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Monostable divide by 5 not working.

Discussion in 'Electronic Repair' started by David C. Partridge, Nov 14, 2005.

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  1. I've a problem that should be simple to solve, but I must be missing the
    glaringly obvious, as the solution eludes me.

    This is a discrete monostable divide by 5 countdown circuit being driven by
    pulses every 1mS on the input, and should drive a pulse every 5mS at the
    output.

    I've checked the transistors (and subbed them just in case), the diode, and
    the resistors, and replaced the 0.1uF timing capacitor, but still the darn
    thing triggers almost randomly (though there is some preference for 5mS).

    Here's the circuit (resistors using : characters) - use a monospace font to
    view.

    +12V
    |
    :
    :3K6
    :
    |
    ------------------------o
    | |
    | |
    | |
    | +12V +12V |
    | | | |
    | : : o--||--o--o-- Output
    | : 2K2 : 61K9 | .2uF | |
    : : : | : |
    39K0 : | | |/ 9K1 : --|<--
    : o--||--o-o-->|--o--| : |
    | | 0.1uF | | |\>- | ___
    | |/ --||-- | ___ .
    ->|-:::::-o-| 22pF ___ .
    10K0 |\>- .
    |
    ___
     
  2. Guest

    Wow, been awhile. It sounds as if you need a 5 stage ring counter.
    (pausing remembering just how to do it)

    Using 5 J K flip flops you hook all the Ts together and in a ring (thus
    the name) you tie the J to the Q and the K to the not Q of the previous
    flip flop.Output at any of the five phases can be tapped at any of the
    Q terminals for a 20% on time, or the not Q for 80% on time.

    How far in the cowbwebs of my mind did I have to go ? The last time I
    thought about ring counters was when I was builing a bootleg cable
    decoder for the old SSAVI2 scrambling method.

    There are also programmable dividers if you want a one chip solution.
    With 7 terminals per FF, you only can get two in a 16 pin DIP so this
    means three chips minimum. If you go with the ring counter you don't
    use the other two, C and cD I think, you should consult the datasheet
    for whatever chips you use to see whether you should ground them or
    leave them float.

    To sum it up, digital division is the only way to get reliable
    operation. If you wind up with three dual flip flops you can also use
    the last one to get on times other than 20% if desired.

    Good luck with it.

    JURB
     
  3. If I were starting from a "blank sheet", I probably would use a digital
    divider chip or several. BUT, this is an attempt to repair to a venerable
    piece of test equipment (Tektronix 184 time mark generator), which I'd like
    to keep pretty close to original ...

    TIA
    Dave
     
  4. Dave D

    Dave D Guest

    You've probably checked the obvious but is the supply to the divider the
    right voltage, well decoupled and free of glitches/ripple?

    Dave
     
  5. Asimov

    Asimov Guest

    "David C. Partridge" bravely wrote to "All" (14 Nov 05 20:38:19)
    --- on the heady topic of "Re: Monostable divide by 5 not working."

    DCP> Reply-To: "David C. Partridge" <>
    DCP> Xref: core-easynews sci.electronics.repair:348583

    DCP> If I were starting from a "blank sheet", I probably would use a
    DCP> digital divider chip or several. BUT, this is an attempt to repair to
    DCP> a venerable piece of test equipment (Tektronix 184 time mark
    DCP> generator), which I'd like to keep pretty close to original ...

    DCP> TIA
    DCP> Dave


    Geez, you guys, this student just wants us to do his homework for him!
    Actually, the circuit is simply a monostable which is not retriggerable.
    Basically, it's a pulse stretcher. It starts by discharging the 0.1uF
    cap through the base resistor of Q2. Because the base is driven hard
    negative, a series diode has been added to protect Q2 from reverse
    bias breakdown.

    The trouble with the circuit is that the input pulses might not be
    properly biasing Q1 when high or low and then the trigger point is
    constantly changing. Also if the input is driven through a capacitor
    then its time constant may be further randomizing the trigger point.

    A*s*i*m*o*v

    .... When I was your age, we carved transistors out of wood.
     
  6. Actually, no, not a student.

    D.
     
  7. Thanks, will check that again, may well be relevant. All the other
    countdowns are working (at least all the way from .1uS to 1mS). It's the
    5mS that's all over the floor.

    Dave
     

  8. Homework? It's the most primitive divide by five counter I ever saw. Can't
    imagine a master to invent such a thing for a question. Actually, as this
    group is about repair, I guess it's an existing circuit. This type of
    circuit is very sensible. Noise and/or disturbances on input or power make
    it fail easily. It really needs uniform inputpulses. As I see no possibility
    for adjustment *and* about all parameters of all components influence
    working, I guess the components need to have high accuracy. The original
    transistors might as well be specially selected. Aging of components,
    especially the transistors, might be caused the problems. As an aside, you
    may need to check or replace the decoupling capacitors of the power supply
    near the circuit.

    petrus bitbyter
     
  9. Agreed it is primitive, but it is an existing circuit put there by
    Tektronix, and yes, the component tolerances are critical on the 0.1uF
    capacitor and the 31.9K resistor - if they are off, you get a divide by four
    or divide by 6 (typically). Here though it looks like anything from 3 to 7
    based on scoping the output, and trying to scope the 0.1uF cap results in
    nothing much that I can interpret as it won't trigger stably.

    In this case, based on what folks are saying, is seems plausible that the
    local supply decoupling caps (which are after all quite old by now) may be
    dying/dead. Alternatively the 1mS input pulses my not be as regular as I
    think (even though they 'scope up very clean).

    Dave
     
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