(Topic ID: 284538)

Bally -17 Stuck ON LED

By oldschoolbob

3 years ago


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  • 15 posts
  • 4 Pinsiders participating
  • Latest reply 3 years ago by Quench
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    #1 3 years ago

    Some may recall I worked on this board back in early summer.

    https://pinside.com/pinball/forum/topic/how-to-determine-what-game-eproms-i-have

    It was working fine. Once I verified the EPROMs I put it on the shelf as a spare. (We’ll call it board A)
    Recently I was working on another board where I replaced an IC socket (board B).

    https://pinside.com/pinball/forum/topic/how-do-you-remove-sip-sockets

    Both boards are Bally -17s – both are jumpered for 2716 EPROMs. When I pulled out board A to compare some voltages and now it has a stuck on LED. I followed the steps in PinWiki. I replaced Q1, Q2, and Q5 (I replaced Q5 last summer but replaced it again). I replaced U8 socket last summer so I replaced all the other sockets. I installed new U9 and U11 ICs. I installed Leon’s test ROM (same one I used for board B). I have 5 volts at U9 – pin 40.
    I checked the voltage across VR1 = 7.77 VDC. That seemed low but I checked board B and it was the same = 7.77VDC.

    IMG_6178 (resized).JPGIMG_6178 (resized).JPG

    Today I checked the voltages in the Valid Power Detector section and compared them with the working board. Green = board A (stuck on LED) Red = board B (working). Everything seems to match closely.

    reset 1 volts (resized).jpgreset 1 volts (resized).jpg

    Then I connected to the oscilloscope to check that the 5 volts was being held low. They both seem to be held low for about 10ms. (Notice that one scan is at 4ms per division and the other is 2ms per division). The yellow probe was on the bottom of R 139 and the blue was connected to TP5.

    Scan of board B - working

    pic_30_1 (resized).jpgpic_30_1 (resized).jpg

    Scan of board A - Stuck on LED

    pic_30_3 (resized).jpgpic_30_3 (resized).jpg

    Everything seems to check out but I still have LED stuck on.
    Anyone have any suggestions what to check next?

    Thanks
    Bob

    #4 3 years ago

    My power supply has one switch for both 5 and 12 volts.

    According to the oscilloscope the 5 volts is being held low for about 10ms - on both the working and non-working boards.

    This is the third U9 chip.

    Is there anything outside the Valid Power Detector that could keep the LED locked on?

    Thanks

    Bob

    #6 3 years ago

    The other day when I followed PinWiki I was getting pulsing on U9 pin 3, 36 and 37. Also got 5 volts at pin 2 (HALT). And pulsing on pin 5 (VMA).
    I have 5 volts at U9 pin 40. I shorted pins 39 – 40 and the LED stays on.

    I’ll check everything again tonight.

    I have not checked activity on U6. I’ll do that tonight also. For some reason I suspect that’s the problem but I used the test ROM and the original MB8516 ROM and still locked on. That’s also the reason I replaced all the IC sockets.

    Quench, check your PM.

    Thanks

    Bob

    #7 3 years ago

    First I tried U6. After probing each pin and logging the results I noticed the LED was off. I turned it off and took a break then tried again. The LED remained on. Then I tried a third and fourth time. The last two attempts had the same results. I don’t think anything was pulsing. (but I could be wrong).

    Then I probed U9. I did U9 twice and both times were the same results. I’m more confident in my U9 results.

    There is a lot of data so I put it into a spreadsheet.
    H = high
    L= low
    P=pulsing light
    0 = nothing on the logic probe

    It looks like the VMA (pin 5) is no longer pulsing. Both times it showed high. I’ll check it again tomorrow.

    Thanks

    Bob

    LOGIC 2 (resized).jpgLOGIC 2 (resized).jpg
    #9 3 years ago

    I just went back to the shop to check the VMA - it's stuck high - no pulse.

    I also checked the continuity from U9 -p13 to U6 -p4. it shows open. but i have continuity from U9 - p13 to pin 4 at U1, U2, U3, U4, U5. something between U5 and U6 - I'll check it next.

    Bob

    #10 3 years ago

    Something very strange going on here. I have continuity from the pad at U5 - p 4 to where the trace goes under the socket at U6 - p 4. But I show open from the trace up to the socket. See photo 1. Also open on the other side of the socket. See second photo.

    IMG_6194 (resized).JPGIMG_6194 (resized).JPG

    IMG_6188 (resized).JPGIMG_6188 (resized).JPG

    I think i found it. Glad I take photos when i do work on any boards. Look closely at the solder pad at U6 pin 4.

    IMG_6166 (resized).JPGIMG_6166 (resized).JPG

    It's broke in half. I'll replace the socket tomorrow.

    But I don't think that's the total problem. I had stuck on LED before I changed the sockets. Would that open solder pad cause the VMA to stay high?

    Thanks

    Bob

    #12 3 years ago

    Tonight I replaced the U6 socket. I checked all the pins for continuity. I plugged in my test ROM. I got a flicker then constant flashing. I connected my logic probe and tested U6. Everything showed high - low - pulsing except 12 = low, 19 = low, 20 = low / pulsing, 21 and 24 = high, 22 and 23 = low. Then I checked U9 pin 5 (VMA), it showed high/low - pulsing. I'm sure it was stuck high yesterday.

    I then put all the original ICs back in and it booted fine. Other than the busted pad at U6 - pin 4 I'm not sure what the problem was. I'll continue testing it for a couple of weeks.

    Thank you all for the help.

    Bob

    #14 3 years ago

    The way I understand Leon’s test ROM (and I’m no expert), is it keeps changing the signals from high to low in one second intervals so you can use a logic probe to see if they are changing. The LED flashes with the changes.

    Quote from PinWiki:

    “This Eprom has a short program which does:

    Set PA 0 to PA 7 and PB 0 to PB 7 of Chip U11 and U10 HIGH and then LOW,over and over again.....(interval=1 second)
    Same for CA2 and CB2 of U11 and U10.

    This program does not lock-up. This means if an output of a chip is bad, the program will continue to work. This way you're able to use a led in series with a 2Kohm resistor,a multimeter or a logic-probe to read all outputs of chips and know if everything works. To have some means of control that the program runs ok , we connect a control-led who will flash in rhythm with the test . The led on the cpu-board will also flash, because it is tied to the signal CB1 of U11, this of course if U11 is working normally.”

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