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Need Guru to look at 5534 scope shot,something is wrong.

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  • #31
    Originally posted by ivconic View Post
    That's why i suspected diodes and base "biasing" voltage. It is "fast" PI machine yet with bipolar transistor and not with fet. It is much more difficult to control bipolar transistor in such conditions, so any possible variations at base voltage would cause also something like "ringing" and indirectly will affect rx too.
    I had similar issues long time ago with such "bipolar" design.
    You actually don't need to resolder diodes to examine them; just measure voltage across them. (that's why i gave you reference 6.75V to start with)
    I am in for the night,I will get right on that. Should I have a look at the base of the buy92's with the scope? or no?

    Comment


    • #32
      Originally posted by Davor View Post
      Ah, so you actually measure the opamp feedback compensated input, and not the voltage on the coil. OK, that makes sense. Trouble with such measurement is that you actually look at the place where a voltage should be different from zero only in times the opamp is saturated, and such measurements will lead you nowhere. This is very different from non-inverting opamp stages where input follows the applied voltage. The inverting one fights back and tries hard to keep its input at zero. What you observe is just the way the opamp recovers after saturation, and it is perfectly OK the way you see it.
      I have a test pin for the coil before the diodes also,I have measured it.I think it is in the first pictures I posted.It looks as it should in my opinion.

      Comment


      • #33
        Originally posted by ivconic View Post
        That's why i suspected diodes and base "biasing" voltage. It is "fast" PI machine yet with bipolar transistor and not with fet. It is much more difficult to control bipolar transistor in such conditions, so any possible variations at base voltage would cause also something like "ringing" and indirectly will affect rx too.
        I had similar issues long time ago with such "bipolar" design.
        You actually don't need to resolder diodes to examine them; just measure voltage across them. (that's why i gave you reference 6.75V to start with)
        I dont feel like I did what you told me to do,but I checked the diode with a fluke multimeter on diode test and here is the result.D16-.215v D15-.607v D12-.609v D13-.210v. should I apply more than 6.75V manually to the base and see if the diode is shunting properly? I looked at the bases on both transistors and they are at 3.48V (VC in the schematic)normally, and they get a negative going pulse to turn on,both of them look the same.Both of them have some noise on the line when the other is switched on,the noise is the highest peak and it does not go above 5Volts.

        Comment


        • #34
          Hi Brian,

          Do you have an original or home made coil on the machine?? Looks like it is under damped, but you already have a lower value damping resistor than original so decreasing the value of the damping resistor should correct the trace, but it will make it take longer for the coil to settle and will also degrade performance. I think you have a capacitance problem with the coil, the C is too high.

          Also some diodes in the feedback loop of the opamp would not go astray to prevent the opamp from going into saturation. This should speed up the opamp settling time.

          I do find it odd that the supplies are not matched for + and - too, perhaps Carl could shed some light on this.

          Cheers Mick

          Comment


          • #35
            Originally posted by Mechanic View Post
            Hi Brian,

            Do you have an original or home made coil on the machine?? Looks like it is under damped, but you already have a lower value damping resistor than original so decreasing the value of the damping resistor should correct the trace, but it will make it take longer for the coil to settle and will also degrade performance. I think you have a capacitance problem with the coil, the C is too high.

            Also some diodes in the feedback loop of the opamp would not go astray to prevent the opamp from going into saturation. This should speed up the opamp settling time.

            I do find it odd that the supplies are not matched for + and - too, perhaps Carl could shed some light on this.

            Cheers Mick
            Its the orig coil,may be it has too much C.I think the diodes in question limit the base voltage to safe levels and ivconic wanted to be sure we didnt have an injured transistor.Yes I am getting very suspicious of the low neg supply.Earlier today as I was cleaning and inspecting the board I believe I found a solder bridge.I say I believe it was a bridge because it took the solder mask off the trace it was stuck to when I scraped it off.This bridge would have bypassed R9 which is the base resistor for Q3 which is the transistor for the neg supply regulator.So if R9 were a short/jumper what would be damaged the lm2904 or Q3 or both?

