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Need a DVA meter, make an adapter for $5

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  • Need a DVA meter, make an adapter for $5

    I needed a DVA meter to trouble shoot a "no spark" problem on my 9.9 HP 1997 Evinrude. I made several post on this problem in the past week as I was seeking some specifications to measure - got some pointers and went to my local library and checked out a Johnson/Evinrude Service Manural.

    The home made DVA helped me conclude I need a new Power Pack - not a component that should fail on an engine with the low hours mine has - I"d estimate about 200 hours.

    The tests I ran with my home made DVA adapter confirm the inputs to the Power Pack are there (Sensor voltage peak measured at 5 volts on rope pull: 1.2 to 9 volts is the spec.) (Charger voltage at 200 volts on rope pull: 150 to 275 expected).

    Then the outputs, the primary current/voltage to the HV ignition coils read zero on both coils on repeated rope pulls.

    The Coils also check out good on resistance measures.

    The above measure call for a DVA meter (which is simply a high impedance DC meter measuring the rectified dive voltage across a capacitor). I have a high impedance VOM so all I needed to do was take a 2 mf capacitor and solder it in series to a diode and a 1,000 ohm current limiting resistor (I decided to add the 1K ohm resister just as a safety measure) Then I connected the above series circuit across the test points: e.g. sensor and charge coils and across the primaries of the ignition coils. The VOM is set on DC and connected across the capacitor. Worked great and all I used was the components, some solider and clip leads. This DVA adapter couldn't cost more than $5 to make, and I had the components on hand, so that cost was buried, free.

    I have not yet ordered the Power Pack, but I am about 90% certain that is the problem - and a $90 investment is parts is a fraction of what it costs to take the engine into a shop, none of which are near my home anyway.


  • #2
    Re: Need a DVA meter, make an adapter for $5

    what you just said went right over my head
    United States Coast Guard Officer #1172938
    Catamount Marine

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    • #3
      Re: Need a DVA meter, make an adapter for $5

      Over your head? Could be my poor writing ability.

      Comment



      • #4
        Re: Need a DVA meter, make an adapter for $5

        post a schematic!

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        • #5
          Re: Need a DVA meter, make an adapter for $5

          I am afarid it has all been posted before.

          Discussions , schematics and photos all HERE

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          • #6
            Re: Need a DVA meter, make an adapter for $5

            Does this look about right? came up with this back in '07 never got around to building it yet.

            http://forums.iboats.com/showthread.php?t=219458 that one without the diode. updated below

            it occurred to me (a non-engineer), that this won't measure the 0.5V timer base output. I actually found a diagram involving two op-amps that supposedly would measure a peak as low as 0.5V but it's kinda worky.
            Attached Files
            It's all about the tools.

            "If the ocean is glass flat and the sun is shining, you open up the special memory compartment of your brain and start recording the smells, sounds, sights and feelings." -- Philster

            "Poets talk about 'spots of time,' but it is really fishermen who experience eternity compressed into a moment. No one can tell what a spot of time is until suddenly the whole world is a fish and the fish is gone. I shall remember that son of a b**** forever." -- N. Maclean

            Comment



            • #7
              Re: Need a DVA meter, make an adapter for $5

              Yep, Jtexas, that's the idea, but I put a 1K ohm resistor in series with the diode, ahead of the capacitor, and I didn't put one in parallel with the capacitor. I don't think there is any need to "bleed" the charge off the capacitor. Using a series resistor will prevent an accidental short and high current. My meter is connected on the right hand side, in place of the parallel resistor and that would make my schematic.

              And yes, it works, and I didn't make it a new "tool", Just some spot solder and some temporary clip leads. I am not in the business of fixing outboard engines and I figure the next time I'll need the tool again will be in the next life.

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              • #8
                Re: Need a DVA meter, make an adapter for $5

                The one I built uses a 1M ohm resistor, thats one million ohm.
                sigpic
                See FAQ Files.
                http://forums.iboats.com/showthread.php?t=299680

                Get the genuine factory service manual.
                www.outboardbooks.com
                www.*****************

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                • #9
                  Re: Need a DVA meter, make an adapter for $5

                  That 100uF, 450 V cap will pop a bit if you get across it while charged. It really only needs to be about 2.2 uF. A 1 meg parallel bleeder resistor will discharge it safely.
                  Please no PMs. My mailbox is full anyway......

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                  • #10
                    Re: Need a DVA meter, make an adapter for $5

                    I agree on the capacitor, I used a 2 microfarad myself. As for the 1 meg resistor, hum, an RC time constant of 2 seconds, long enough not to interfere with making a reading and short enough to discharge the capacitor in a few seconds. Still I didn't use one as the meter I had across the capacitor also discharged the capacitor in a few seconds.

                    The adapter worked great on measuring the Sensor and Charger coil outputs to the Power Pack, but I just realized when I used it to measure the Power Pack output to the coil primaries I did that with the wire open circuit, that is not attached to the primary coil. I got no voltage output and thought "bad Power Pack". Looking at the service manual I see there is a factory tool that they recommend using and it shows a resistance to ground - but no value given. I will go back tomorrow and measure that again with a resistance to ground and I'll try a low value maybe 10-100 ohms. The primary coil has very little resistance, but it does have inductance that slows the ramp up of current flow into the coil. But I ramble, I think there is a possibility that the SCR in the Power Pack doesn't operate if its load (the primary) is open circuit. It wasn't a fully open circuit the input of the "adapter" connected the line to ground through the diode, the series resistor I use, and the capacitor in parallel with the meter. The resistor I used was 1K ohm and that may not draw enough current from the Power Pack SCR to turn it "on". I can't say I really know the specifics of how an SCR works. This is important as I'm using that measurement to decide if the Power Pack or the Coil is what is killing the spark delivery.

