Rcs on janome mb4 went black but seems to be working

Question edited for correct maker name and to what I think you thought you were saying. :>D
Question moved form Cellphones to Elna Sewing Machines.

On the back of the display there is a capacitor that controls the brightness of the display. If you can find and electronics technician to replace it, it will fix it. Otherwise, you just buy a new display board. These is one here for $40

https://www.ebay.com/itm/Elna-9006-Envision-LCD-Replacement-Back-Light-Kit-NO-SOLDERING-REQUIRED-/302572186374

This say the light is adjustable

TEST & SET MODES - Janome Service Videos

http://www.janomeservicevideos.com > 2015/02
PDF
MODE(S). Janome model(s):. MB4. Elna models: 940 / 9900. Screen Calibration ... Brightness: Press the '-' and '+' keys to make the screen brighter or darker ...
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Verify that you are threading it correctly. Thread nesting will happen often because the thread path is not correct.

Since you just had it serviced, don't hesitate to take it back. Perhaps something was missed.

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Inside the machine there are many parts that have wires connected together. If one of them have been taken apart by accidental bumping it may cause this. By carefully opening up the machine and looking you may see the problem. However, touch yourself to some metal before messing with it. It is a better idea to let a pro take care of it.

could be a computer problem, needs to be serviced

Make sure you r sending design in correct format from memory mb4 reads only .jef format

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.

IF THE GEARS ARE PLASTIC, THE LOCKING MAY HAVE CAUSED THE GEARS TO JUMP A TOOTH AND THREW THE TIMING OF THE MACHINE OFF. IT MAY NEED TO BE SERVICED.

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)

You need a bobbin like this:

You can puchase them from this link
http://www.sewingpartsonline.com/bobbins-janome-kenmore-elna-102261103.aspx

Janome have one standard plastic bobbin that fits almost all their models except
1600P, 1600P-DB, 1600P-DBX and MB4.
So if you see packets of plastic "Janome" bobbins for sale they will be the right ones.