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  1. #1
    Join Date
    Jul 2007
    Posts
    9

    Default syncro 250 pedal will not vary output, but will work as switch

    I have a syncro 250 (Model JK54xxxxx)that I have never actually gotten the pedal to work on. Tonight I finally spent some time with the cover off, and a meter. I have a pdf of a service manual, but it does not have my wiring diagram in it, though it does have the schematic, and a section on measuring voltages and wave forms. I did not bother getting the scope off the shelf to look at ave forms because the machine works perfect in stick mode, and it semi-works in tig mode if I leave amperage control at panel and just use the pedal as an on off control ( with contactor set to remote)

    Measuring the pedal while disconnected from machine it looks like it is fine, 0-1k varying resistance, and A-B is on off as it should be.
    I measured every voltage from RC55 and RC54 on the main board. The guide( TM353 in footnote, t353e.pdf filename) has the pins numbered, not lettered. My board seems to have letters, but the sequence seems to match up for expected values, except for 2 things:
    RC55 pin 4 always reads 0v, but should be 0-4v as arc force is adjusted with the switch on. RC55 pin 10 does vary from 0-4 with the switch off.
    and the really relevant thing:

    RC54 pin 9 - should be 0-10v varying with amperage adjustment with contactor on. RC54 8 reas fine from 0-10 as amperage is adjusted with machine knob.
    RC54 pin 9 does not vary, and reads -5.8v with the pedal connected.
    When I disconnect the pedal it reads -10v.
    That pin 9 wire is labeled 106 in my machine, which I traced to pin C on pedal connector RC1. ( should be 0-10v signal in from the board?) Measuring on the back of RC1 I can measure from E to D and the value goes from 0v to -5.8v as I fully depress the pedal.

    It seems like my root problem is that I am getting -5.8v from the main board instead of +10v. Any one ever dig this deep into a machine to figure out why, and if it can be fixed?

  2. #2
    Join Date
    Jul 2007
    Posts
    9

    Default

    RC54 Pin 9 is labeled BJ, connects to remote pin C.
    The trace on the circuit board leads through an inductor RFC4 to two components:
    transistor labeled Q13 pin 1 with number 2n2219a ( pins 1-emiter, 2 base, 3 collector and metal case) ~ $.45 online
    AND
    to a 14 pin IC labeled A4 pin 14 with numbers RC4136DB RAYC8830 ($4.50 online)

    Which is a package of 4 high gain op amps of type 741. Pin 14 is amp D negative input, pin 13 is positive input, and pin 12 is output.

    I dont know what any of this means sadly. I get ~4 ohm resistance through the inductor which seems normal since it is just a really long piece of wire.
    I suspect as these are the first components inline with this signal, and the whole rest of the board seems to be working, that one of both of these components has died.

    The maximum values for the pin of the transistor are only in the 40v range, and when I had my meter in the guts of the machine, hitting the pedal on and off I could see a 130v spike that settled right back to the -5.8v value I was seeing.

    At a service center, is the actual board worked on or is it just tossed and a new one used?
    http://www.plccenter.com/en-US/Cart/...itemId=2465080 $123.50 is looking like a pretty good option, though just swapping out both of these components semi-blindly would be **** satisfying if that would fix it... My concern is maybe there is something else that is killing them via that voltage spike I saw when hitting the pedal.

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