(Topic ID: 214767)

Sys 9: stray voltage on logic circuit on space shuttle


By Tbirdmatt66SS

1 year ago



Topic Stats

  • 2 posts
  • 2 Pinsiders participating
  • Latest reply 1 year ago by CadillacMusic
  • Topic is favorited by 1 Pinsider

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#1 1 year ago

Hello,
This is an update to an earlier topic I started about no 5volt logic.
I now have the +5vdc getting to the board. However, I severed many of the +5vdc traces to isolate areas of the board. In the lower right corner of the board around ic chips U50, U28 and Q51, I am getting (point) .3 or .5vdc at pins 16 on the chips and .3vdc at the collector side of transistor Q51. When I disconnect the -12vdc grey wire at 1J18, the stray voltage disappears.??? I thought U48 was suspect, but all it’s outputs check out against a working sys 9 machine. I also disconnected power to U47 audio amp.
Feel like I’m chasing a ghost, Any help is much appreciated.
Thank you,
Matt

#2 1 year ago

...do you even need -12? I don't see where it's even used. Why you're not getting 5v is more of an interesting question. What's the reading on the other side of R78?

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