Tag: repair tip

#0023: Repairing short circuit damage within a ribbon cable

Otto

#0023: Repairing short circuit damage within a ribbon cable

Preamble

Sometimes you might come across damage within a ribbon cable similar to the example. The minor burn damage on the example featured was done by a liquid causing a short circuit between two exposed copper pads. As it burned, it created a break between an exposed pad, and it’s respective trace. Cutting the circuit in the process. The short also caused some of the other pads to oxidise, and some minor burning of the ribbon cable’s plastics. This will be a quick tutorial on repairing such a fault.

Please note: I have no images before the initial cleaning and prepping stage. This is because I was halfway through this repair, when I decided that it may be good to document it.

Tools and materials:

  • scalpel
  • tweezers
  • isopropyl alcohol
  • cotton earbuds
  • soldering iron
  • lead solder
  • copper strand from a wire
  • side cutters
  • multimeter

Step 1: Identifying and logging the damages

This should always be a first step before attempting a repair. The reason for this is that the initial pre-work cleaning, is likely to clean away a lot of contextual clues about the location and severity of all the damages. A good visual inspection and initial assessment can save time later on, due to not having to track down any circuit damage that got masked or hidden by cleaning.

Step 2: Cleaning the local area

First thing I did after identifying the location of any potential damage I wanted to repair, was to clean it. In this case I needed to first scrape off the more obvious patches of oxidation and burnt/melted materials using a scalpel. Then thoroughly clean off the pads of the ribbon cable using isopropyl alcohol and a cotton earbud. I paid special attention to the tiny burn hole next to the most damaged pad, making sure to remove any conductive materials from it by scraping it out thoroughly.

Step 3: Test to confirm faults

Using a multimeter in continuity mode, I tested for continuity between the pads surrounding the burnt spot. Perhaps it still contained conductors (such as pieces of the broken pad) that may cause a future short. After being satisfied that it did not; I focused on the particular pad that sustained the most damage, and tested for continuity across this pad and it’s respective trace to see if it was still connected. It was not. All other pads had continuity with their respective traces.

Step 4: Fixing confirmed faults

After identifying only a single fault; this being a break between a pad and it’s trace. I moved to repair it. Firstly I endeavoured to bridge the gap by just tinning across it from the pad to it’s trace. The thought process was that that the mere mass of the solder itself would be enough to bridge the tiny gap. It did not. After that initial failure, I decided that I required a bridging medium; something for the solder to adhere to. In this case I decided to use a single copper strand from a wire.

Using a scalpel, I removed some insulation from the ribbon cable trace above the broken pad. This was in order to have something on that side to comfortably solder the copper strand to. After which, I soldered the wire whilst using a pair of tweezers to hold the tiny copper strand down in place.

Step 5: Cleaning up after a repair

The next step involves cleaning up any messes I might’ve caused during the repair. In this case, whilst trying to initially tin the broken pad, I also tinned the neighbouring pads accidentally. In trying to remove the bulk of the solder, I caused further damage by starting to burn the plastic that the pads are set into. In the end I decided to just leave it be. The repair needed to be functional, not aesthetically pleasing. I did consider using a desoldering braid to remove all the solder, however I was very likely to cause more damage trying, so I opted to just leave it be.

In the end the after-fix clean consisted of just clipping off the excess wire with a side cutter and cleaning off any flux residue with an isopropyl dipped cotton earbud.

Please note: There is no electrical connection between adjacent pads. The burnt plastic between the pads just looks like solder. I told ya it was ugly.

Step 6: Testing the repair

Next. I performed a quick continuity test on the repair with a multimeter; both testing for continuity across the pad to it’s trace; and testing for a lack of continuity across neighbouring pads. These tests take basically no time to do and can set my mind at ease. I do these types of tests even when a visual inspection indicates that it’s not necessary.

The real test is putting the device back into service and seeing if it functions as expected. In this case the ribbon cable that I repaired was from a laptop’s integrated keyboard. Although the ribbon cable fitted more snugly into it’s receptacle or socket than I wanted. It still fitted and functioned properly. The keyboard was fully functional after this repair. Huzzah!

Closing thoughts

I apologise if this article came off as a little patronising; especially given the quality of the example repair, the missing “before” picture, and the fact that it contains mistakes front and centre. Generally, I find step-by-step guides like this one difficult to write, without sounding either needlessly pedantic or excessively didactic. Anyway, even if the exact specifics are not useful to you, I hope the general steps would be. Identify. Clean. Test. Fix. Clean. Test. Then Profit. That is if it works, if not: then go back to testing for faults.

Thank you for reading.

