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Mirco Challenge PCB Repair

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Share any logs or helpful information you may have to repairs of arcade PCBs, Monitors etc

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  • #16
    Oh wow what an amazing find, John! Thank you very much for the link! Funnily enough I was looking on their site just the other day for something else!

    it is great how you can play 1, 2 or 4 players and you can play with 1 paddle or 2!

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    • #17
      Hi jbtech thank you so much for all the awesome info! I am from the UK and recently purchased a Mirco 4-player Pong table. Some of the pots were a bit stiff, but I managed to clean them out and they are all super smooth now. The issue I am having is everything works (monitor, sound, pots, scoring, and game logic etc) except there is no collision detection on the balls when they hit the paddles, they just pass through. There are no obvious issues on the PCB, but it may require a closer inspection. I wanted to ask in your experience, do you know which ICs or areas would be a good point to start checking for this issue?

      Cheers,

      Tora

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      • #18
        Hi Tora,

        I haven't had that exact issue but Kaizen mentions a similar issue in one of his repair logs:-

        https://www.aussiearcade.com/forum/a...b-repair-log-1

        Aside from first making sure the power supply voltage is correct, most other issues will be due to the TTL logic ICs failing, with various symptoms so keeping these logs will eventuallly help someone who experiences the same symptoms with their machine.

        As no circuit diagrams or other service information for the game PCB have been shared by anyone as far as I know, we have to locate the source of any new issues the hard way, either tracing the circuit from a known point until a signal is not as expected or just checking the inputs and outputs of every IC with a logic probe or oscilloscope and looking for stuck outputs or invalid voltage levels (i.e. not TTL High or Low) until we find a fault.

        My own PCB is back on the bench while I sort two new issues, the first was a limited range of movement of one of the paddles, caused by a faulty ceramic capacitor which measured about 20kOhms resistance (should read as an open circuit in a resistance measurement) on the input to the LM339A voltage comparator.

        The second is unusual, although the collision detection is working and the game plays correctly sometimes there is no sound when the ball rebounds from the lower boundary.

        So I'm tracing back from the sound output to find the collision detection for the bottom boundary and then trying to see where the signal goes missing or is of marginal level.

        In this case I'm tracing back from the sound output at IC location G3 pin 8. The three inputs to this 7410 are the tones for collision with the bats (pin 9) Serve tone (pin 10) and top / bottom boundary (pin 11)

        So in my case tracing back from pin 11 via ICs at locations G1 and E3 to F9 the collision detection for the bottom boundary appears on pin 3 of the 7400 IC at location F9.

        Likewise, tracing back from pin 9 of IC G3 should eventually lead to the collision detection for the paddles. Unfortunately this method is very time consuming and requires good note keeping to avoid confusion.

        It's definitely a job for someone familiar with this (old) technology so if in doubt look for a repairer with a good reputation in your area. The UK VAC forum may assist you in this regard as well.

        I will post more details of my current repair here shortly and will definitely make a note if I see a potential cause of the issue you mentioned along the way.

        Hope this helps in some way. P.S. love the photo of the Illinois Revenue stamp (appears to have expired though)

        Regards, John
        more from John's Retro Workshop - www.jbtech.linkpc.net

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        • #19
          Hi John,

          Thanks so much for your help! Yes that definitely helps me get started, I will take a look at Kaizen's link also. It's nice to know other people are working on these machines. I will start with ic G3 and work my way through.

          Haha yeah might be a bit overdue. I will let you know if I find the source of the collision issue.

          Cheers,

          Tora

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          • #20
            Hi all,

            Here's a little update with the Mirco Challenge PCB repairs. The latest problem I've been working on was the occasional absence of a sound when the ball rebounds from the lower boundary. Interestingly the collision detection was working so the issue had to be somewhere between there and the sound output.

            Tracing the circuit back I found the origin of the collision signal for the lower boundary at IC F9, a 7400. It was difficult to get a good look at the quality of the signal with my analog oscilloscope which has no storage ability, the signal not having a regular repetition rate. The levels looked OK though.

            Following F9, 2 gates of IC E3, another 7400 forward the signal (combined with the top boundary collision) to G1, a 74109. Attempting to locate the exact point where the signal was being missed, only occasionally - while keeping the ball in play proved to be very time consuming and ultimately inconclusive so, taking a punt I decided to replace E3 with a 74LS00, also installing an IC socket.

            Testing again, the lower boundary sound was still occasionally missing. This transferred my suspicion to IC F9 itself - even though the collision detection was working and the ball rebounding, by the time the pulse reached E3 at the other end of the PCB it was occasionally failing to trigger a sound via E3 and G1. Perhaps the issue here was more to do with rise time than voltage level.

            In any case, replacing F9 with another 74LS00, installing an IC socket in this location as well, appears to have solved the problem and all the sounds now seem to be working correctly.

            For each part of the PCB which I've worked on, I've sketched a diagram of the relevant parts of the circuit. Although far from complete the diagram is taking shape and may help with troubleshooting other PCBs with similar issues.

            While I have the PCB on the bench it also seems like a good opportunity to try and trace the collision detection for the paddles, which is working perfectly on my PCB. Hopefully this may help torakai with the issue above and may even help me or someone else down the track...

            So here we go.. Because the collision triggers a sound we can once again trace back from the sound output at IC G3, a 7410. The paddle collision tone arrives at pin 9, this in turn comes from IC E2, a 7400. Its two inputs are the tone frequency on pin 2 and the collision signal on pin 1.

            Tracing back from there we reach the other half of IC G1, a 74109, in this case pin 6 which is a Q output. Checking its inputs now I find the collision signal arriving on the preset input, pin 5. The incoming pulse length must be quite short as it doesn´t show up clearly on my oscilloscope display but is detected by the trigger circuit. Switching to my cheap but funcional logic probe the signal shows up on the 'pulse' LED each time the ball strikes a paddle.

            Now tracing back further via a 7404 inverter at IC D3, 7402 NOR gate at IC G2 all the way to a 7410 3 input NAND gate at location C9, this appears to be the origin of the collision detection between the ball and paddles

            When tracing a circuit back to its origin we must look for the output of a previous stage though we may also locate other inputs which are fed the same signal. If we trace back to an open collector output there is also the possibility that multiple open collector outputs could be connected (i.e.wire ANDed) together. In this case the combied signal would be the product of the first output AND the second output AND any others hence the term.

            The 3 inputs to IC C9 appear to contain the position of ball and paddles, producing an output when the three inputs coincide, i.e. all are true. Of course if any of these 3 signals were missing there would be no collision detected but I suspect in that case the characters on screen would also be affected. If the paddles and ball are all present and correct but there is no collision detected my bet would be the 7410 at location C9 is at fault

            As yet I haven't taken the time to draw up the final circuit diagram but have hand drawn a rough draft which will have to do for now, here it is: Regards, John

            Click image for larger version

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            Last edited by jbtech; 8 May 2021, 09:31 AM. Reason: Corrected error in draft schematic
            more from John's Retro Workshop - www.jbtech.linkpc.net

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