Diagnose / Repair Modern Electronics -- A DIY Guide: Reverse-Order Reassembly

You fixed it. Congratulations! Now it's time to put everything back together. Just screw the boards down, plug in the connectors, close the lid and you're done, right? Well, sometimes, but not performing the reassembly methodically can lead to all kinds of trouble, from failure of your repair to new damage, and even to danger for the product's user. You're sewing up your patient after the operation, and it's important to put in the stitches carefully and avoid leaving a scar.

Common Mistakes

It might seem absurd to think that one could reassemble a machine and have parts left over, but it happens all the time. You snap that final case part into place, breathe a sigh of relief, glance at the back of your workbench, and there it is: some widget you know belongs inside the unit, but you forgot all about it. Hmm, is it really worth all the trouble to backtrack, just for one little, seemingly non-essential item? Sigh….

Time to pull the whole mess apart again. It's very easy to forget to replace a bracket, a washer, a shield, a cover or even a cable. The unit might function without one of those pieces, but it's not going to work completely right.

Putting the wrong screws in the wrong places may have no consequences, but it also could seriously damage the device. If a screw that's too long presses against a circuit board, it might short whatever it touches to the chassis, hence to ground.

You flip on the power and, voilà, you have a new repair job on your hands. See the definition of magic smoke in the Glossary.

Overtightening screws can strip their heads, making it very hard to get them out again. It can also break plastic assemblies and cause cracks in the case.

Undertightening screws may lead to their falling out later, possibly jamming mechanisms or shorting out circuitry if they're internal screws.

Our memories can really fool us sometimes. You're certain that part went over here, but now it doesn't quite fit. So, you press a little harder, because you know that's where it goes. Snap! Oh, right, it went over there, not over here. If something doesn't want to fit together, it probably belongs somewhere else. This kind of error happens more with mechanical parts than with electronic components.

Manufacturers try to make internal connectors different enough from each other that only the correct cables will fit. In products with lots of connectors, though, there may be ambiguity. CRT projection TVs, for instance, have so many connectors that there's just no way to make them all distinct, and sometimes a color code is all you have to guide your reassembly. It's not hard to get one wrong. Guess what happens when you turn the set on.

Even when you put ribbon cables in the right connectors, it's easy to damage them. They're delicate and easily torn or folded hard enough to snap their printed copper conductors. The connectors are fairly breakable too, especially the tiny ones in pocket-sized products. Those plastic sliders that hold the cables in place snap off without much pressure. Worse, the entire connector can break its solder joints and fall right off the board, just from the stress of being pushed on while the cable is inserted and clamped down. I've seen that happen on digital cameras and pocket video gear.

Getting Started

To begin reassembly, reverse the order in which you took the machine apart. When the unit has multiple boards, you'll need to get the inner ones reinstalled first. If a board that'll wind up under another board has connectors, put the cables in before covering up that board.

In older gear, take a look at the ground lands on circuit boards, where the screw or the metal bracket makes contact. They exist to connect those points to ground via the chassis, and a poor connection due to oxidation or corrosion can seriously affect the product's performance. Clean them up with some contact spray or, in extreme cases, fine sandpaper. Make sure the screws are tight, so contact will be reliable, but don't overtighten to the point that you might crack the board. Heating and cooling in larger items, and physical stress in portable devices from being bounced around, can cause cracks later on if the screws are extremely tight.

A little sealant on a screw head is better than pushing the limits of tightness.

Manufacturers and pro shops use a type of paint called glyptal to keep screws from loosening. Swabbed around the edges of the screw head, it’s highly effective. You can use nail polish. Don't glob it on; just a little smear will do fine. Be careful not to cover components or their leads, and let it dry before closing the case, so the outgassing won't remain inside. I use red polish so I can see where I've been, should I open the unit again later on.

Placement of wires and cables is called lead dress, and it can be surprisingly important. When you took the unit apart, you may have noticed that some wires were tacked down with hot-melt glue or silicone sealer. If the manufacturer went to the trouble to do this, there was a reason. Maybe the wire needed to be kept away from a hot heatsink that could melt its insulation. Perhaps a cable carries a weak, delicate signal that would receive interference if it got too close to some other element of the machine. This can be the case with the cables going to video head drums. Or maybe the reverse is true: the cable would cause interference to other sensitive circuits. Wires carrying high voltages, like those used to run projector lamps and LCD backlights, may need to be kept away from all other circuitry to avoid not only interference but the possibility of arcing. The closer a high-voltage wire is to ground, the more those devious electrons want to punch through the insulation and get there. Give 'em time and they just might.

