The following descriptions and associated pictures are offered as observations on what we do to maintain and repair our ULT. Copying what we do may void your warranty and you (not us) are responsible for problems which occur if you break your machine while trying to adjust it. If you're not mechanically inclined it may be wise to get help from a family member who is handy with small tools. Always unplug the machine prior to undertaking any work on it.
Bobbin Sensor
The ULT notices when the bobbin is running low and warns the user with a beep and a window message displayed on the LCD screen. This warning is sometimes erratic and cleaning the sensors involved may improve things. If sufficient lint collects on the sensors then the warning will never appear and you will run out of bobbin thread.
The amount of thread on the bobbin is sensed using a LED (light emitting diode) whose beam shines through the race and the shuttle onto a photo sensor on the other side; the path of the beam is shown by the yellow line in the picture (the LED is visible at the right end of the line). The sensor (which looks similar) is located under the little shelf on the left in the picture and is hard to see except with a mirror. The race is cut away to allow the beam through and the shuttle has an opening on each side of the bobbin such that the beam passes through it adjacent to the center core of the bobbin. Naturally, lint collects on the LED and photo sensor. Crack a Q-tip about 1/2 inch from the end so it forms a right angle and use this to carefully wipe the lint from both the LED and the sensor. It's a good idea to clean these each time you clean the shuttle and race, i.e. often. Also check the holes in the shuttle and clean if needed.
Bobbin Alarm Adjustment
On some ULT's the low bobbin alarm occurs when there is considerable thread remaining (it alarms too early) so users sometimes ignore the warning and then run out of bobbin thread. The bobbin alarm sensor may be adjusted to set the amount of thread remaining on the bobbin when the alarm and frowny face appear. There is some variability to when the alarm occurs vs the amount of thread remaining on the bobbin. This is caused by the necessarily loose fit of the bobbin in the shuttle, so it isn't possible to alarm precisely at the last foot of thread. Brother's setting is conservative, ensuring that you won't ever run out of bobbin thread. If you are more adventurous and prefer to push the limits, adjusting the bobbin sensor may be just the ticket :-)
The first adjustment technique (immediately below) allows setting the low bobbin alarm as accurately as the ULT can detect and verifies the setting as part of the procedure. You use a nearly empty bobbin as a model and set the ULT to alarm when the bobbin reaches a similar level while stitching. Some owners would like adjust the detection level but are intimidated by the apparent complexity of the procedure. It is possible to adjust the level more simply if you can settle for less precision, as described later.
Getting Ready
Remove the bobbin arm cover for access to the sensor, as shown in the picture. Locate the silver screw which retains the black plastic sensor -- the red phillips screwdriver in the picture shows this screw's location. The sensor is held in a black plastic bracket which has a slot on the right end; the grey flat blade screwdriver shows the slot's location. Look things over to get the idea of where everything is before proceeding and read this whole section before taking any action. There is only one large silver colored philips screw in the area pointed to by the red screwdriver and there is only one piece of black plastic with a slot in it where the grey screwdriver points so it is straight forward to locate the correct points. In addition to the screwdrivers, you will need a test bobbin containing the amount of thread you would like to just set off the alarm (similar to the one in the picture). Clean the LED and sensor as described above before attempting adjustment.
NOTE: When diagnostics mode is active, the machine operates much differently than normal. BE CAREFUL -- if you bump the Start/Stop button the needle will move even with the foot up!!! For safety, remove the needle beforehand.
Preparing to Adjust the Bobbin Sensor
To activate the ULT's diagnostics page, hold the Start/Stop and Backtack buttons in (they're adjacent) and turn the power on, holding these buttons depressed for a few seconds until the diagnostic screen appears. The screen will show an array of numbered buttons; press button 10 to start the bobbin sensor test. A screen will appear with "Lower Thread" and either a "+.+" or a "0.0" near the center. With the bobbin removed, rotate the handwheel VERY SLOWLY counter-clockwise until the +.+ changes to 0.0 (the needle will just be starting down); this indicates that the sensor can see the LED through the shuttle slots. There is a slight delay so if you rotate too fast you will miss it; don't turn the wheel backward, just go around again. You can verify sensor operation by putting your finger into the shuttle to simulate a full bobbin, which changes the screen to +.+ .
Making the Adjustment
Insert the test bobbin containing the amount of thread which is to just set off the alarm. Make certain that the screen continues at 0.0 -- if the screen changes to +.+ then the low bobbin has not set off the alarm (your setting is OK unless your machine alarms too late). Loosen the sensor retaining screw about 1/3 to 1/2 turn (red screwdriver in our picture). Use a flat blade screwdriver in the slot to lever the sensor gently to the right(sensor moves towards the front of the machine); I used the large spring as a fulcrum - go slowly and gently - if you need force you haven't loosened the retaining screw enough. This is the tricky part: push the bobbin gently toward the rear while levering the sensor and simultaneously watch the 0.0 on the screen: when the 0.0 changes to +.+, stop. If you move the sensor too far, insert the screwdriver blade between the end of the sensor and the case and gently twist to push the sensor toward the rear until the 0.0 appears and try again. One final detail: cover the area with a piece of cardboard or dark cloth to minimize light hitting the sensor as you are making the adjustment. Any extra light will affect the adjustment since the sensor is photo sensitive.