#0021: Repairing an LCD with missing segments

Otto

#0021: Repairing an LCD with missing segments

Preamble

This is a quick guide to repairing a specific fault found on undamaged low information monochrome numerical LCDs. Such the ones present within calculators. As such it will not go into detail about the functioning of LCDs in general, types of LCDs available, or any other information outside of the scope of simply repairing the missing displayed segments fault.

What is an LCD?

An LCD or Liquid Crystal Display, is a type of flat panel display. At its most basic an LCD operates by using the properties of liquid crystals coupled with polarisers. Polarisers are a type of optical filter that only allow light waves to funnel through them in a particular orientation. In other words, they remove light scatter; only allowing it through in a uniform manner. This, coupled with the liquid crystals’ property of altering their physical orientation when in the presence of an electric current; means that the narrow beams of light that make it into the crystal solution can either be allowed to pass through, or blocked, depending on the orientation of the crystals within the solution.

The specific type of LCDs we are dealing with are low information monochrome single line seven-segment displays. These types of simple LCDs are typically used in devices that predominantly output numbers. But may also display static symbols, such as the “E” in calculators for numbers that are too large to display without index notations. These types of LCDs are most commonly associated with pocket calculators. However they have been used with such devices as: alarm clocks, multimeters, solar charge controllers, battery monitors, household mains electricity meters; and I have even seen them used as a display on an electronic keypad lock.

I think this type of LCDs popularity is mostly due to it’s relative simplicity and low operating costs. It follows the KISS design philosophy. Keep It Simple Stupid. If a device would not notably benefit from a more complicated display, if all that it displays are simple numbers and basic symbols; then there is little reason to incur the (production and operating) costs of increasing design sophistication beyond this type of display.

As for the mechanics of how liquid crystals work, I like to (keyword) imagine a matrix of magnetic rods. At rest, the viewer can only see them from the top, and considering their microscopic size, this renders them essentially invisible. Whereas when a current is passed through them, the entire matrix of rod shaped crystals reorientate themselves to reveal the entire length of each of the rods. This greater surface area against the polarised light from the viewing angle, makes them appear opaque. There’s more to it than that, but that’s the general mental model I use to conceptualise the process. Although strictly speaking it isn’t accurate.

So how does a basic monochrome seven-segment LCD actually display information?

An LCD of this type is mapped with discrete segments. These primarily consist of seven dashes arranged in a number ‘8’ pattern. These are the core seven segments that are used to display numbers. Additionally LCDs have segments in the shape of static symbols; such as a period on calculator, or a colon on an digital clock.

Every discrete segment is given a set of electric probes. These probes are designed to allow a current to pass across the segment’s liquid crystals. This is how the liquid crystals within each individual segments are switched on and off. It operates in an analogous way to seven-segment LED displays. I.e. they both require an a electric current to be passed across each individual segment in order to activate it. Additionally, this electric current is controlled by a display controller IC (Integrated Circuit). Which translates any numerical values into display data (active segment and inactive segment map) that it uses to power it’s display accordingly.

example of an LED display
LCD segment map
segment circuit animation

Missing segments fault (on undamaged LCDs)

First of all, I specify that the LCD is undamaged because if the LCD you are attempting to repair is damaged, (e.g. has a crack across it); then chances are this fault is not the major contributor to your LCD’s malfunctions. That being said, missing segments on undamaged LCDs are likely caused by a break in the particular missing segment’s individual electric circuit supplying it.

Earlier I likened LCD seven-segment displays to their LED counterparts. This is because both require an individual IC controlled circuit that connects with each discrete display segment. Its just that with LEDs, its a lot easier for people to understand what is happening, when some of a displayed number’s LED segments fail to turn on. The failure to activate can be intuited due to a break in it’s branch of the circuit.

In this case the break in the segment’s circuit is usually caused between the LCD module and the underlying PCB; which hosts all device circuitry, including the display controller. This break usually occurs within or around the bridging material between the LCD and PCB. Namely the elastomeric connector (trade name: “Zebra strip”). This black and pink rubber like material is a soft electric conductive material that conducts electric signals across the naked pads of the PCB to the LCD and vice versa. It does this by having many tiny channels (or layers) of conductors and insulators, that alternate across it’s black strip. This black strip is then sandwiched with a pink insulator that runs the length of the outer sides of the elastomeric strip. This configuration allows the elastomeric strip to act like a large assortment of miniscule wires that electrically connect together whatever pads or traces that they touch at either of their ends.

