7 Segment LED display problem - air hockey scoreboard

Discussion in 'The Projects Forum' started by chadecoen, Feb 15, 2016.

  1. chadecoen

    Thread Starter New Member

    Oct 14, 2015
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    My kids air hockey table scoreboard display lost part of the 7 segment LED displays. Segment "B" I believe on all of the displays, the clocks, home, and visitors score.
    I had opened up the unit years ago and replaced the speaker so I thought I would just open it up and find a loose solder joint and fix it. Well, couldn't find a broken joint. Problem is intermittent. I thought I "fixed" it by pressing on various parts of the board when the displays lit back up. But then they went out again and I am unable to pin point a bad connection. I took out the board and re-flowed every joint on it but same problem exists. Can sometimes get it to light back up but ultimately goes out again. Looks like same segment is out on every display. There are three 7 segment displays on each side of the scoreboard a count down timer, a visitors score, and the home score.

    I have looked around and am unable to source a replacement scoreboard.

    Any suggestions?
    [​IMG]
    [​IMG] [​IMG] [​IMG]
     
  2. AnalogKid

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    Aug 1, 2013
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    The 7 transistors in a neat little row, each with a 100 ohm resistor, almost certainly are the segment drivers. If you have 8 more transistors without collector loads, those are the digit drivers. First guess is that one of the segment drivers has a bad transistor or a loose end cap on a 100 ohm resistor or or or. Looking at the voltage across the 100 ohm resistors should identify the channel to investigate.

    ak
     
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  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Looks like a Harvard air hockey controller. Rebuilding our table was one of my first Microcontroller projects.

    You were smart to stop before it gets too much damage. Those transistors are some Chinese 8051 or something similar. Essentially any 2N2222A or bc337 should work fine.

    The Harvard design draws way too much current from the Microcontroller pins and burns those pins. Good to hear that you can still get it to work. I would replace the B segment transistor and also bump all of the 100 ohm resistors to 330 ohm.

    Note, not all transistors have the same pinout. bc337 (CBE) is revers from 2N2222 (EBC). You'll have to figure out what pinout those transistors have.
     
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  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Segment B is missing, it will be one of the flexible connectors that has a loose or broken resistor link, the transistors will be for the common Anodes, its not those.

    You can temporarily short the segment to another like A, to find the fault and move the short further along its path working back to the chip pin.
     
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  5. chadecoen

    Thread Starter New Member

    Oct 14, 2015
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    WOW, thanks everyone. I will be stopping by the store sometime today for some parts and hopefully dig in tonight. Looks like I just need to find and replace the bad transistor and go ahead and replace all the resistors with 330 ohm. I am so excited to have this direction. I really didn't know what I was looking at. Thanks again everyone.
     
  6. GopherT

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    Nov 23, 2012
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    Before you go too far, can you describe what exactly caused the B segment to come back on? Were you messing with the pcb, the connectors, the display?

    Also, if you move on to replace parts, DONT OVER HEAT THE PCB. these boards are not really meant for rework and the copper pads can come loose from the board when heated too much and then you have a real mess. If you don't have one, use a temp controlled soldering pencil and solder wick to remove solder or a good solder sucker/desoldering station (re-work station).

    To minimize stresses on the board, you can try to splice in a 220 ohm (or 33o ohm) resistor across the exposed lead on the vertically-mounted 100 ohm resistor. (Clip it in the middle of the exposed wire and bend the top wire up straight. Then splice the new resistor across the two segments).

    Good luck.
     
  7. AnalogKid

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    Don't think so. Next to the 7 transistors with 100 ohm resistors are 8 more with 1K resistors. My guess is that those are the 8 digit drivers with 1K base resistors. If the segment drivers are emitter followers, they would be the same transistor type as the digit drivers and not require base resistors, typical ultra-low-cost design practice.

    ak
     
  8. GopherT

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    Nov 23, 2012
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    @chadecoen

    Put a hold on my comment. According to @AnalogKid, they may have changed their design a bit since I re-did my scoreboard. Mine had 100 ohm in to the transistor base and then 100 ohm on the collectors of the segment drivers.

    As analog kid said in his first post, you need to measure voltage across the transistors to confirm where the problem may be.
     