            Comment


            • #36
              HI Brian,

              After the solder bridge was removed was there any change in the - supply voltage? If anything were damaged it would most likely be Q3. If you apply some extra load to the supply does it compensate to keep the correct/same voltage? If the supply voltage is low this could be causing a problem with the tx transistors not turning off properly perhaps.

              If you have the original coil, then it is not likely there will be a problem there.

              Cheers Mick

              Comment


              • #37
                Originally posted by Mechanic View Post
                HI Brian,

                After the solder bridge was removed was there any change in the - supply voltage? If anything were damaged it would most likely be Q3. If you apply some extra load to the supply does it compensate to keep the correct/same voltage? If the supply voltage is low this could be causing a problem with the tx transistors not turning off properly perhaps.

                If you have the original coil, then it is not likely there will be a problem there.

                Cheers Mick
                No change in the voltages after it was removed,it looked old and I dont think it was from me.I figure the damage is done so it does not suprise me that the voltage didnt change.The tx transistors are pnp devices and their bases are pulled up to VC the pos supply voltage which is 3.48V and I believe this is the correct voltage. Which brings me to another point I should have mentioned this machine runs on 6 volts.It used 4 D cell batteries originally.I run it on AAs,and it will even run on a 9v.So to demonstrate earlier today my batt pack measured 5.7 volts and the output of the 7661 was -4.7V so you couldnt have a 5v supply on this thing,after the losses in the inverter chip etc..I know the answer is in the voltage divider connected to the non inverting input of U1A I am just to stupid to figure out what the neg supply voltage is supposed to be.

                Comment


                • #38
                  Ok I got to looking at the divider( R7 and R8 ). at the input of U1A.I found an resister divider calculator to do the math.So U1A is going to be output high until the non inverting input matches the inverting input 0V,so we consider that pin to be zero volts if the regulator is working,then we say VC is 3.48V then do the math of the resistor divider and we get VE = 2.9V which is roughly what I have at 2.82V. So before I put my foot in my mouth I applied the same math to the positive regulator,D4 a 1.2v reference diode,connected to the non inverting input of U1B,and another divider to the inverting input.assume once again the center of this divider is 1.2V given that the regulator is working,insert the resistor values in the calculator and you get VC = 3.34V and this calculation is made with no respect to the battery voltage.Only the 1.2v ref diode and the resistor divider values are used. So there was no room for error like the above calculation for the neg,where I had to enter the VC value. So isnt 3.3V a pretty standard control voltage? I think I am solid now on the voltage supplies.

                  Comment


                  • #39
                    I am clueless after reading your fresh infos.
                    Loooks like some component turns to higher tolerance than it should be.
                    How old is detector? Used much?
                    I assumed coil is original.

                    Comment


                    • #40
                      Here you can find how old detector http://www.geotech1.com/forums/showthread.php?t=12357
                      Here scheme with better resolution http://www.geotech1.com/forums/showthread.php?t=18157

                      Comment


                      • #41
                        HI Brian,

                        Have you checked the 1k series resistor on the input path to the preamp to make sure it is 1k. Although this would not be causing the under damping problem anyway as the parallel diodes have stopped conducting by the time the problem shows up. Hmmmph!

                        Have you checked that the coil ground(0v) is all connected properly and that the damping resistor is connected to it properly?
                        Have you checked that the coil is wired correctly to the plug?

                        As far as I can tell the tx transistors seem to be working properly, the power supply seems ok too.

                        Can you check when the first sample is being taken by the 4016 (pins 5 & 13) compared to the preamp output?

                        Do you have access to another coil?

                        Cheers Mick

                        Comment


                        • #42
                          Originally posted by Mechanic View Post
                          HI Brian,

                          Have you checked the 1k series resistor on the input path to the preamp to make sure it is 1k. Although this would not be causing the under damping problem anyway as the parallel diodes have stopped conducting by the time the problem shows up. Hmmmph!

