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Defective PCB/scroll saw

Discussion in 'Troubleshooting and Repair' started by KTW, Oct 2, 2015.

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  1. KTW

    KTW

    273
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    Feb 22, 2015
    Looks like the transformer is pooched.
    Old board 120vac in 6 out.
    New board 120vac in 42.5 out.
     
  2. 73's de Edd

    73's de Edd

    3,105
    1,312
    Aug 21, 2015
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    Sir KTW . . . . . . .


    I certainly would not be expecting any AC on those test points, just pure-pure DC.
    You can see the little square 4 pin cased bridge rectifier on the low voltage side of the RED power transformer, then it dumps into the LARGEST electrolytic in the board, a short distance away, that is being mounted just aside to a TO-220 cased three terminal regulator.
    Initially check across that MAIN electrolytic filter of the systems low voltage power supply in your meters DC function mode . . . . .THEN check across it, using the meters AC function .
    If there is still that high level AC presence . . . . . then, might the filter have declined in capacitance so severely as to now be creating "ripple city" ?
    Or possibly, your meter does not tolerate a superimposed DC element.
    To confirm that, take your AC reading again thru a series inserted ( minimally a .1 ufd, or more hopefully, a 1 ufd paper ) capacitor to D.C. voltage isolate, when you are taking your AC reading again.

    On that 3 pin connector, the pin closest to the board edge. is receiving the main LVPS positive voltage.

    The center pin is "close" to ground potential . . . . just being so, because pix resolution prohibits my making out its associative resistive elements.

    The last pin closest to the 4538 is the one that "communicates" with the 2 input nor gate of one of the two contained one shot circuit blocks.

    We are now just awaiting the arrival of the "assembly" . . . . . that can enlighten us even more .


    73's de Edd


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  3. 73's de Edd

    73's de Edd

    3,105
    1,312
    Aug 21, 2015
    Looks like the transformer is pooched.
    Old board 120vac in 6 out.
    New board 120vac in 42.5 out.


    "Dat don't seem wight to me . . . no . . . dat just don't seem wight to me" . . . . .a la Junior . . . . . of Red Skelton fame.

    42.5 . . . . . .even unloaded . . . . for the amount of electronics on that board that it has to support.

    The tenet for transformer failure is REDUCED voltage output along with its heating up, then odor and then "smoke city"..

    Best now look at the filter that I just mentioned and get its voltage rated spec, as well as looking at the three terminal regulator for its 78?? marking to see what its basic voltage out is being.



    73's de Edd
     
  4. KTW

    KTW

    273
    15
    Feb 22, 2015
    The capacitor has 6.4 volts AC on both sides, no DC reading is attainable.

    capacitor.JPG

    Everything downstream of the transformer seems to be a lower voltage than what the new board exhibits.
    The voltage regulator is an AN7805, I believe it to be 5volt.
    The regulator also reads 6.4 volts AC both sides, no DC.
    The bridge rectifier seems to feed the MC14538BCP Monostable Multivibrator through a series of resistors.
    I'm not sure if I have a paper cap, I'll have to hunt.
    It could be the AC ripley affect, believe it or not but I'm not certain at this point.
     
  5. KTW

    KTW

    273
    15
    Feb 22, 2015
    Yet another revelation.
    Using an analog meter I've got 120 volts going into the transfomer and nothing coming out.
     
  6. Alec_t

    Alec_t

    2,965
    804
    Jul 7, 2015
    Whichever you believe, it certainly looks like the transformer has had it :(.
     
  7. KTW

    KTW

    273
    15
    Feb 22, 2015
    I pulled the transformer from the board and I'm now reading 16vac across the transformer.
    Reading across the transformer give me 15+ volts AC, the problem was trying to get a reading one side to ground.
    It appears as if the transformer is working to produce close to 16vac, the new board transformer tests close to the same across the transformer in circuit.
     
    Last edited: Apr 3, 2016
  8. KTW

    KTW

    273
    15
    Feb 22, 2015
    From the transformer 16volts AC to 16volts DC at the cap and 16volts DC at the regulator coming out of the regulator at 5volts DC, both boards are the same.
     
  9. KTW

    KTW

    273
    15
    Feb 22, 2015
    The capacitor is 25volts and the outside pin of three has 5 volts on it.
    He hasn't brought the saw to me yet.
     
  10. Bluejets

    Bluejets

    4,870
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    Oct 5, 2014
    Sounds to me like you need to be careful of where you are taking your measurements from with respect to common or negative probe.
     
