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  • Originally posted by Jerry View Post
    Hi Simon,

    Here are a few pictures showing what happens while adjusting the TX oscillator. The changes with just a little distortion in the TX sine wave is interesting. The bump produces noticeable ringing. The upper trace is the TX signal measured at J1 and is set at 5 V/div. The lower trace is U101b, pin 1 and is set at .2 V/div. Same setting apply to all pictures.

    First picture is the TX oscillator (top signal) tuned for a clean sine wave.

    Second shows the TX oscillator with about as much bump at the top as was there before I added the adjustment pot.

    Third is somewhat in between with just the slightest of bump at the top. Even with that, there is still ringing.

    Interesting at how the Disc pulse amplitude goes up with more distortion of the TX sine wave. I suppose the next question is how much is needed for good discrimination. I am too wiped out right work on it tonight but it gives me quite a few ideas.

    Thanks for suggesting this, it is interesting.

    Jerry

    Now that I think about it, I did not even look at the Disc knob settings..... whatever it was, it is the same for all pictures. Will check some more tomorrow.
    Hi Jerry:

    I've seem some crazy waveforms at u101b pin 1, but yours looks particularly unfamiliar - can you confirm you are measuring what I think, which is: the output pin of LF353 that connects to the DISC pot?

    I think maybe you are scoping pin 2, the inverting input of U101b (LF353). The output is more revealing as to how it affects the sync pulse.

    Also, to really see what is driving the sync pulse, try scoping U102a (LM393) pin 2, the inverting input of the comparator connected by resistor to DISC pot wiper. Then as you turn the DISC pot, you can really see why the phase shifts where it does. Maybe that is what you meant to do anyway.

    Regarding the "double" sync pulse that happens at one extreme of the DISC pot -- I had originally thought it was due to all the "ringing" in the LF353 output. Today I checked (with dfbowers' TGSL) and noticed that the double pulse happens when the comparator is driven purely by the TX oscillator signal. My guess it is due to the oscillator voltage going beyond the "rails" of the comparator and causing the glitch when the DISC pot mostly selects the oscillator signal.

    Regards,

    -SB

    Comment


    • Originally posted by simonbaker View Post
      Hi Jerry:

      I've seem some crazy waveforms at u101b pin 1, but yours looks particularly unfamiliar - can you confirm you are measuring what I think, which is: the output pin of LF353 that connects to the DISC pot?

      I think maybe you are scoping pin 2, the inverting input of U101b (LF353). The output is more revealing as to how it affects the sync pulse.

      Also, to really see what is driving the sync pulse, try scoping U102a (LM393) pin 2, the inverting input of the comparator connected by resistor to DISC pot wiper. Then as you turn the DISC pot, you can really see why the phase shifts where it does. Maybe that is what you meant to do anyway.

      Regarding the "double" sync pulse that happens at one extreme of the DISC pot -- I had originally thought it was due to all the "ringing" in the LF353 output. Today I checked (with dfbowers' TGSL) and noticed that the double pulse happens when the comparator is driven purely by the TX oscillator signal. My guess it is due to the oscillator voltage going beyond the "rails" of the comparator and causing the glitch when the DISC pot mostly selects the oscillator signal.

      Regards,

      -SB
      Hi Simon,

      Just walked out to the shop to confirm what I was hooked up to and you are correct, it is Uio1b pin two that I took pictures of. I must have been more tired than I thought, I just hooked it up quick, snapped a few pictures, shut everything off and called it a day.

      It is raining here today so I think I can get some more pictures. I will check U101b output pin 1 and also U102a pins 1 and two.

      However the last series of pictures is still interesting in that it shows a huge gain in the pulse along with increased ringing with increased distortion. So it was still useful in that sense.

      Will post more later this afternoon.

      Jerry

      Comment


      • Originally posted by Jerry View Post
        Hi Simon,

        Just walked out to the shop to confirm what I was hooked up to and you are correct, it is Uio1b pin two that I took pictures of. I must have been more tired than I thought, I just hooked it up quick, snapped a few pictures, shut everything off and called it a day.

        It is raining here today so I think I can get some more pictures. I will check U101b output pin 1 and also U102a pins 1 and two.

        However the last series of pictures is still interesting in that it shows a huge gain in the pulse along with increased ringing with increased distortion. So it was still useful in that sense.

        Will post more later this afternoon.

        Jerry
        Ok. BTW, I think the ringing mostly happens when the LF353 "saturates" due to out-of-bounds input, and clips the output, going nonlinear. I'm guessing that your input signal you scoped suddenly spikes when the op amp is decidedly outside the linear range. The original TGS had a lower TX oscillator voltage which I believe did not cause this condition -- we've basically "super-charged" our TGSL oscillators by removing the regulator JFet.

