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  • Help With Surf Pi

    My surf pi threshold is not steady,it pulses the speaker every second or so.The output of the preamp shows this as zero jumps around,it is not dead flat.Now garbage in garbage out right? The input of the preamp has voltage spikes on the rising edge of the tx pulse,the spikes are constantly moving in amplitude and go negative and positive.The same thing on the peak of the flyback. Now the input to the preamp is protected by the 4148 diodes and 1k resistor and the tops of the tx and flyback pulses are around 800mv tall,the voltage spikes are going above the tops considerably. Let me take some screen shots and show this,I have read several post's of people complaining the surf pi does not give smooth thresh,one guy described it as motor-boating. Do I have bad diodes or input resistor?
    Attached Files

  • #2
    I bread boarded two new diodes and 1k input resistor and I get the exact same wave form.

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    • #3
      Is your ICL7660 synchronized to the transmitter, or perhaps instead I suspect it's running just a few CPS higher or lower?

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      • #4
        Originally posted by porkluvr View Post
        Is your ICL7660 synchronized to the transmitter, or perhaps instead I suspect it's running just a few CPS higher or lower?
        In the silverdog kit pin 7 is connected to ground through a 220pf cap,which must be a mod for what ever reason.Now the two Whites schematics I have show pin 7 to V+ with a 1n5819,and to pin 3 of the 555 through a 10k resistor.I would have rather done it this way but I dont have the diode,and I dont know enough to make substitutions yet.I see it is a schottky diode.I suspect that has something to do with it if its not the whole problem.I was playing around earlier with letting the 7660 run at its own freq, 10k for one,and 12k for another I have, and that was a can of worms.So I put in the 220pf cap.Thats where I am at,I have terrible sensitivity 4" on quarter.But how could it be sensitive when the preamp output will not settle to 0.

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        • #5
          This looks like RF noise pickup to me. While looking at the preamp output, try rotating your coil around in different directions.

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          • #6
            Carl's right, that does look alot like RF or noise pickup.

            On the subject of the diode, I think the diode is there simply to keep the input to the osc pin from being pulled above +5V that serves as the 7660 supply voltage, possibly causing damage.

            I would think just about any diode should work if you wanted to try synchronizing - but - being as how you have let two different 7660 "free run" and still had problems, I now doubt that synchronization is the answer you are looking for.

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            • #7
              Or make another coil and twist it into a figure 8 shape and watch the CRO the noise should be reduced.


              In that second picture if I'm reading the setting correctly it's showing a TX of 50us per div and the amp is taking 50+us to decay, if that is correct thats the reason for the poor sensitivity 50+us to decay is too slow. All the problems could be just the coil and the fact your inside with a lot of external EMI.



              Mick

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              • #8
                Ok I will try all those suggestions,I think my scope runs on batteries also maybe I can take it out in the woods and check it,that would be *****in.Yes the tx is 50us, and yes it takes way too long to decay. And it has the 390 damp resistor in it now,and its still that long.I started out with a mosfet irf9520 100v,but I switched to the kit supplied irf9640 because its 200v and I wanted to see if letting the flyback go higher would get it to decay faster.I have seen it decay fast.And I have sampled at 15us with a 280uh/3ohms coil,and the 9520 mosfet.

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                • #9
                  Originally posted by Carl-NC View Post
                  This looks like RF noise pickup to me. While looking at the preamp output, try rotating your coil around in different directions.
                  If I hold the coil head high,and rotate it at just the right angle I can get the preamp output to settle down,the voltage spikes on the input side do not change.When I get the output as smooth as I can the threshold is better but still oscillatiing about 1hz or so,not useable at all.I took the thing outside and no difference. I can clock the 7660 but after that I dont know what to do.

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                  • #10
                    hello brian,

                    just been reading this thread , all sounds very similar to a problem i had on a home made detector.

                    1Hz clock signal in the house , breaking threshhold as regular as clockwork so to speak.
                    i never did find the source , but went round house turning everything off , even took batterys out of electromechanical clocks.

                    only to say it never happened in the field .

                    you say turning the coil can help reduce noise , i even tried to locate source of noise using detector !!

                    my only conclusion was as i said 1Hz clock signal , but not from my house , poss from next door.

                    good luck finding yours.

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                    • #11
                      Originally posted by DOOLEY View Post
                      hello brian,

                      just been reading this thread , all sounds very similar to a problem i had on a home made detector.

                      1Hz clock signal in the house , breaking threshhold as regular as clockwork so to speak.
                      i never did find the source , but went round house turning everything off , even took batterys out of electromechanical clocks.

                      only to say it never happened in the field .

                      you say turning the coil can help reduce noise , i even tried to locate source of noise using detector !!

                      my only conclusion was as i said 1Hz clock signal , but not from my house , poss from next door.

                      good luck finding yours.
                      Maybe the people next door have an invisible dog fence,I am going to try limiting the coil current and maybe put the irf9520 back in to see if I can get the delay down to reasonable time.

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