Need help to identify lamp in board for LED backlight - sewing machine repair

Discussion in 'Technical Repair' started by cdc3030, Dec 31, 2016.

  1. cdc3030

    Thread Starter New Member

    Dec 31, 2016
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    HI,
    I have two identical sewing machines, that have both now failed with the same problem. It looks like the bulbs have burnt out on the boards for the touch screens for the LED. They are older computerized Brother machines, so are useless without the screens. The screens work...but are invisible.

    We tested the bulbs in situ for continuity, and they all (8) seem to be dead. I can take them out (soldered in) and replace... but am not sure what they are. My big question is...if I get the wrong bulb, what impact will it have on the rest of the board? The replacement seems simple enough, but getting them wrong...I dunno.

    these machines are much loved, but not able to be repaired by Brother. Too old.

    Thank you

    (I hope the pictures come through, I will try again if I messed up.)
     
  2. DickCappels

    Moderator

    Aug 21, 2008
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    1,519
    I am not sure, but from the photo below, I suspect this is an incandescent rather than an LED. The vertical wires would be the filament support/connections and the silvered appearance of the top is the result of the Edison effect.

    upload_2017-1-1_13-6-58.png

    It is possible that the board originally had LEDs and a LED driver circuit which might have been the original purpose of IC5. A switch from LED to incandescent might have been a cost cutting measure.

    The two bulbs have the appearance of being connected in series. If you can find the total voltage across the lamps by poking around with a voltmeter you can just rig something up with a pair of bulbs with voltage ratings that add up to the voltage you find across the two.

    If you are adventurous you can try replacing them with LEDs and a series resistor if you can get enough light without the resistor getting too hot.
     
    cdc3030 likes this.
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    I too reckon that they're filament lamps; two pairs, each pair having two in series. Since they are used to edge-light a slab of plastic the omni-directional light pattern of a filament lamp is much better suited than the narrow beam of a LED. Judging from the mounting/wiring arrangements of the lamps I don't think LEDs would ever have been used.
    Pea bulbs, as used for dolls' houses and railway modelling, might be a suitable replacement. What voltage is supplied to the lamps/board?
    Alternatively, if space permits, then the omni light pattern could perhaps be replicated by replacing each lamp with a triangular/circular array of bright SMD LEDs.
     
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  4. cdc3030

    Thread Starter New Member

    Dec 31, 2016
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    I don't know the voltage required for the lamps. I had considered just bypassing the bulbs and rigging up a separate LED light source....something like a strip that would be used in a laptop back light would fit...rather than replacing those bulbs...which would need to be replaced every time they burn out. As I said, I have two machines...both of them are dead, so I can afford to experiment on one board and mess up, so I may replace the bulbs on one, and if that does not work, rig up a sep light source and see if side lighting that plexiglass from some other source would work. I will try plugging the whole thing back in without reassembling it and see if I can check the voltage on the bulbs first. (this board is part of a series of boards, which are all screwed to the case...which are attached to the sewing machine...so a bit challenging to reattach and power up, but I will see if I can get it to work)

    I may be back with more questions, thank you!

    c
     
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    A DMM should tell you that. Or a service manual/schematic if you can source one.
     
  6. cdc3030

    Thread Starter New Member

    Dec 31, 2016
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    Ok. I have removed bulbs, figured out what I need to replace them....figured out voltage of the bulbs and ordered them.

    I have spent more time looking at the boards, and I am a bit more confused.

    So. with the bulbs removed, I could test the voltage by bridging the gap with the board plugged in and powered on. Both holes had power...so the missing bulbs did not kill the circuit. (makes sense, right? Or a burnt bulb would kill the whole circuit)

    But, then I started thinking more. I dont know squat about touchscreens/lcds. from what I can gleen, this is a resistive touch screen. Until I started messing with it...the touch screen portion of the....screen....worked. The DISPLAY part didnt.

    So, I thought it was all about the bulbs, but it isnt. The bulbs are burnt out. For sure. but, also....the display part of the touch screen is not showing anything.

    The touchscreen is thin layers of plastic, stuck together, and stuck to a plate of glass. That plate of glass has no electronics on it....so must just be a rigid background for the trouchscreen. And, the display must be actually in those plastic layers. The backlight (bulbs burnt out) helps to illuminate the images...but I would think that they would be visible if I shined a light on the screen. They are not. So, the touchscreen touch part works, but the image part doesnt.

    So, it seems that there is more at issue than just bulbs....unless the bulbs actually CAUSE something else, or are the result of something else.

    Now, the thing is, I mentioned, this has happened in the same way, on 2 machines. Both were subject to shaking/moving damage.

    I have tried googling all this to death, but I just don't understand touchscreens/lcd's enough to figure out where my logic is wrong. I am just making all this up. Are my assumptions above correct? Is there something that I cannot simply see that would explain this? Do the lightbulbs provide resistance that would be important somehow, and need to be there? (as far as I can tell, it seems the display and the touchscreen are on different boards....connected by a tape)

    Is there some way for me to trace the issue so I can figure out/pinpoint the problem? I plan to replace the bulbs when the arrive...but not sure that will make any difference at this point.

    (As for schematics, I have searched, but cannot find any resources available. It is a Brother PC6000 sewing machine)

    Thank you
     
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