The circuit break in this missing segments fault could be caused by two main things. Firstly, it could be due to an electric insulator getting in between the elastomeric strip and the exposed pads of the LCD and PCB. This could come in the form of a build up dust or grim, or even oxidation of the exposed PCB pads. To repair this, just clean all the pads and the elastomeric strip itself. I recommend using isopropyl alcohol and a cotton ear bud or cue tip. Just saturate the bud with the alcohol and scrub until it’s clean. Then reassemble the device and test.

Alternatively, this fault could also be caused by a separation between the elastomeric connector and it’s adjoining contacts. I.e. it has lifted off or away from the pads that is it supposed to be pressed against. This is usually caused by vibration. Typically, there will be a method of mechanically tightening or pressing the elastomeric connector against it’s pads. Such as a screw or adhesive, which with time and vibration (and maybe a little heat) can become undone enough that it allows the connector enough space to move away from it’s pads. To repair this, just re-tighten the screw or re-secure the elastomeric connector with tape if needs be.

The fault in the case with the example unit; I believe was caused by both a build up of dust between the contacts and the elastomeric connector, and also the connector physically separating from the LCD. The separation was caused by a loosening of the self tapping screws which held in place a bracing bar. This bracing bar applied pressure to the assembly consisting of the PCB, strip, and LCD, which sandwiched them together and kept them in place. That got loose, the strip moved slightly, and then dust got into the gaps that it created. After a good cleaning and tightening, it now works flawlessly.

Closing thoughts

When it comes right down to it, this is as simple as a repair really gets. No replacement of parts; just a basic disassembly and cleaning. It is essentially maintenance.

In my opinion, this type of repair is especially good for an aspiring repair tech with no confidence. The tools needed are basic, there are no additional parts (i.e. expenses) required, and the device being worked on it likely inexpensive; so there should be little in the way of consequences of failure. Such as fear of damaging a device, which may put people off from just ‘aving a go. Essentially the repair has little in the way of friction that may prevent a person from trying. It a good confidence builder.

Lastly, I just wanted to raise awareness encase you ever come across this type of fault in the future. It is easily repairable; and hopefully you’d be more inclined to at least give it a go, rather than discard the unit and purchase a new one as is usually the case for these types of cheaper mass market products.

Th-th-th-th-that’s all folks!
Thanks for reading.

#0014: Tip for reinserting self-tapping screws into plastic housings

Otto

#0014: Tip for reinserting self-tapping screws into plastic housings

What is a “self-tapping” screw?

Whenever one removes screws from a device – especially a mass produced consumer grade device with a plastic housing; then chances are that the screws that were removed were “self-tapping” screws. As the name suggests, these are screws that are designed to cut their thread into the inner diameter of the plastic hole of the device housing that they are inserted into at the factory.

All self tapping screws have a few distinct qualities in common. These include: a relatively deep thread in order to cut into and grip the plastic, and a wide pitch between these threads in order to more effectively slide into the material. Additionally most self tapping screws tend to come to a sharpened point in order to to aid in aligning the screw as it is inserted into the hole.

So what’s the actual tip?

Well, when reinserting a self-tapping screw into it’s plastic receptacle. It is good practice to initially rotate the screw counter-clockwise (loosen) until you feel it pop or click into place. This process aligns the screw with the pre-existing thread, and in doing so means that when the screw is screwed in, it will slide into the already established thread and not cut a new thread over or across the old one. When aligned correctly, the screw should screw into the hole with very little resistance.

Why use this method?

The reason why this is important, is because repeated re-boring of the plastic hole will eventually strip it out of any threading. This is because each time a new thread is cut into the plastics it removes material; which in turn incrementally widens the diameter of the hole in the process. This can continue to the point that it can no longer effectively hold the original screw within it. Avoiding re-tapping the thread every time you reinsert the screw will minimise your damage footprint when repairing/working on the same devices repeatedly. Which is always good.

Video demonstration

Please note: although it may look like I am using force to reinsert the screw, I am not. It’s just the awkward angle and set up that may make it appear that way.

What to do with an already stripped or overlarge hole?

There are a few remedies to take if this advice is coming to late. The quick and dirty solution is to use a larger size self tapping screw that then can cut a new thread into the widened hole. After which one has to then absolutely makes sure to always realign the new larger screw with it’s thread before reinserting it. This is because any further damage to the plastic hole may make it entirely unusable. E.g. a plastic stand-off splitting entirely due to it’s enlarged hole.

The more intensive repair requires filling the hole with something that the original sized screw can then bite into. The most likely materials used here are either hot glue for a “temporary” fix; or melted plastic of the same type as used by the device for a more permanent solution. If you use the melted plastic route: I recommend filling the hole entirely, then drilling a new hole of the correct diameter. Followed by finally using the original self tapping screw to cut a new thread into this virgin material.