Rebuild Approach Schematic & other Info

Discussion in 'The Projects Forum' started by steward, Aug 17, 2011.

  1. steward

    Thread Starter Active Member

    Jan 21, 2011
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    Back in January Ken Moffet & others were waiting for a schematic from me http://forum.allaboutcircuits.com/sh...ad.php?t=48762 . To be fair, at that time we were basically tossing around ideas of starting completely from scratch.

    Included in this present post is a pdf that hopefully will show enough so I can use my my lab grade soldering & desolderer & isolate the most likely cause of the rare problems that I have had with the specific board & its functions.

    On two boards --- they have lost the ability to allow selections. The "off" led is on but no other outputs or leds --- even with the proper 24VAC timer load input. Hopefully some of you farther along can help me isolate the problem based on the key components. The large number of more common capacitors, diodes, resistor components that I did not show must have something to do with the 24VAC nature of the board I believe --- such as many bridge rectifiers. Hopefully, some of you can at least comment on what the most likely flow of the current or pattern that takes place based on the linked data sheets of the key components used & on what the specific board needs to accomplish & also based on the hi res pics!

    Another board --- has 2 outputs #10 & #11 stuck on even when the load from the timer or bypass is not on. #10 & #11 corresponding leds are also stuck on when the board is powered on.

    I did get some advice from a recent post to trace the exact connections that includes so many --- so it is really tough doing that --- since there are connections both on top & the bottom of the board. Also the data sheets are going over my head on those 4 more specialized components that I identified & provided links to the datasheets. The mediafire link on the attached pdf shows the hi def detail of the board.

    http://forum.allaboutcircuits.com/showthread.php?p=388536#post388536

    I don't know if "all about circuits" allows prizes to be given for a solution to a problem but I would sure be openminded about that if it would help. Normally, a board like this would not be worth saving but we have already --- including some on this forum invested some & who likes to give up too easily? Like "History Detectives" I exhausted every possibility of finding the fab shop that designed & made this board even though it was part of the "top of line" offering of a self service car wash coin box. The "good faith" efforts were all made in writing to everyone who had any previous connection or potential connection. I got a "somewhere in Northern Minnesota" but no chance of finding them or allowing them to broker for me --- apparently the installer of the equipment lost all records of the board. It seems they replaced the boards with something less user friendly --- triple decker mechanical rotary switches.

    Since we have 10 of these boards we know of their extremely good quality as evidenced by --- only 3 problems over a 24 year period. The timer & bill & coin drop & front panels have already been arranged to fit with this board so you can see why we would like to isolate the component or combination of components that are giving the trouble.

    There is absolutely no sign of a burn on the boards to give us a clue & the waterproofing conformal looks like new on them. The waterproofing conformal --- we will have to dab off with a q tip (Toluene) or something when we finally isolate the area based on how the data sheets are interpreted & based on normal functions that should occur. Hopefully, that will happen with some of you folks assistance &/or brainstorming. If nothing else --- hopefully --- it will be a learning experience.

    Steward
     
    Last edited: Aug 18, 2011
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The links you put in the PDF file don't work.
    Can you simply put the links in your post?
     
  3. wayneh

    Expert

    Sep 9, 2010
    12,123
    3,047
    Have you examined every electrolytic capacitor? Your pictures just aren't clear enough to tell if any of them may have failed. Look for bulging, splits, or any evidence of high internal pressure.

    I don't know the probabilities, but checking the caps is relatively easy and many electronics have suffered from the bad caps that were made some years ago. They're also easier than most things to fix. I just got done repairing my 12V to 120V inverter, which had suffered a blown cap.

    Unfortunately for your situation, a blown cap often leaves a mark as it spews out its guts. You don't seem to have any of that, but maybe that coating hides it.
     