        The TX signal "blip" also can make a really hefty blip in the LF353 output, but it doesn't actually ring I think if the LF353 is operating in a linear mode; but it can cause some weird DISC phase shift motion sometimes.

        -SB

        Comment


        • Discrimination Issue Solved!

          Well I made a lot of progress this afternoon and I think I made some discoveries as well.

          I checked the scope signals at U-102 pin 2 and also at U-102 pin 1. Overall, the output of U-102 (Pin 1) proved to revealed the most information that was useful for my problem. I have noted before that this square wave (The Disc Pulse) would only move so far while turning the Disc control clockwise and it would hit a point were it would stop advancing in step with the Disc knob.

          I Added a 68 pf Cap across feedback resistor R17 and now the Disk Pulse moved much further to the right as compared with the TX pulse. This increased my discrimination by a lot, too much even. With the 68 pf cap on R17, I can discriminate out U.S. Quarters! This is more than what I was looking for so I reduced it to 22 PF and that will disc out zinc pennies and not quite silence copper pennies and dimes. Exactly what I was looking for!

          It did not settle the issue of shifting where nickels discriminate out. It used to be five on my very crude scale of 0 to 9 and now it is a little over six. (6.1) I may revisit the TX operating frequency but I can live with what I have now. Especially since a little further experimenting may have given me a good work around.

          As of right now, my TGSL discriminates as follow. On a scale of 0 to 9:

          Small Foil ........... 5
          Nickel.................. 6.1
          Pull Tab............... 6.3
          2 Euro Coin ........ 7.0
          Zinc Penny ......... 7.5
          Copper Penny...... 8.0
          Quarters cannot be discriminated.

          Notice that the discrimination scale is compressed into about half of the rotation of the Disc control. With that in mind, I shut the detector off and started measuring DC resistance between the wiper of the Disc Pot R18 and ground. I discovered that from 0 to about 2.2, the reading stayed at a constant 3.3K. This has me puzzled since I not see a 3.3K path to ground on the schematic. Will have to check R19 for proper placement tomorrow.

          Sticking with that 2.2 number a little further, earlier I had made a note that the Disc pulse would not start moving to the right until it reached 2.2 on the disc scale. This is also the same point where the pulse actually started to move the double blip in the Disc pulse went away!

          I was going to mention another possible change but will hold off until I find out for sure the status of R19.

          I knew this was going to get a bit long, so I dumped the pictures of U102 pin 1&2 into single images. The first one is captioned and is of the input of U102, pin 2. the second is of the output at pin 1 and the captions for disc settings would be the same.
          I also included a pic of the targets I used as test subjects for the discrimination test.

          It rained most of the time I was taking the pictures and I had to wait a bit so I would not get wet walking back to the house. The last picture is probably the best rainbow I have ever seen.

          A good way to end the day.

          Jerry
          Attached Files

          Comment


          • Interesting find Jerry may follow that mod at somepoint on my machines and see if i get the same result.
            Did you go to the end of that rainbow and check for gold?

            Regards

            Comment


            • Reference my last post. I had turned in for the night but got to thinking about that 3.3 k resistance reading on the Disc pot wiper. It really isn't apparent at first glance but there is a 3.3k dc path to ground through the coil. Coil isn't shown on the schematic so I missed it earlier.

              This should pave the way for the mod i had in mind. which would be to replace the 100k with a 50 k linear pot plus 47k of fixed resistance. This would have the effect of band spreading the disc dial so that things like foil and coins would be spread out over the entire range of rotation instead of being compressed in the upper half only.

              Will think on it but it is looking good.

              Jerry

              BTW No pot of gold Dave

              Comment


              • Originally posted by Jerry View Post
                The last picture is probably the best rainbow I have ever seen.

                A good way to end the day.

                Jerry
                If you look to the left of the photo, you will see that it is in fact a double rainbow.
                Maybe there were two pots of gold.

                Comment


                • Junk Parts

                  I did some more checking on the 100K Disc pot looking for the reason why I was getting a constant reading of 3.3k for the first 20% or more of the knob rotation as described in my post #1039.

                  As it turns out, the pot is defective. The resistance reading should change from either end relative to the wiper as the shaft is turned with no dead zones. This pot, which is brand new does nothing for the first 20% as the knob is rotated clockwise. Checking from the other end, the resistance starts to change immediately as the shaft is turned. I suspect that something is wrong with the internal setup when they were made. I have several other identical pots that do the same thing.

                  So the mystery of why the Disc pulse would not move and it had a double bump until that point is solved. Chalk it up to bad parts!