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                    • #11
                      Re: Need a DVA meter, make an adapter for $5

                      I tried to get the parts from Radio Shack and they had all but one and couldn't get the last one. Just can't remember which one.

                      Comment



                      • #12
                        Re: Need a DVA meter, make an adapter for $5

                        Originally posted by Jerry_NJ View Post
                        ...
                        The resistor I used was 1K ohm and that may not draw enough current from the Power Pack SCR to turn it "on". I can't say I really know the specifics of how an SCR works. This is important as I'm using that measurement to decide if the Power Pack or the Coil is what is killing the spark delivery....
                        it's my understanding that an SCR is similar to a transistor, but it "latches"...a voltage to the gate causes it to conduct, and it keeps conducting while there is sufficient forward voltage, regardless of the gate status. I don't remember if current is also required to keep it turned on.

                        now, how that relates to the size of your resistor, and you'll use that information, I dunno......would like to hear about it sometime, if convenient. Hopefully I'll won't actually "need" that info for myself any time soon.
                        It's all about the tools.

                        "If the ocean is glass flat and the sun is shining, you open up the special memory compartment of your brain and start recording the smells, sounds, sights and feelings." -- Philster

                        "Poets talk about 'spots of time,' but it is really fishermen who experience eternity compressed into a moment. No one can tell what a spot of time is until suddenly the whole world is a fish and the fish is gone. I shall remember that son of a b**** forever." -- N. Maclean

                        Comment



                        • #13
                          Re: Need a DVA meter, make an adapter for $5

                          I know just enough to be dangerous, but somewhere in the back of my mind I remember something from OMC school about the inductive kickback from the coil being necessary to turn the SCR back off. Sorry, I flunked that part of the test.

                          Comment



                          • #14
                            Re: Need a DVA meter, make an adapter for $5

                            Thanks, I'll make some more tests.

                            I am a graduate EE with 40 years of design experience... no, not true, the last 30 years was as an engineering manager. And this came home hard when I started thinking about a "peak" detection scheme. I wanted to do some transient analysis to decide on the capacitor and resistor values. This requires differential calculus (Laplace transforms and the like) and I realized I just don't remember. Anyway, that said, I believe Radio Shack has the parts. Be flexible on the capacitor value, anything from 0.5 to 5 uF should work and the resistors are a matter of dispute and it seems anything can work, but if you use a series resistor (my design) keep it at 1,000 ohms or less and if you use the parallel resistor 1,000,000 ohms should be fine, but 500,000 ohms will work about the same from a user perspective. It is my guess that Radio Shack may not have diodes with a 400 volt reverse polarity operating range... almost all electronic these days are low voltage, less than 50 volts. In the old vacuum tube electronic days, high voltage was common. I think the diode I'm using doesn't have a 400 volt rating. If the diode is cheap (not sure what that means, to me around $1 is cheap, $10 isn't) buy the highest voltage they have if it is at least 200 volts and try it. Or if they have 200 volts, buy two and put them in series, bingo, 400 volts.

                            As for inductive "kick" I'd see that as more an initial condition then an end state. That is I know the inductor will not allow current to immedately start flowing, there is a current build up related to the RL (ohms times henrys) and that's why the R is so low on the primary, I believe. But, there may be a rebound, when the field starts to collapse when the Power Pack capacitor voltage falls below the voltage across the primary coil. In fact the SCR may turn off when the voltage across the primary coil matches the voltage on the power pack capacitor - it may be the sudden shut off and rapid collapse of the primary coil current that causes the spark plug to fire. The voltage in the secondary is proportional to the magnitude and rate of change of the current in the primary. I'm sure you've noticed what happens when you unplug an inductive load, e.g., an electric motor. As I remember it, this is due to the sudden change in the current (unplug) and the associated rapid collapse of the associated magnetic field... a reverse polarity voltage induction in the same wires/coils.

                            Fun, just like being back in school, hope I didn't fail.

                            All of this is interest (to me anyway) but my real goal is to fix my 9.9 HP Evinrude. The tests I've made with my home made DVA adapter indict the Power Pack, but that is only because I can not read any voltage (peak or otherwise - I haven't yet put an oscilloscope on the wire) on the primary drive out of the Power Pack.. but then I made the test with the DVA adapter being the only termination and hence my wondering about how the SCR works, or doesn't.

                            Jtexas: is that your Chief Boatswain mate on the front of your boat? Does he handle the trolling motor

                            Comment



                            • #15
                              Re: Need a DVA meter, make an adapter for $5

                              Yeah, Dusty will operate the trolling motor sometimes, usually at the worst possible moment. If I could only get him to back the truck down the ramp.....
                              It's all about the tools.

                              "If the ocean is glass flat and the sun is shining, you open up the special memory compartment of your brain and start recording the smells, sounds, sights and feelings." -- Philster

                              "Poets talk about 'spots of time,' but it is really fishermen who experience eternity compressed into a moment. No one can tell what a spot of time is until suddenly the whole world is a fish and the fish is gone. I shall remember that son of a b**** forever." -- N. Maclean

                              Comment

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