basic question on z80 cpu and 5 volt line

Discussion in 'General Electronics Chat' started by tester272001, Oct 31, 2012.

  1. tester272001

    Thread Starter New Member

    Aug 24, 2012
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    What is the tolerance on a +5V line for this older series processor? For instance will a 4.6V do the job or does it need to be 5V exactly? I am diagnosing a dead Pacman board and the it reads 4.6 volts. I have another similar board and it is +5.1 - it makes me think that either the regulator/associated circuitry is off or one of the ICs is dragging down the 5V line. Perhaps someone on here has done a bit of tinkering with the Bally Midway boards ?
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,418
    3,355
    Some specs show Vcc = 5.0V ± 5% (4.75V - 5.25V)
    others show Vcc = 5.0V ± 10% (CMOS) 4.50V - 5.50 V

    Do you have the actual part number and manufacturer?
     
  3. tpny

    Member

    May 6, 2012
    216
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    might could put an ammeter at the 5v power input to see how much current is being drawn; compare readings between two units. maybe the 4.6v unit's got a short somewhere..
     
  4. tester272001

    Thread Starter New Member

    Aug 24, 2012
    18
    0
    The ammeter is a good idea. I will see if I can locate the exact model of the z80 too. This board was "worked on" by someone before and it had TWO solder bridges. On was a DEAD short on the 5V line the other was on an IC (took a while to find those!). Fixed both of those shorts and replaced a couple IC that were questionable. The main diff. was the +5V line. There is also a 16V line and that is good on both boards. Thanks for the suggstions. I will report back...:)
     
  5. tester272001

    Thread Starter New Member

    Aug 24, 2012
    18
    0
    BTW - this is a Z80A CPU - I tried plugging the cpu into a known good board and no dice. So that cpu is probably toast. (its on a daughter card and pin 11 is getting +5v just all garbage on the video out). Still got to do the current reading as tpny suggested. When removing the jumper for 5V isolation my voltage readings vary by only .5 volt on the bad board compared to 2.5V on the good one. (for instance 7.4 vs 5.5) - I also scoped the caps and only 400mv of ripple (even on the known good board) so I think the caps are ok. Used an esr meter and they are in spec. I am suspecting the regulator/pass transistor. I also am considering jumpering the 5v with an external +5 supply to see if the voltage drops still (that would prove if the supply if at fault of the downstream ICs.
     
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