  11. KTW

    KTW

    273
    15
    Feb 22, 2015
    Yes, you're right about that.
     
  12. 73's de Edd

    73's de Edd

    3,105
    1,312
    Aug 21, 2015
    .



    Sir KTW . . . . . . .


    From the transformer , 16volts AC (converts) to 16volts DC at the cap and 16volts DC at the regulator (is) coming out of the regulator at (/as) 5volts DC, both boards are the same.

    That statement above agrees with a perfectly normal power supply situation . . . . . for both boards .
    Your common connection for any negative ground metering reference will be the negative terminal of that main electrolytic mentioned.


    Now . . . . .

    I'se gots a question . . . . . . What . . . IS . . . your avatar ?


    73's de Edd


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  13. KTW

    KTW

    273
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    Feb 22, 2015
    Thanks Edd;
    I was using what I will refer to as a known ground away from the board thus the erroneous readings.
    I'm not sure why the readings vary between the two boards using the same ground?
    Still waiting for the saw to show up so I can see what's left to look at.
    The board appears to run wide open until it receives a signal from the motor to slow things down.
    The avatar is my first born electronic project about a year ago.
    I built a circuit with the help of others here that controls a light through a PIR and a light sensor.
    I use it under the bed, it lights up the floor at night when motion is detected.
     
  14. KTW

    KTW

    273
    15
    Feb 22, 2015
    de Edd

    Where oh where are you tonight, why did you leave me here all alone?

    Hey Edd, look what I found!
    sensor.JPG

    What's the proper name for this sensor and do they still make them?
    Can it be tested, are they known to fail?
    A perforated disc spins inside this unit and I'm guessing detects the rpm from the motor shaft.
    This could be it.
     
  15. KTW

    KTW

    273
    15
    Feb 22, 2015
    Could it be a
    Opto Optical Sensor?
    It doesn't have any markings so does it matter which one I buy as long as the distance is the same?
     
  16. KJ6EAD

    KJ6EAD

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    Aug 13, 2011
    It's a photomicrosensor, but as configured with an encoder disk it's an optical encoder. They are readily available but the good ones aren't cheap. A magnetic encoder is much more affordable and may be adequate.
     
  17. KTW

    KTW

    273
    15
    Feb 22, 2015
    Thanks KJ6EAD, more research.
     
  18. 73's de Edd

    73's de Edd

    3,105
    1,312
    Aug 21, 2015
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    Sir KTW . . . . . . .

    That is what I told you was coming down the pike, back at my post # 18 . . . look back and defresh your memory bank.
    Usually they mold an arrow into the housing on the LED portion telling you the light path between the two . . .thereby differentiating the LED portion
    versus the photo sensor portion.
    With three leads . . .that will be having one shared lead . . . to test it just initially make a likely assumption that the center wire of the three is the common.
    Then, you plug the unit into the 3 pin orange connector on the board and power up the unit.
    Hold DC metering probes between center pin and one of the side pins. Move an opaque card in and out of the the wide center slot.

    Either:
    1 . . . . .There will be a constant voltage reading . . . irregardless if the card is blocking the IR beam between the units or not . . . therefore you are monitoring the
    steady voltage supply to the LED of the unit.
    2 . . . . .There will be voltage swing as you block and unblock the IR beam, you are on the photo-sensor portion and you have a functioning unit.

    With a disqualifier being, if there is being but a minute voltage swing.

    I'm having a high expectation of the unit being good.

    Waiting to see what you find now.



    73's de Edd



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  19. KTW

    KTW

    273
    15
    Feb 22, 2015
    sears.JPG
    Ok, thanks for that Edd.
    When I popped the bottom plate off I find a broken connecting rod to a counter weight and now I'm thinking this is why the unit is running wide open.
     
  20. 73's de Edd

    73's de Edd

    3,105
    1,312
    Aug 21, 2015
    .


    Sir KTM . . . . . . .

    Behold . . .what doth mine eyes detect . . . . . at the apex of yon RED arrow . . . . .is that being a "potty" metal failure ?
    With that combination of a rotating shaft from the motor connecting to an eccentric to produce a repetitive reciprocating linear action . . . . as is needed for the up and down drive of the jig saw blade.

    Its significance ? . . . . just observe the current mechanics to see if this would have resulted in the motor NOT NOW spinning the optical interrupter disc.

    I am thinking not . . . . just a loss of blade action and a LOT of noise, if the motor driven end of the broken connector rod repetitively slams into anything.

    As for failure explanation . . . .was this unit being continually used to cut fine intricacies into a 6 inch thick maple block ?




    73's de Edd


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