  4. steward

    Thread Starter Active Member

    Jan 21, 2011
    38
    0
    Thanks SgtWookie for letting me know! Hopefully, these from the top down are all the links for the datasheets etc.

    http://www.onsemi.com/pub_link/Collateral/MC14001UB-D.PDF

    http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00000534.pdf

    http://www.mouser.com/Search/ProductDetail.aspx?qs=me8TqzrmIYXle%252baGidGxHA%3D%3D

    http://media.digikey.com/pdf/Data Sheets/Fairchild PDFs/2N5172.pdf

    [SIZE=1][COLOR=#0000ff][FONT=Times New Roman][SIZE=1][COLOR=#0000ff][URL]http://www.mediafire.com/kingkoinmike#1mct4rf78mm5x,1[/URL][/COLOR][/SIZE][/FONT][/COLOR][/SIZE]

    By way of wayneh's post --- it looks like the files open to the public mediafire. com is OK[SIZE=1][COLOR=#0000ff]

    [SIZE=1][FONT=Arial][COLOR=#0000ff]Steward -- Mike[/COLOR][/FONT][/SIZE]
    [/COLOR][/SIZE]
     
  5. steward

    Thread Starter Active Member

    Jan 21, 2011
    38
    0
    Wayneh,

    I have a large magnifier light & so far we have not seen anything that resembles a blown capacitor or similar component. We will keep looking but no sign so far. Thanks for the suggestion though.

    Being in the much more wet environment than normal -- we did have situations of the leds & outputs sticking occasionally but that was rare & really was not a problem because we would swap it out with a good one until the board would thoroughly dry by the next day & then it would always work perfect again. I thought I would mention these rare temporary incidents --- so everyone has that bit of info too --- in case it has some bearing on the sluething. I really never had a clue to what part would get wet internally enough that would actually cause that.

    Steward --- Mike
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Wet environments when a board hasn't been sealed will cause corrosion. Corrosion will eventually cause problems in the way of poor connections, even problems like eating into aluminum parts (like electrolytic capacitor cases) to the point where the internals are exposed to the atmosphere. If an electrolytic capacitor gets so much as a microscopic pinhole in it, the electrolyte inside will rapidly dry out, and the capacitor will no longer function.

    Boards used in a wet environment should be conformal coated to prevent the moisture from attacking the components, or simply causing shorts. I think you mentioned something about the boards having a clear coating on them before. If the coating has become damaged, water will be able to get at the components again and cause problems.
     
  7. steward

    Thread Starter Active Member

    Jan 21, 2011
    38
    0
    SgtWookie & others,


    Thanks for the info. Since the boards did not fail after one of the rare incidences of water droplets --- I do not believe the conformal sealing has ever actually failed. There is no sign of corrosion that we can see --- hopefully the pics verify that somewhat. Pic #3 has a misleading camera light reflection & is too blurred --- I will try to remove that specific pic later on as that is the one pic that would not be worth looking at. If we get to the point where we remove a small area of the sealed coating --- what conformal coating would you recommend over a previous coat?

    Hopefully, the focus will be to understand the board better based on:

    1. What the board needs to accomplish revealed by the schematic & links to previous posts.

    2. What the datasheets of the key more specialized key component datasheets shown on schematic say. These datasheets went over my head in terms of just going in with a mulimeter & checking the board that way while powered on.

    3. Hopefully, the pictures will show the proximity of those key components to capacitors, diodes, resistors, & the 24vac inputs & outputs. From that I am hoping to understand better how the board functions. After that step of further understanding --- I am hoping to zero in on the most likely area or component that could cause the previously stated symptoms.

    I wish I knew more about the relevance of all the additional components besides the 4 datasheet components. To my less trained eye it looks like the designer & fabricators went to extra lengths with the heat sinks based on the large protusions on each end of the board --- shown on the pics but not on the schematic.

    I did try to verify all the outputs with my Fluke 12 multimeter & I got 24vac false readings which I realized --- when I also used a 24vac load on the outputs. The 24vac load was an incandescent 24vac load with 2 leads on it. One lead to the output & the other to the neutral-ground. In other words the testing has to be done with a load & not just with the multimeter seeing voltage.

    I also have a B-C-E multimeter that can test a transistor in case someone explains how one of those 12 transistors close by the 12 leds could possibly be the cause of one or more of the symptoms described.

    Steward - Mike
     
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