  9. chadecoen

    Thread Starter New Member

    Oct 14, 2015
    14
    2
    Not sure exactly. Yes, it came back on while pressing on the PCB. I was pressing on the larger raised green part of it. Never seems to happen while pressing the exact same spot but has happened several times while pressing in that general area. Could not pin point a "bad" spot to be the cause.

    I'll have to source one of those. I have some very basic solder tools. I will go ahead and stop by store and pick up some parts just to have. I will try and dive in with a meter tonight to put together a little more specific repair plan.

    Again, I really appreciate the help here. You guys are awesome.

    Measuring voltage across the transistors........ which ones? all of them? And what am I looking for measuring voltage? Like measuring engine compression in an eight cylinder, just a lower number than the rest or zero or ?

    Does a marginally better pic help?
    [​IMG]
     
  10. AnalogKid

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    Based on both a hasty freshman generalization and a massive assumption not supported by fact - it looks to me like the 1K resistors are going to the center pins of 8 transistors, so I'll stick with those being base resistors to the digit drivers.

    ak
     
  11. GopherT

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    Nov 23, 2012
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    I agree now that I see the clearer photos. The 1k likely go to the base pin of this SS9012 PNP transistors (too shaded to see in the earlier photos).
    The other bank of 7 transistors (assume NPN) has a 100 ohm resistor, I assume those limit current out of the collectors and on to the LED (and back through the PNP mentioned in paragraph above).

    Now, my concern is that there is no resistor between Microcontroller and NPN. I would really need photos of the bottom of the board. To trace out the rest of this mess. On the board I replaced, there were no 1k resistors, just 100R. The Microcontroller pins were fried and had no output (which I blame on excess current).
     
  12. chadecoen

    Thread Starter New Member

    Oct 14, 2015
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    10/4 guys, will pull it apart and take better photos ASAP. Thanks!
     
  13. AnalogKid

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    Aug 1, 2013
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    PNP - surprised. Still (clinging to my theory with bloodied fingernails), the segment drivers could be PNP emitter followers, not requiring base resistors.
    ak
     
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  14. chadecoen

    Thread Starter New Member

    Oct 14, 2015
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    See if this helps at all. More slightly better photos. I paired the top and bottom pics of the pcb and took a couple close ups.

    [​IMG]
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  15. chadecoen

    Thread Starter New Member

    Oct 14, 2015
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  16. Dodgydave

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    Jun 22, 2012
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    The 1K transistors are pnp these are the common Anode feeds, they are ok leave them alone,

    the 100ohm transistors are pnp feeding the segment Cathodes, so start looking at these seven transistor pulses with a dvm.
     
    Last edited: Feb 17, 2016
  17. AnalogKid

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    Check the markings on the two groups of transistors. It doesn't make sense to me that the designer would have base current limiting resistors for the PNP's but not for the NPN's. Quickly - my fingernails are getting sore.

    ak
     
  18. GopherT

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    Nov 23, 2012
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    I was pretty new to electronics when I rebuilt the scoreboard controller. Am happy to hear that you find the design odd as well. My board had the 9012 transistors (which I assumed were SS9012 PNP transistors) and the others were marked 8050 (assumed to be SS8050 npns).

    I no longer have the original board - I rebuild everything onto a perf oard using a PIC, I wish I would have kept everything I later found out how many people have the same problem with this brand of air hockey table. After 5 years of use, my air hocky table is now the platform for out pingPong table and that has become the staging airia for my electronics projects.
     
  19. chadecoen

    Thread Starter New Member

    Oct 14, 2015
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    Hang on AK!

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  20. AnalogKid

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    Aug 1, 2013
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    Looks good to me. Now, back to the original problem. If one segment is missing from all characters, it has to be that segment's driver circuit:

    Something on the uC mezzanine board
    uC output connection from uC mezzanine to main board
    Transistor
    Resistor
    Connector on main board
    Cable
    Connector on display board

    This is a multiplexed display system, so unless you can get 8's on all characters, the segment outputs will be variable square waves. Scope? Start with the ends of the resistors toward the uC, and compare a known good output to the suspect output. Depending on the characters being displayed, the pulse train timings will be all over the place but the amplitudes should be consistent. The known good signal will show you what a signal should look like in terms of amplitude, high side flatness and noise, rise and fall times, etc.

    ak
     
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