My Janome mb4 will not power up when turned on. The light in the needle bar comes on just for a second then off.

This may prove helpful...
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Change your needle and if it continues to happen, change the thread. Most likely your needle and thread are not compatible, ie the thread is too large for the needle's eye. Also, inexpensive thread will shred.

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Have you hit a hoop while sewing? Is the needle jammed up and will not come down or down and will not go up? You need to make sure that you have no thread in the bobbin area. Even a tiny little piece will lock it up. Have you kept it oiled ? There are several places that need oiling. Bobbin area and needle bars are the two main ones. Make sure your needles are all the way up

sometimes the screws will vibrate loose on the needle bar .

Do you have the MB-4 instruction Manual?

http://janome.com/siteassets/support/manuals/embroidery-models/inst-book-mb-4en.pdf

If your embroidery is distorted be sure you are using the right stabilizer for the fabric you are doing the embroidery on and enough for the density of the design.

I hope that helps.

Humm, sounds like the needle change sensor is telling you that the machine is not properly centered on ANY needle position. It is between needles. There should be a knob to turn that move the head from needle to needle. once the needle position has been centered.

Cheers,
Kelly
San Diego, CA
Embroidery Services

Unfortunately you are probably not going to be able to fix this.
It needs a sensor adjusted (probably), or a component replaced.

You may have already got it fixed or just don't use needle #1. If you have hit a hoop it will cause this. Take the front cover and top off the machine. At the top of the 4 needle bars there is a screw. Look to see if the screw on needle #1 is lower than the rest. You should be able to compare with the other 3 needle bars. Loosen this screw and slide it back up even with the others. This lets the needle bar go lower and hopefully this will fix your problem.

This is usually caused by the y axis movement system. Occasionally this system can come out of sync, and cause the issue you described. Your y axis pulse motor may be going bad as well. In either case it would be best to take your machine to a Janome dealer for repair.

checks that the strip wire and strip wire sensor, this worked well.
(the strip threads is the small spring by which you spend the thread to thread)