo buzzer elements are often permanently affixed to the back cover of some designs. Brief contact with water does not damage piezo elements at all. Just be careful not to physically damage the element by striking it with something hard like a metal tool, toothbrush handle, etc. Note that during the drying-out process water will usually collect behind the piezo element and may need a shot of compressed air to aid drying.
I Can See Clearly Now
The clear plastic lens cover over LCDs can be quite a challenge. Some multimeter users apparently just throw their unit right into the toolbox with the screwdrivers, wrenches and utility knives. You have to accept that lenses damaged in this way are unlikely to ever look new again. But the appearance and clarity can usually be improved with a few applications of plastic polish. I am not an expert at this, but I get decent results by using products from the auto parts store intended for modern headlight lenses, like PlastX. Just apply the polish and rub vigorously with a soft cloth. It’s a good idea to do this before cleaning the rest of the case, so that the polish residue can be removed with water.
For really tough cases, I have used abrasive powder cleansers like Bar Keeper’s Friend or even very fine grit sandpaper to smooth out bad scratches and solvent damage. (Mix a few drops of water with the cleanser to form a paste.) This will severely haze the lens, so be prepared to spend a lot of time with the plastic polish to restore clarity. This process will probably leave a lot of light scratches in the lens, but in some cases this is much preferable to the original appearance.
Most of the time, polishing by hand is sufficient, albeit tiring. On occasion I have used a rotary tool (Proxxon IB/E) with a polishing wheel, at it’s lowest speed. But you have to be very careful doing this, because using a high speed tool can create enough heat by friction to melt or soften plastic. And if the metal spindle pokes through the polishing wheel, it can make yet more scratches. Yikes.
This gallery illustrates the clean-up of a Fluke 189. The condition wasn’t too bad, but the Fluke signature yellow over-mold was discolored and grimy, and the probes were very dirty.
Dear Readers, Come Clean
If you have favorite products and/or tips for doing this kind of clean-up and restoration, please feel free to share them in the comments below. I’m especially interested in tips to remove markings made with permanent marker (Sharpie), and experiences with different kinds of plastic polishing systems (Novus, etc.)