Introduction: Roomba 400 Series Disassembly and U2/U4 Charging Fix

Is your Roomba 400 not charging? When you plug it in, does it light up but when you press the button you get no response? Here's the disassembly guide as well as a guide to fixing the U2/U4 Problem. This will also give you a chance to clean out all the Dust that builds up inside, you could clean out the inside of the brushdeck and preform the sealed bearings mod, and much more. In addition, once you open it up, you can see and clean all of the dust balls inside. Literally, the inside is usually PACKED with dust. That dust is harmful to the Roombas in many ways.

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Step 1: Intro to Disassembly

All Bumper and Cover retaining screws must be removed before attempting dismount of either subassembly. The two subassemblies must mechanically disengage Chassis fittings as a set; at least for the first few millimeters of movement. Steps in HoB apply to the following Discovery models, in which the Handle is mounted on the Bumper and interleaves with the Cover: Roomba-Discovery, #4210Roomba-Discovery Replacement, #4211 Roomba-Discovery SE, #4220Roomba-Scheduler, #4230 Note: We have not had hands-on experience with the Discovery SE, or Scheduler models, but those bots are considered mechanically identical to the #4210 Discovery, which is known to reward owners when the HoB process is used. Models #4210 and #4220 that were produced prior to, roughly, mid-2005, came with some removable parts that are called "Fender Skirts" in these documents. It can be useful for an owner to recognize whether his Roomba has Fender Skirts, because their presence provides much easier access to the drive-wheels; and also explains why two very long screws are used just forward of the drive- wheels. The image below has pointers to the joint features which indicate presence of a Skirt. A Fender-Skirt is a removable panel which spans the distance between those joints. Roomba-owners who purchased either of these Discovery models after mid-2005 should inspect the left and right sides of their bot to see if "Fender Skirts" exist on their particular unit. If no Skirt-joints are visible, no Skirts exist on that Roomba.

Step 2: Mechanical Disconnects

To access the interior of a Discovery #4210, Remove the Bin assembly, then turn Roomba on it's back and remove the Brush-Guard, Main-Brushes (with end-bearings), and Battery. Place removed items clear of the work area. Remove 4 Bumper screws. With the front wheel nearest you, look at the bumper's bottom edge and note the four screws, marked "Bumper" in the following image (and that are all in half an inch deep holes) two just in front of the twirly brush and two the other side. To access the interior of a Discovery #4210, Remove the Bin assembly, then turn Roomba on it's back and remove the Brush-Guard, Main-Brushes (with end-bearings), and Battery. Place removed items clear of the work area. 2. Undo (#1 Phillips bit) the four screws, and turn Roomba over momentarily so they fall onto your workbench.(hopefully covered with a towel, to stop them bouncing). 3. Put the four screws somewhere where they won't get lost, and note that they are slightly, but noticeably thinner than the other screws you are going to remove, so don't mix them up.

Step 3: Dislodge the Cover.

1. With an upside down Roomba with its front-wheel towards you, note the two screws that are between each tip of the black bumper and the front of each side wheel. 2. If Disco is fitted with removable Fender-Skirts (which is why these screws must be long) these two screws need be only loosened by four turns (each will stay with its Skirt), and the Cover will no longer be connected at those two points. With Skirts, those screws are both about 13-mm long, and so are different to all the others in this set of eight. Without Skirts present, they are all the same length. 3. Next remove the two screws each end of the battery cavity. 4. Then the two screws deep down in front of each end of the black brush-deck (positioned between the screw you just took out and where that long screw was). 5. While working this left-right row, take a look at mid-span to verify there is no screw in a central position. If one is found, it should also be removed. 6. Then, look at the back end of the Roomba and each side of where the Bin fits, you will see a screw that is sunken deeper in a hole than the other two screws you will see...remove ONLY the two sunken screws....invert Roomba and shake the screws out. 7. From the main lid you should have six short screws and two long ones, count them to make sure they are all out [unless two remain in their Skirts, or one was found at the center of the brush-deck cavity], then proceed.... 8. Put the eight screws somewhere where they won't get lost, and note that they have differing lengths, so use a memory-jogging scheme to store them, a scheme that will ensure they go back in the same depth holes from which they came.

Step 4: Separate the Inner & Outer Bumper-sections

1. With Roomba back to inverted, put a finger under the left-side, black-bit of the bumper just in front of the fixed, wheel-protecting brush, and with thumb or the other hand, push the grey bumper downwards to dislodge it. A visual hint (but happens to be a view of the right-side features) is in the pictures. 2. Repeat this on the twirly brush side by sticking your finger in the cavity that is on the twirly-brush side, just outboard of the square, silver looking, charging-pad, and pushing the grey bumper downwards as before. The grey bumper cover should now be loose, and we now have to disconnect it.