The picture shows two screwdrivers; this was done to reduce the number of pictures -- only one is used at a time, so you don't really need 3 hands :-)
Checking and Finishing Up
When properly set, pulling the test bobbin toward you in the shuttle will make the 0.0 appear, pushing the bobbin toward the rear will make the +.+ appear, this is the slight loosness mentioned earlier.
When you're happy with the adjustment, tighten the retaining screw. Then, re-check the adjustment by pushing the bobbin forward and back (keep the end of the machine covered to exclude extraneous light) to verify that both 0.0 and +.+ appear. If all is well, turn the power off to exit diagnostic mode and replace the bobbin arm cover.
What it Does
When adjusted as described, the alarm should occur when the bobbin in the machine has the same amount of thread as the sample bobbin used for adjustment. We set our machine to alarm when the outside diameter of thread on my Nebs bobbin measured 0.370 inch vs Brother's recommended minimum of 0.448 inch (11.4 to 11.8 mm); when we have more experience with this we may reduce it further. The Nebs bobbins may have a smaller center than Brother's bobbins, accounting for the disparity in settings. This small difference in setting reduces the thread left on the Nebs bobbin by quite a bit.
Limitation
Bobbins are not precision devices made to NASA standards. The centers vary slightly in diameter so don't use a bobbin which has only one layer of thread as your test bobbin and expect to alarm all bobbins with one layer left -- the differences between bobbins plus the loosness of the bobbin in the shuttle will not allow this level of precision. Once you get the hang of it, it is easy to do this adjustment. So, creep up on the final setting in stages, verifying that the results are repeatable on several bobbins before adjusting for even less thread remaining.
A Simpler but Less Precise Way to Adjust the Low Bobbin Alarm
Gain access to the low bobbin sensor by removing the bobbin arm cover as described earlier. Look through the opening where the red screwdriver points in the picture to locate a silver phillips screw holding a black plastic piece - it should be the only silver phillips visible in that area. Loosen this screw 1/2 turn. Locate the slot toward the end of the black plastic piece, as pointed to by the grey screwdriver in the picture. Put a flat blade screwdriver into this slot in the black plastic piece. Move the handle of the screwdriver toward the rear of the machine and allow the shaft of the screwdriver to contact the vertical coiled spring shown near the screwdriver; press the handle of the screwdriver gently toward the rear of the machine and the blade of the screwdriver will lever the black plastic, moving it towards the front of the machine. Move the plastic piece about 1/16 inch toward the front of the machine to reduce the diameter of the thread remaining on the bobbin by 1/16 inch - generally, move it less so you can "sneak up" on the desired setting by adjusting it again after trying it with the first adjustment. Re-tighten the silver phillips, replace the cover and try the machine to see what level the bobbin alarms at now. Repeat as needed to arrive at the desired setting - the adjustment is not difficult to make once you have done it the first time so having to repeat it isn't a hardship. Also, once you have done this, the precision method will make more sense and you might then try that.
The picture shows two screwdrivers; this was done to reduce the number of pictures -- only one is used at a time, so you don't really need 3 hands :-)
If you are not comfortable making this adjustment, then choose a bobbin as described above to serve as the model for your minimum and ask your dealer to make the adjustment for you.
The amount of thread on the bobbin is sensed using a LED (light emitting diode) whose beam shines through the race and the shuttle onto a photo sensor on the other side; the path of the beam is shown by the yellow line in the picture (the LED is visible at the right end of the line). The sensor (which looks similar) is located under the little shelf on the left in the picture and is hard to see except with a mirror. The race is cut away to allow the beam through and the shuttle has an opening on each side of the bobbin such that the beam passes through it adjacent to the center core of the bobbin. Naturally, lint collects on the LED and photo sensor. Crack a Q-tip about 1/2 inch from the end so it forms a right angle and use this to carefully wipe the lint from both the LED and the sensor. It's a good idea to clean these each time you clean the shuttle and race, i.e. often. Also check the holes in the shuttle and clean if needed.
Remove the bobbin arm cover for access to the sensor, as shown in the picture. Locate the silver screw which retains the black plastic sensor -- the red phillips screwdriver in the picture shows this screw's location. The sensor is held in a black plastic bracket which has a slot on the right end; the grey flat blade screwdriver shows the slot's location. Look things over to get the idea of where everything is before proceeding and read this whole section before taking any action. There is only one large silver colored philips screw in the area pointed to by the red screwdriver and there is only one piece of black plastic with a slot in it where the grey screwdriver points so it is straight forward to locate the correct points. In addition to the screwdrivers, you will need a test bobbin containing the amount of thread you would like to just set off the alarm (similar to the one in the picture). Clean the LED and sensor as described above before attempting adjustment.