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    Thread: Outrun ic95

    1. #1
      Arcad3's Avatar
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      Outrun ic95

      Hi all,

      Unfortunately I am facing major problems with my Sega Outrun (Upright)...
      The game won't boot anymore after replacing IC95 (SN74ALS138AN)

      I found out that pin 6 on IC95 was only 1.82VDC and did connect it to the VCC (pin 16, 5,25v) to make pin 6 a HIGH or 1.
      I did this because according to the datasheet of the sn74als138an chip 2vdc is the minimum for a HIGH and 0.8vdc is the maximum for a LOW. It made pin 6 look suspicious.
      Also according to the logic table pin 6 (G1) on low means that the outputs are not able to change at all.

      sn74als138an CHIP.png
      sn74als138an logic table).png

      Since the game stil doesn't boot after the repair... I am wondering if someone with a working Outrun cab can meassure the voltage (vdc) on IC95 from pin 6 (red probe) to pin 8 (black probe) and reply the answer so I can confirm that the 1.82vdc was wrong?
      THANKS!

    2. #2
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      Quote Originally Posted by Arcad3 View Post
      I found out that pin 6 on IC95 was only 1.82VDC and did connect it to the VCC (pin 16, 5,25v) to make pin 6 a HIGH or 1.
      So what you say is pin 6 is tied to pin 16 but you read only 1.82V on it.
      It must be +5V unless the connection is poor (cut/corroded trace).
      Are you sure of that connection?
      [EDIT] I got it you forced it to +5V. That's not good, it's probably driven by another chip.

      Quote Originally Posted by Arcad3 View Post
      I did this because according to the datasheet of the sn74als138an chip 2vdc is the minimum for a HIGH and 0.8vdc is the maximum for a LOW. It made pin 6 look suspicious.
      Also according to the logic table pin 6 (G1) on low means that the outputs are not able to change at all.
      Are you sure it's not driven by something else rather than being tied to Vcc?

      Quote Originally Posted by Arcad3 View Post
      Since the game stil doesn't boot after the repair... I am wondering if someone with a working Outrun cab can meassure the voltage (vdc) on IC95 from pin 6 (red probe) to pin 8 (black probe) and reply the answer so I can confirm that the 1.82vdc was wrong?
      THANKS!
      If the signal is driven it makes no sense to measure voltage as signal is just constantly changing state and what you measure is the mean/average value.
      Only a scope can show what it's doing.

      [EDIT 2]
      Quick checks you should do if game doesn't boot:
      - check for a valid clock on the main CPUs (68k)
      - check vital signals on the CPUs, especially /RESET and /HALT (active low)
      - check program ROMs
      - check work RAMs
      etc.

    3. #3
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      Yes I forced it indeed On the Outrun schematics pin 6 is connected to an arrow going upwards (symbol) I have measured two more pins from other ic's which were also connected to an upward going arrow symbol and both were 5v...

      Can it be that an upward arrow symbol is driven by another chip?


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      Quote Originally Posted by Arcad3 View Post
      Yes I forced it indeed On the Outrun schematics pin 6 is connected to an arrow going upwards (symbol) I have measured two more pins from other ic's which were also connected to an upward going arrow symbol and both were 5v...

      Can it be that an upward arrow symbol is driven by another chip?


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      I had a look at the schematics, you're correct it's tied to Vcc (if schematics are correct).

    5. #5
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      Thanks! Good to know... I will keep it like that then.

      Unfortunately this was not the reason for stopping the pcb from booting and later even kill my crt (?). First stripes were flashing on the crt, then the fan stopped and also the monitor stayed black.


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      Power issue?
      Also you +5V is a bit high for my liking. All original games should run fine at +5.00V (I made tests on hundreds of OG PCBs and they all played fine at +4.85V).
      Some DRAMs starts to die at only 5.15V (but don't worry Out Run isn't concerned).

    7. #7
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      Outrun ic95

      Hmm until now I haven't found any dead ic's but I have probed the Rams for the sound and all lights of the probe (high, low and pulse) went on without flashing. On other rams the probe lights were not lightning up all together and they were flashing, like there was activity. I don't know what it means when all lights stay on on the probe.
      Also the IC for the sounds and the sound cpu felt really hot...


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      A logic probe isn't enough to make a good diagnostic. For instance a completely dead RAM can show activity on its data pins cause signals are bussed. In this case what you see is activity coming from somewhere else.

      What's the brand of your logic probe? Don't tell me it's Eistar.

    9. #9
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      Ooooh I didn't know that! How do you test ram then?

      On mine there's know brand written, it only says LP-1. Since the probe looks similar to the Eistar I guess it is...
      It was a cheap probe... Don't they meassure the right values?


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      The skinny 6264 rams on Outrun pcbs have a high failure rate, both 68k cpus use them as work ram. Replacement skinny rams aren't to easy to find these days either.

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