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Current Circuits, DVM meter will short out when measuring on current circuits

Discussion in 'General Electronics Chat' started by Billy Mayo, Dec 17, 2013.

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  1. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Aircraft? It makes sense now.

    Anyway, I looked at the U19 and U20 stuff and I'll bet the transistor goes to leds and to a +24 V supply.

    So, you really have one easy test to make. Do a lamp test with lamps present and check the B-E junctions of the transistors. Without the LED's, it's gonna be tough to troubleshoot.

    The S/R FF's have some sort of Reset and they latch an error condition.

    That should be relatively easy to troubleshoot, although R is a pulse and is common, so any fault on that line could mess up all of the IC's. S is a momentary input.

    The inputs on R & S have to have nice edges.
     
  2. Billy Mayo

    Billy Mayo Member

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    But it is connected to an output of a gate PLUS a pull down resistor on the input why is that? the output of the gate that driving it is a LOW state

    How do you guys troubleshoot logic circuit when there is that BUSS line, If I disconnect a diode or transistor or resistor off of that BUSS line , the whole circuit gives different voltages because everything it TIED to that buss line

    So I do I troubleshoot if I can't disconnect anything that it TIED to that buss line?

    If something is shorted to ground or VCC or shorted to some voltage , how do i track that down, when components like that are tied to a BUSS LINE?
     
  3. Billy Mayo

    Billy Mayo Member

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    How do I do a Lamp Test with lamps in the circuit? they aren't LEDS or Lamps for sure, they are square and colored

    What do you mean by they latch an error condition? how so and why?

    What makes the have nice edges? and RC network? does the RC network make the edges sharper or rounded?
     
  4. dave

    Dave New Member

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  5. audioguru

    audioguru Well-Known Member Most Helpful Member

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    CR30, CR31 and R110 make an OR gate. When either or both inputs are high then their output to the input of inverter U7A is high. All those parts make a Mickey Mouse NOR gate.
    R110 to ground makes the inverter input low if the inputs to the diodes are also low then the output of inverter U7A is high.

    U7A feeds a "box" called U12 but the schematic has no part number for U12.

    Why did you lift D31? Did you measure the voltage (with a shielded 'scope probe and cable) at its cathode and anode before lifting it?
    When R110 makes the input of U7A low then should the output of U7A be high or low? Do you know?
     
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  6. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    J11-37 is a LAMP test. Some positive voltage there causes the "square things", I would guess to light although it could go to something else.

    The stability of the power supply voltages is first.

    What is the symptom? Lights on when they aren't supposed to be or lights off when they are not supposed to be? Random lights on each power-up?

    What happens when the LAMP TEST signal is present? Do all lamps light?

    Your measurements show 0 and 15 V which is a good sign.

    The resistors are not really pull-downs on the output, but they provide a place for the leakage currents to go. They are required for the circuit to the right of the FF.

    Look at the datasheet for the SR Flip Flop (FF). The suffix BF is a ceramic package.
     
  7. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    AG:
    Has to be a Classic.
     
  8. audioguru

    audioguru Well-Known Member Most Helpful Member

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    You need to learn about flip-flops and latches.
     
  9. Billy Mayo

    Billy Mayo Member

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    When you use Diodes and Resistors to make an OR gate or AND gate, what is this called?

    Cause I need to find schematics on the Diodes and Resistors and get the truth tables for these

    Yes both diodes have LOW logic stages, but how does a LOW logic stage get passed the anode of the diode? doesn't it BLOCK the Low logic stage from getting in and to the U7 input?

    4017 to a 4043


    To Isolate the board, to see if the short or problem is before or on that board, plus i can inject a HIGH STAGE to see if I was going to get the Comp fail light to TURN OFF

    It turns on with you enable the flip flops and that BUSS line with everything connected to it

    The Flip Flop that is connect to Q10 that turns on and off the Comp Fail light , should be at Zero volts , but i'm getting +14 volts

    I have changed Q10, Diode, flip flop, U12, U7 , CR13 , nothing has worked , the +14 volts is still on the flip flops output for Q10 , it should be Zero volts on the output of the Flip flop going to Q10
     
  10. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I stole it from Don Lancaster's Cmos Cookbook. (smilie)

    Where did the smilies go? I did a "set compatibility with Internet Explorer 11" to see schematics then my reply text became tiny when I type it and the smilies are gone.
     
