LCD backlight mod on cheapie Mastech MY64 DMM - Page 1
Forum section: electronics / testgear

author: tooki
author_rank: Super Contributor
timestamp: September 08, 2015, 01:23:26 pm
content: So as some of you may have seen inanother thread of mine, I picked up a Keithley 197 bench DMM that I'd like to outfit with a backlight. My first crack at the inside showed that this would be a bit tougher than I hoped, due to tight clearances in the display module. So I decided to first practice on the cheapie rebadged Mastech DMM somebody gave me. And I'm very glad I did, as some parts were trickier than I expected, even with the Mastech's ample interior space.I didn't document this as I went along, unfortunately, but I'll disassemble it here and show you what I did.Since the Keithley is a bench meter, and I don't use the Mastech much, I didn't make any attempt to have separate backlight control, so it's simply wired to the "on" side of the power switch. I can always mod the mod in the future...So without further ado, here's the result:And here's the same thing with the aftermath of the workbench visible.Closeup of the display. Note that while the illumination is verycleanand usable, the uniformity sucks. More on this later.For comparison, here's the finished mod alongside a 7 month old Fluke 87V with its backlight on high:And for reference, what it looked like before the mod:Now, to the teardown. Here's the naked meter. Note that the display doesn't look very different turned off. Indeed, if Ihadgiven the backlight independent on-off, it would still be eminently usable in daylight without the backlight.Same, with backlight on:Some closeups of the displayNow let's peel the display apart layer by layer.The removed bezel, with the matte frame laying inside. (Note the broken-off tabs that hold it all together. They broke after being opened and closed many, many times while experimenting. Luckily enough tabs remain to hold it together just fine.)After assembly, it turned out you could actually see the LEDs leaking light from outside the matte frame, so I added some strips of black paper to mask those off. If I had black tape, I would probably use that instead and apply it not to the matte frame, but to the inside edge of the bezel itself.Bezel removed, turned on (display is blank because there's no pressure against the zebra strip):Here the LCD panel itself has been removed, showing the diffuser film I used. This layer lays in direct contact with the rear polarizer of the LCD panel.This film is not a proper LCD optical diffuser film. If I get my hands on a dead TFT that I can harvest proper optical diffuser film from, I'll swap it out and see how it works. The material I used is actuallyinkjet backlight film, sold for signage applications. I have a few sheets laying around that I got at a trade show nearly 20 years ago!! It's got a glossy side and a matte (printable) side. Note that this film is extremely delicate, and though it can be handled just fine with clean, dry, oil-free hands, liquid spatter (even just microdroplets of spit from blowing dust off) will leave permanent marks, and the matte surface is easily damaged by scratching and scuffing. The matte side normally goes down (toward the backlight), but with edge light at close quarters like this, the matte surface actually cast shadows, so I inserted it matte side up, which actually ends up being a great transflective surface should I ever give the backlight independent control.The meter's LCD panel of course had to have its reflective film removed. Annoyingly, the contact adhesive remained stuck to the panel, not the film, so I used Goo Gone to remove the adhesive, followed by an initial wash with dish liquid and water to remove the Goo Gone. The adhesive itself, interestingly enough, was actually pale purple colored. I guess that's to neutralize the natural greenish tinge of the polarizers used.Here's the next layer: the paper background diffuser, which lays in the floor of the frame, about 2.5mm below the LCD panel (and the diffuser film). This is nothing but ordinary copy paper trimmed to size.Finally, the bottom of the frame, where most of the work went into. I haphazardly lined the back with foil (actually it's alcohol wipe packages, with thin double-stick tape attacked to the back to make it sticky). If this had been a very clean application without seams and overlaps, the paper diffuser above might not have been necessary.I very carefully lined the edges with foil. It turned out that the edges are very sensitive to clean application. Same alcohol pad wrapper, sliced to 2mm wide strips. And of course, what you also see here are the LEDs themselves. TheThey are 3528 SMD white LEDs, to which I soldered 28ga stranded wire harvested from a damaged HDMI cable, and then the soldered leads bent to a 90 degree angle so the LEDs would lay flat on their edge. (Yes, I understand the irony of attaching leads to SMD LEDs...) The cutouts for the LEDs were made with an Xacto knife, as the plastic is rather soft. Before the entire foil treatment, I put strong sticky tape along the inside of the LED holes so I could insert the LEDs and have them held flush with the inside surface, then I hot glued the LEDs in place. This tape was then removed and the reflective foil attached.Here's what the LEDs look like from the outside:Next, the small PCB I used to wire it all together. It takes 9V in and runs three parallel sets, each of two LEDs and one 180R resistor in series. (Total of about 43mA/0.4W power draw. Oh well. IKEA has cheap batteries!)And here's how it's wired to the board. The negative (purple here) is wired directly to the ground/-battery, the positive (yellow) is wired to the hot side of the power switch.As I mentioned above, the uniformity isn't great, it's very, very obvious that it's edge-lit. I am very curious as to whether a proper LCD optical diffusion film would work better. An interesting thing I noted about these LEDs: they must not have a diffused surface too close to them, or the color shifts strongly toward yellow. I assume what's happening is that blue light that should be escaping is instead bouncing back to the yellow scintillator, causing it to glow more than it should. This effect does not happen if held to a transparent light pipe, so conceivably one could optically bond these to the edge of a piece of acrylic to edge-light it. You could then avail yourself of frustrated internal reflection and laser etch a pattern of dots to release light where it's needed. (No, this isn't speculation, it's exactly what many edge-lit LCDs use, and it's what Apple uses to backlight its keyboards. A plastic edge-lit film has few "escape dots" at the edge near the LEDs, and lots of them in the middle where the LEDs are far away -- and the dots are only below the keys, so light isn't wasted lighting opaque keyboard frame!)But I think that might be overkill for a $30 meter.Some other lessons learned:Clean frequently when doing this stuff. It's amazingly easy to get dust in somewhere, hence having to open the damned thing over and over.Be careful with the bare LCD. To my surprise, itsfrontsurface was surprisingly delicate, and I ended up micro-scratching it with tissue when cleaning it for final assembly. Luckily this really isn't noticeable.Use alcohol copiously to degrease everything before attempting to glue it, be it sticky tape or hot melt glue (why doesn't anyone teach us this in grade school?!?).