If the manufacturer tacked wiring down, put it back the way you found it. Hot melt glue is somewhat flammable, and it melts with heat, of course, so it isn't used much in larger products. Now and then, you may find it in smaller items that don't carry much voltage or produce significant heat. To tack wires back down into it, you can melt the blob with your plastic-melting iron, avoiding any other wires, or drip a little more glue on top from your glue gun.

More often, you'll find silicone sealer used to secure wires. The type used is called RTV, and it's best to replace it with the same kind, because it offers the correct insulating strength. RTV is available at most hardware stores, and electronics supply houses carry it too.

Now and then, and especially in small-signal RF stages operating at very high frequencies, you'll see blobs of wax covering transformers and capacitors. The wax holds the parts to the board and dampens vibrations that can cause noises in signals or frequency instability in oscillators. In some circuits, the capacitance of the wax may affect circuit operation, so it's best to remelt and reuse the original wax. It's not the same stuff that's in the candles on your dinner table.

Even if the manufacturer took no extra care with wiring, you should pay attention to the issue to achieve maximum product reliability and safety. Could a power supply lead touch a heatsink? Is the cable from a tape head going right by the power supply? And, perhaps most important, is any wire or cable placed such that it'll get crimped by a circuit board or part of the case when reassembly is complete? The sharp ends of a board's component leads can go right through a wire's insulation, with disastrous results. Crimping caused by the case can break the wire or cut through the insulation and short it to the chassis. With a wire carrying unisolated AC power, a crimp could even present a shock hazard. If the case halves don't mate properly, a wire is probably in the way. Don't just squish them together and go on.

These scenarios may sound farfetched, but they're really pretty common. I always make it a point to watch the wiring as I close up the case, imagining where things will be and how they'll press on each other before I actually snap the halves together or tighten the final screws.

Reconnecting Ribbons

Insert ribbon cables into their sockets carefully. It isn't hard to put them in wrong, which can lead to anything from no operation to circuit damage. Most ribbons have bare conductor fingers only on one side. If you get one of those in upside-down, the product won't work, but it's unlikely to cause damage unless the socket has U-shaped contacts that touch both sides of the ribbon. There are some like that.

Double-sided ribbons offer more opportunity for calamity. How do you know when they're in the right way? Many are keyed with a notch at one end so they can't be inserted upside-down. Some are not, though. I sure hope you heeded my advice in Section 9 to mark the darned things! If not, see if the cable has a bend or curvature suggesting its original orientation.

As discussed in Section 9, some ribbon connectors have no latches, and the ribbon just slides in. That type requires some force for proper insertion. With such a connection style, the ribbon cable will have a stiff reinforcement layer at the end.

Even when you find one, take a good look at the connector to be sure it has no latch that slides in or flips up, because some latch-type connectors accept reinforced ribbons too. If you see no latch, grasp the cable's reinforced tab, carefully line up the ribbon with the connector and press it in firmly.

Connectors with latches are easier to manipulate. First, be sure the latch is open.

Slide it out or flip it up. The flip-up kind will stay up while you insert the ribbon, but the slide style has an annoying habit of going partway in before you want it to, blocking full reinsertion. One end may slide in, resulting in a crooked latch. If that happens, pull that end back out, remove the ribbon and try again. You might have to hold the slider's ends with one hand while you insert the ribbon with the other.

Latch-style connectors require almost no insertion force. Gently slide in the ribbon until it stops. Don't press firmly here. Look down at the top of the connector and verify that the ribbon isn't crooked. Then close the latch while holding the ribbon in place with your other hand. That's easy with flip-up latches and a little harder with sliders. Occasionally, I've had to close sliders one end at a time with a thumbnail or a screwdriver while holding the ribbon in my other hand. To avoid a crooked result, it's better to close the ends at the same time, but now and then you gotta do what you gotta do.

When the latch is closed, look again at the exit point and make sure the ribbon is straight. You may see the edges of the bare conductors sticking out. That's fine as long as they're even and you're sure the ribbon is in all the way. Many of them are designed that way.