  11. Billy Mayo

    Billy Mayo Member

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    Yes all lamps go on and then off , they blink

    Well you turn on different switches on the Test fixture and then I flip a big switch when leaves on the Computer Fail light, it shouldn't be on , it should be OFF

    The flip flop output of the computer fail light is at +14 volts, it should be at zero volts i think
     
  12. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    So, you probably have a pulse at pin 4 on U20 at the instant COMP FAIL turns on that should not be there.
     
  13. Billy Mayo

    Billy Mayo Member

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    The BIG SWITCH on the front panel turned on the Enable buss to the flip flops and to that Lamp test Buss? you called it , the lamp test buss is what the diodes and transistors are connected to

    The BIG SWITCH is SW1-1 that goes to CR28 and CR25 which is going to ARIC and ARID op amps that goes to that BUSS LINE

    The MEMORY Enable reset switch button ( J11-39 ) , switches off and on the Enable voltage to the Flip flops , that's the Enable buss

    Here is the picture again:
    http://s1348.photobucket.com/user/B...Lightstayson3_zps20ad3b01.jpg.html?sort=3&o=1
     
  14. Billy Mayo

    Billy Mayo Member

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    Maybe, but I changed U20 , Twice

    I changed Q10 , CR19, U20 , U12 twice also

    The 1 hz clock looks good to going to U12

    Something is making U20 has +14 volts on Pin#2 , Pin#2 should be at Zero volts not at +14

    Because +14 volts will turn on the COMP fail light

    I even Disconnected R57 , 30K resistor and U20 still has +14 volts

    What is that buss line called or what is it doing?
     
  15. audioguru

    audioguru Well-Known Member Most Helpful Member

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    It is called "making a logic gate with diodes and a resistor". The DTL logic family was like that but the diodes and resistor were inside an IC.

    It is obvious what they do just by looking at them.

    Another example that you do not know what a diode does. You do not even know which direction the signals flow because you have it backwards. I would explain it but I do not think you know which is the anode and which is the cathode and which way a diode conducts.

    You need to learn their truth tables. I simply look in my Cmos Cookbook.

    Another example that you do not know what a diode does and how it works.
    You did not say how you made a high stage and where you injected it.

    "with you enable" is very poor English. Maybe you mean, "when you enable".

    Simply look at the truth table for the set/reset flip-flop 4043 then see what is making its SET input High.
     
  16. Billy Mayo

    Billy Mayo Member

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    Yes but how do you tell which is a OR gate or AND gate ?

    And why did the designer want to have a OR gate infront of a inverter or a NAND gate?

    What am I missing? I thought a Logic Low can only go to the cathode of the diode

    The input signal is on the anode , the anode it connect to a Multiple connect that has inputs , the input going to the anode is a Logic LOW , so it can't pass through the diode right?

    If i put a logic low signal voltage on the anode of a diode will it pass thought the diode , no right? only through the cathode
     
  17. Billy Mayo

    Billy Mayo Member

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    SW1-14 TEST Switch connects , turns on or off the BUSS LINES , the enable buss for the flip flops and the Buss line that the diodes and transistors are tied to

    The Memory Reset button J11-41 turns On the RESET buss line to the flip flops

    The Comp Fail INPUT J10-44 goes to CR31 diode to R110 , 68 K resistor to U7A

    The Comp Fail Input is a Logic low zero volts, how does zero volts or logic low go through a Diodes Anode?
     
  18. Billy Mayo

    Billy Mayo Member

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    What is ARID output doing? what kind of circuit is it?

    ARID op amp output pin#14 is going to that buss line, what is that buss line doing? what is it called please?
     
  19. Billy Mayo

    Billy Mayo Member

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    U7 is a 4011, NAND GATE
    U12 is a 4017, Decade Counter
    U20 is a 4043, R/S latch, tri-state output I'm gussing?
     
  20. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    How does ARID pin 14 get to -14 V? Suspect CR11 at this point.
     
  21. Billy Mayo

    Billy Mayo Member

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    When I Turn on TEST Switch, SW-1 14 , it does to CR28 diode which does to ARIC op amp and to ARID

    When I turn on the Test switch SW- 14 , The ARID pin#14 outputs +15 first, than -13 volts , than +15 volts , it goes through this 3 step cycle really fast

    I think it Turns ON and OFF those switching transistors that are all tied on that buss line

    By doing 3 steps +15 , -13, +15 it turns on the CR19 diode and Q10 switching transistor i would think this is how it works

    Isn't that the whole point of that buss line connected to ARID pin#14? is to switch on the diodes and switching transistors? i'm just guessing
     
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