author: TorqueRanger
author_rank: Frequent Contributor
timestamp: September 11, 2015, 02:03:02 am
content: Awesome job and I hope it works out for you ... I was thinking to do the same mod to my fluke 87 II or just buy the upgrade ??

author: retiredcaps
author_rank: Super Contributor
timestamp: September 11, 2015, 05:59:18 am
content: Quote from: TorqueRanger on September 11, 2015, 02:03:02 am+1. Good jobQuoteWhat do you mean doing the same mod? All the 87s have backlight. Or I am missing something?

author: TorqueRanger
author_rank: Frequent Contributor
timestamp: September 11, 2015, 06:41:49 pm
content: Quote from: retiredcaps on September 11, 2015, 05:59:18 amThey do but I wanted to upgrade to the led version instead..

author: tooki
author_rank: Super Contributor
timestamp: September 13, 2015, 11:31:39 am
content: Thanks for the kind words, guys!But yeah, TorqueRanger, I'm confused -- all Fluke 87 series have LED backlights. In early ones it's green, later it's white, but that's it. (There are differences in the LCD panels themselves, e.g. digit size, but that's neither here nor there.)

author: tooki
author_rank: Super Contributor
timestamp: September 13, 2015, 11:45:34 am
content: As an unrelated aside, I thought you guys might enjoy this picture I came across on the Swiss local equivalent of eBay. Something tells me the seller is not a test equipment specialist...