Step 5: Bumper Electrical Disconnect

While holding the loose external-bumper, invert Roomba and place it on it's wheels again, facing away from you. The following steps should safely de-mate the Bumper-Cable: 1. Carefully lift the Bumper and look underneath it. At the front and in the middle you will see a small connector with seven wires, and tiny little "ears" ("Ledge", in the photo) at each end, next to the wires -- Note that the connector can only be connected one way around....more about that later. Picture of what should be seen is attached. 2. Carefully, lightly, "pick" one ear (or "ledge") downwards with a fingernail, and move it a tiny bit, say 2 mm. Then "pick" the other ear and move it about the same amount. Repeat this, and the connector should slide down and apart with gentle wiggling, not much force is required (you are trying to slide the connector apart evenly, without bending the pins), just be gentle and patient. 3. When the connector is out, do not attempt extracting the Bumper (wait for additional instruction), however, note that the bit of the connector fitted to the external-Bumper has a sort of gap, or window that lets you see the pins and the other half of the connector has cast on ribs that fit into this gap, as seen in second photo. This is what stops the connector being connected the wrong way round, but BE WARNED we are sure if you really tried very hard, you could ham-fistedly manage to force it together the wrong way round, so make sure you don't! The wire colours starting at the left are: red, blue, black, white, blue, red, brown.

Step 6: Roomba-Cover & Bumper Mechanical Separation From Chassis

Put Roomba back on It's wheels, facing away from you this time, then:

1.Since no screws are retaining the Cover, gently lift it away from the eight chassis' sockets by lifting the Cover and pushing down on the Chassis at a sequence of points around the rim. Do not plan on lifting the Cover very far, it has a cable fastened to the chassis, or hooked to the chassis; and its forward- facing 'prongs' will still be engaged with bumper parts.

2. With the Cover now free to move upward, attention must be shifted back to the external Bumper assembly, since its needed upward motion will not be blocked by the prongs of the Cover. Start at the left end of the Bumper, and lift it vertically. Wiggle it as necessary to slide apart the engaged screw-bosses. There is a view of those bosses (as seen after Bumper & Cover dismounts) attached.

3. As lifting causes the wanted separation, allow the Roomba-Cover to elevate to the extent useful, and work your way around to the right-end until the Bumper assembly can be taken away.Store the Bumper away from the work area.
The Roomba-Cover will next be disconnected and removed from the assembly. In preparation for that work, Rotate Disco end-for-end, so it faces you, right- side up.

Step 7: Roomba-Cover Electrical Disconnect

1. There is another connector to disconnect, this time its what's known as a ribbon cable and goes from the control-buttons in the Cover to the main processor board. Lift either the forward Cover-edge, or rear edge, and view the ribbon-cable, following it to the where it is secured to the chassis' main, left-right bulkhead. The cable may be found spot-bonded with HMG, (hot-melt-glue) to a shallow channel-shaped notch, e.g., ```|__|```, or simply fitted into a 'lazy-C' (see the photo) pass-through with hooks at its top, along the upper edge of the indicated bulkhead.

2. Remove the cable from that tie-down point. The scene should look like picture 1. 

Step 8: Electrical Disconnect Cont'd

1. If the service-loop in that cable permits it, carefully flip the Cover over and lay it down on the Chassis Assembly, so it appears like picture 1. 

That operation greatly enhances access to the cable-connector. De-mating this one can be the same deal as before.....gently flick the ends, and note that the holes and cast-on lumps (a rib on each narrow end-face that get it the right way round) are much smaller and daintier this time, but once again they should still work, especially if you use your brain.
Also take note that friction-force of engagement is more than twice that of the Bumper's connector (16- contacts here, vs. seven there), so not everyone's finger nails may be up to the job.Some sort of special connector de-mate-tool can make the task very much easier in such cases. Two useful de-mate tools are described in De-Mate Tools.

After the connector's de-mate, remove the Cover to safe storage away from the work area.It should now be clear, that full access to any electro-mechanical subsystem is readily available. Attention may now be directed to that task which required access to Roomba's interior... For our purposes it is the U2/U4 Charging fix! 

Step 9: U2/U4 Fix

What causes this? While the cause is not completely determined, one reason may be charging your completely discharged battery.

In most cases, both the U2 AND the U4 mosfet need to be replaced. This part will not only fix it, but make your Roomba less susceptible to this type of damage in the future. The whole procedure should only cost 3 dollars!

To remove and install this part, a SMD Rework Station will be used. (An Aoyue) Just use your hot air soldering iron to desolder the part, dispose of the part so you don't get it mixed up, then put solder paste on the pads and install your Mouser part. The Parts you need to be replaced are on different sides of the board,  and labeled U2 and U4 hence the name. First picture is of U2, second is U4. Schematic made by RobotReviews Member Gordon:

To reassemble, just follow the steps backwards. 
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