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(Topic ID: 206066)

Gottlieb System 1 memory RAM issue

By ripple

2 years ago

Topic Stats

  • 8 posts
  • 4 Pinsiders participating
  • Latest reply 2 years ago by noflip95
  • Topic is favorited by 1 Pinsider


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    #1 2 years ago

    I have a System 1 board I am working on that appears to not be able to save any info to the RAM. The 5101 RAM is tested good. It will only show garbage in the memory. I can attempt to clear the audits to 0 in the self test but it won't clear when I go back and check the audits . I won't display the credits or high score , it just shows garbage. I think no information is being sent to the RAM. The board has had a lot of work done. Any ideas on an easy fix would be appreciated.

    a72 (resized).JPG

    #2 2 years ago

    Does the machine play?

    Replace that yellow thing with a real battery.

    You should familiarize yourself with this:

    #3 2 years ago

    The board works 100% except the memory issue. The battery is only temporary.

    #4 2 years ago


    It's normal that the memory doesn't works properly

    Indeed, if you look at the datasheet of components, it is said that the 5101L RAM require at least 2 volts to keep data in stand-by mode. There is also some CMOS circuits in the circuit (Z1, Z2 - 4081 + 4528) powered by VAB. The VAB voltage is directly from the battery, only thru a 62 ohms resistor, and the schematic say "3.6 v battery".

    The "yellow thing" on your board is a single cell of a Ni-Cad (Cadmium Nickel) battery. The voltage delivered by this unique cell is about 1.2 volts (could be up to 1.4 when well charged). Sure this is TOO LOW to ensure a normal retention for the 5101L.
    The voltage is simply not enough to power the components.

    #5 2 years ago

    The battery is 3.6V and I am getting nearly 3.5 volt to the RAM. Also I have had a 5101P in there and it did not work either. I did some poking around with my meter on the Z1 and Z2 and got a funny reading on the Z2 when compared to another board. Will see if I can find a Z2 chip and replace it. The board is a back up and not in use. I saw the other topic on 5101 ram in system 1 boards and was inspired to take it out of storage and see if I could get the memory working. Thanks for the help.

    #6 2 years ago

    Per Clay's repair guide, the blue caps next to the 5101 can go bad and cause this. This fixed one of my boards that did the same thing, but not another which I haven't figured out. Hope that might get you a better result.

    #7 2 years ago

    Will check those caps, I missed that in the guide.

    #8 2 years ago

    OK, this yellow stick is a "special backup" nice for tests only

    Yes, the problem can be in the memory protection circuit, around Z2. Just a little explanation how its works :

    The RAM 5101 and CMOS (Z1, Z2) are powered by VAB source (the battery @ ~3.5 volts). Their functions are the following:

    - Z1 (4081) act like a big AND GATE between three signals : a basic chip select from the decoding, and two others signals Q1 and Q2.
    Q1 and Q2 are protections at power-up and power-down.
    Their role is to disable the RAM access (and especially write), when the power become unstable. This prevent spurious access at startup and stop.
    Q1 and Q2 are only single pulses at power change, but remain stable after.

    - The "zener group" (CR38, CR37, R156, R157 and Q5) is a voltage threshold detector. It detect when the +5 volts drop (at power-off) or rise (at power-on), and then trigger the 4528.

    -This 4528 circuit is a double monostable: It generate a delayed pulse on Q1 and Q2. The monostables are separate, one is used with positive edge, and the second is used with negative edge. It's simple, one is used for power-on, one is used for power-down. The duration of the pulses is determined by the couples C31/R159 and C32/R158.

    What to check ?
    As you have a voltmeter and another board, you can compare some signals.

    First, check that the return-protect diode CR26 is good. When power is off, the +5 volts line should be 0 ! if you get ~3.5 v, then the battery power all the board (and also the ram protection as mentioned above - so detection could be inefficient).
    Check the power voltage at Z1 and Z2 (pins 8 / 16), should be VAB (~3.5 volts).
    Check the voltage at the 4528 triggers entries (pins 4 and 11, when power is on, and off) - could show a problem at the "zener group".
    Check the stability of the Q1 and Q2 signals. This can show a problem with Z2 or R/C couples.

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