Special ribbons used for hard drives and other very dense applications can have two sets of fingers, one behind the other. These will always have latch-style connectors. Be absolutely certain the ribbon is fully inserted, so there's no chance the wrong set of fingers could make contact with the mating pins in the connector.

Oops!

If you were unlucky enough to break the latch on a sliding-style connector when you removed the ribbon, don't despair. The object is to get pressure on the conductive fingers so they make good connection with the socket. Find some thin, soft plastic from, say, the bottom of one of those little pudding cups in which you keep screws.

Cut the plastic into a rectangle that just fits into the socket and sticks out a few millimeters. Trim carefully so the edges line up well with the edges of the socket, without a gap. Now put the ribbon in and wedge the plastic piece in to replace the broken latch. Be absolutely sure to insert it on the side of the ribbon that does not have the conductive fingers, or the plastic will block the connection. I got that wrong once and went around in circles for hours trying to figure out why that confounded shortwave radio wouldn't turn on anymore! If you get the thickness right, it'll take a little pressure to slide in the plastic, but not a lot. If it slides in very easily, it may not put enough pressure on the ribbon to make good connections. The few times I've had to do this, I used forceps to push in the plastic piece. Needlenose pliers will work as well.

Flip-up latches are much harder to repair. It might be possible to modify the socket by melting a piece of plastic over it and converting it to a sliding arrangement, and then using the plastic insert approach, but it'd be a difficult modification to pull off, considering the size scale of some of these connectors. Trivial as it may seem, a broken flip-up latch often means the end of the product unless you can scrounge a latch from a parts unit.

Layers and Cups

Here's where those pudding cups come into play. If you used them as I suggested back in Section 9, your innermost layer's screws will be in the cup second to the top, just under the empty protective cup. Put that layer's boards, shields and assemblies into place and fasten them with those screws. If the screws are of different sizes or styles, you should have taken digital photos or made a drawing and placed it in the cup with the screws. Be certain to use all the loose parts in the cup, because you won't be able to reach that layer once you reinstall the layers covering it.

When you're ready for the next layer, pull the empty cup and put it aside, exposing the next set of screws. Continue on with the next layer, and so on. When you're all done, you should have a nice set of empty cups ready for the next project. If you have screws left over in the final cup, remember to check under labels and rubber feet for hidden holes.

It’s a Snap!

Those nasty hidden snaps are much easier to close than they were to open! Line them up carefully and apply pressure until they pop into place. The edges of the case should fit smoothly. If there's a bulge, either the snap isn't all the way in or a wire is caught underneath.

Sometimes a case has to be snapped together at one end before the other, even if it didn't come apart that way. Look at the style of snap, and how it fits together should be apparent. If you get it wrong, you might break a snap, but it's not a big deal. Heck, they can break even when everything is done right. Often you can live without one or two. If a snap's loss makes the case wobbly, it might be worth some careful repair with your plastic-melting iron. Be sure to pop the case apart first; trying to melt plastic near the outside will almost certainly result in very visible damage. Even from a half-inch away, the iron puts out enough heat to soften and deform many plastics. A repaired snap is weak, so you may get only one chance to close up the case properly.

Still, it's better than nothing. If you hear something floating around inside the unit after you finish putting it together, the snap has broken off. Open the case and remove the plastic piece.

Screwing It Up Without Screwing It Up

As I mentioned, screws should be reinstalled carefully to avoid damaging them or the plastic into which they are screwed. Phillips screw heads are especially easy to strip, and trying to remove one is mighty frustrating once you do. Insert all the screws in a layer, or on the outside of the case, but don't tighten them down before the last one in that layer is in its hole. Sometimes you'll need to remove one because it's the wrong length or you suddenly realize the black one went here and the silver one over there. Or, an internal bracket doesn't quite line up and you need to open the case again before going on. Once you have them all in, it's time for the final tightening.

How tight is right? Hold the screwdriver with your fingers, not in your palm. Turn the screws just until they stop, and snug them in ever so slightly. That's it. Don't twist until you can't twist any farther.

Done!

If everything fits together well, you should be ready to fire up the unit and consider your repair complete. Be sure to bench-test receivers, projectors and other heat-generating products if the work you did could possibly make them run too hot. You'll want to bench-test a projector whose fan or ballast circuit you repaired or replaced, or a receiver that needed new output transistors. A digital camera or an MP3 player, of course, won't require that extra step. Now go show off your work and bask in the glory of a job well done. You've earned it!