                          Have you checked that the coil ground(0v) is all connected properly and that the damping resistor is connected to it properly?
                          Have you checked that the coil is wired correctly to the plug?

                          As far as I can tell the tx transistors seem to be working properly, the power supply seems ok too.

                          Can you check when the first sample is being taken by the 4016 (pins 5 & 13) compared to the preamp output?

                          Do you have access to another coil?

                          Cheers Mick
                          Yes, those are good questions. Great chances that there is some trivial problem there.

                          Comment


                          • #43
                            Originally posted by Mechanic View Post
                            HI Brian,

                            Have you checked the 1k series resistor on the input path to the preamp to make sure it is 1k. Although this would not be causing the under damping problem anyway as the parallel diodes have stopped conducting by the time the problem shows up. Hmmmph!

                            Have you checked that the coil ground(0v) is all connected properly and that the damping resistor is connected to it properly?
                            Have you checked that the coil is wired correctly to the plug?

                            As far as I can tell the tx transistors seem to be working properly, the power supply seems ok too.

                            Can you check when the first sample is being taken by the 4016 (pins 5 & 13) compared to the preamp output?

                            Do you have access to another coil?

                            Cheers Mick
                            Well I will check all those things again,the coil wires are coax and they plug directly into the board,the coax socket is soldered directly to the ground plane on the pcb,and they are gold plated.I have had the damping resistors out a couple times and the board is fragile,and double sided so I have a couple spots that dont look perfect but I am pretty sure there are all solid solder connections,I will double check.I dont have another coil,would be nice.However you know there are two coils in this search head and they both measure almost identical in inductance and resistance it is hard to imagine there is a problem.I have shortened the coax and reused the coil side connectors, so I will put my work under the microscope today.I do recall it was very tedious,the coax plugs are tiny about 1/4" diameter outside.I can move the sample with the trimmer,I dont know where it is presently,I will check that and the second sample also. What if I bread boarded an 5534 and fed it all the corresponding pins of the in circuit chip,except for pin 6 of course,and check the output? Would that be viable? And Carl if you are reading this I dont mean to ignore your initial suggestion but I think in the past I have had a pot soldered in to the damping resistor sockets and the mystery pulse didnt go away even with very low R. I have to buy some sockets before I destroy this board,it will not tolerate much more desoldering of the damping resistors.

                            Comment


                            • #44
                              Hi Brian Deese. You had a detector in working order, but you were not satisfied with the detection of small objects (aluminum cylinder 10mm tall,8mm diameter at 20cm... its too slow to signal...) http://www.geotech1.com/forums/showthread.php?t=17571
                              What did you do after?

                              Comment


                              • #45
                                Originally posted by maikl View Post
                                Hi Brian Deese. You had a detector in working order, but you were not satisfied with the detection of small objects (aluminum cylinder 10mm tall,8mm diameter at 20cm... its too slow to signal...) http://www.geotech1.com/forums/showthread.php?t=17571
                                What did you do after?
                                The preamp output has always looked like it does now,(it crosses zero one time too many)it does not look like typical PI.It looks like it is ringing in the output of preamp but not in the input of preamp or measured directly at the coil,that is the purpose of this thread to determine why that is.Now the machine will work just as it always has if I set the sample pulse on the decay curve,I did that this morning actually,I wanted to check the detection distances while adjusting the preamp offset,and I got no gains by offsetting it,so I set it at zero.I changed some timing resistors so that I have control over the sample time,and width,and tx width,and freq. I have adusted it every way possible and the "ringing" is consistent.I didnt know much when I posted that thread,and now I have built a couple PI's and have a little more knowlege.It does respond slow,I had to study and find the vco stage in the schematic and if you lower the value of C31 it will respond faster but I am not sure it is as stable.I have the original value in it now and when the sample pulse is in the right place and you hold the auto tune it clicks at one click per second,solid,stable no variation.An iron ring 50mm diameter will increase the freq to 2 clicks per second at about 30cm which is not terrible,so there you have it.It works but I dont think the preamp output is supposed to look like that.

                                Comment

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