System 11 MPU no VMA signal

(Topic ID: 200481)

System 11 MPU no VMA signal

By ekthoren

1 year ago

Topic Stats

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


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#1 1 year ago

I need some help. I have a system 11 (not an a, b, or c the first one) board. It does boot but not properly. I have Leon’s test eprom. When I hook it up it does not cycle through the tests. When I check the signals on U15 6802 chip I get only .11volts on pin 5 VMA and 5.0 volts on pin 37 signal “E”. Pins 2,3,8,35, and 40 all have 5.0volts as does my working system 11 board that I’m comparing this too. Pin 39 is 2.0volts on both boards. I’m using a dmm not a logic probe. So I’ve changed u37 and u32 and their is no change. I’ve also changed u15 and their is no change. I’m not sure where to go from here and any help would be appreciated.

#2 1 year ago

If the E (external clock pulse) pin on the cpu is not pulsing either the CPU is dead, the crystal+oscillator is not working, or a chip connected to CPU E pin is dead (shorted high).

Check the E pin of the other CPU. Note the Hitachi vs Moto CPU type jumper. When using an external oscillator like the sys 11 cpu does, the hitachi and moto cpus have different needs (pull up vs floating of one of the internal oscillator pins I think it is).

#3 1 year ago

Ok disregard this thread. I somehow managed to kill the 6802 when I was doing testing because when I replaced it (again) the VMA signal came back. Now I’m back to my original sound problem which I never mentioned in this thread.

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