                  The double bump issue is a matter of the 3.3k resistor at R19. I think increasing that slightly will make the double bump just go away and I don't think it will affect anything else with the discrimination circuit.

                  I plan on changing the Disc pot to a 50K and increasing the resistance of R19 to 47k to start with and see what that does to the band spread on the Disc scale. I am sure I will have to play with the numbers a little.

                  Now that I think a little more about R19, this is part of what Simon accomplished with his voltage divider mod which he posted here: http://www.geotech1.com/forums/showp...9&postcount=73

                  Jerry

                  Comment


                  • Originally posted by Jerry View Post
                    I did some more checking on the 100K Disc pot looking for the reason why I was getting a constant reading of 3.3k for the first 20% or more of the knob rotation as described in my post #1039.

                    As it turns out, the pot is defective. The resistance reading should change from either end relative to the wiper as the shaft is turned with no dead zones. This pot, which is brand new does nothing for the first 20% as the knob is rotated clockwise. Checking from the other end, the resistance starts to change immediately as the shaft is turned. I suspect that something is wrong with the internal setup when they were made. I have several other identical pots that do the same thing.

                    So the mystery of why the Disc pulse would not move and it had a double bump until that point is solved. Chalk it up to bad parts!

                    The double bump issue is a matter of the 3.3k resistor at R19. I think increasing that slightly will make the double bump just go away and I don't think it will affect anything else with the discrimination circuit.

                    I plan on changing the Disc pot to a 50K and increasing the resistance of R19 to 47k to start with and see what that does to the band spread on the Disc scale. I am sure I will have to play with the numbers a little.

                    Now that I think a little more about R19, this is part of what Simon accomplished with his voltage divider mod which he posted here: http://www.geotech1.com/forums/showp...9&postcount=73

                    Jerry
                    Hi Jerry:

                    Good sleuthing!

                    You may find that increasing R19 is not a good solution and prevents full phase shift (off the top of my head). That's because the pot is supposed to allow variable weighting between two signals that are 90 deg phase shifted:

                    Asin(phi) + (1 - A)sin(phi + 90) = wiper signal

                    where A varies from 0 to 1. This creates a signal whose phase varies from 0 to 90.

                    If you increase R19 to, say, 100K, you may get something like:

                    Asin(phi) + (1 - A/2)sin(phi + 90) = wiper signal.

                    This won't make the full phase shift.

                    That is partly why I put the divider before both signals. The other reason was to prevent the LF353 from overdriving and causing the ringing and clipping of the LF353 output signal.

                    You may find that just replacing the bad pot (plus your capacitor phase shift) may be all you need -- most people's TGSLs seem to work great with really ugly looking signals at u101a pin2 (input).

                    Note that by adding the capacitor, you probably shift the whole range of the DISC control -- so what you gain at one end, you lose at the other. I don't know if that is good or bad or indifferent. Maybe you can tell us.

                    Loved the rainbow and landscape in general -- and the idea of toiling like a mad scientist in the shed while the weather rages outside - perfect.

                    -SB

                    Comment


                    • Originally posted by simonbaker View Post
                      Hi Jerry:
                      You may find that just replacing the bad pot (plus your capacitor phase shift) may be all you need -- most people's TGSLs seem to work great with really ugly looking signals at u101a pin2 (input).

                      Note that by adding the capacitor, you probably shift the whole range of the DISC control -- so what you gain at one end, you lose at the other. I don't know if that is good or bad or indifferent. Maybe you can tell us.
                      -SB
                      I think just changing the pot and adding the cap is a good idea for now. Now I just have to find a 100K linear pot that does not have a dead band at the end. I have several new pots on hand and all of them have the same problem to some extent. It is ironic, the pot that shows the least dead band is the one I removed that was under value. The problem with that one is the threaded collar is smaller in diameter than the present one so I have already enlarged the hole in the front panel.

                      I don't know if I am going to have time to work on it tomorrow or not but will report back when I get everything done.

                      Talking through these problems is a help in thinking them through. I appreciate your help.

                      Jerry

                      Comment


                      • Hi Jerry
                        some have used multiturn pot for the gb on here trouble is there so exspensive, maybe it would be worth trying one on disc if you have one.

                        Comment


                        • Originally posted by satdaveuk View Post
                          Hi Jerry
                          some have used multiturn pot for the gb on here trouble is there so exspensive, maybe it would be worth trying one on disc if you have one.
                          Hi Dave,

                          I did that and it works great for ground balancing. Now I still need to figure out the best method to use. The reason I have not tried it for Disc is I like having a single turn knob that I can turn with my thumb until the beep goes away and then glance down to see where the Disc pointer is. I don't even have to look at my detector otherwise.

                          Thought about it some more last night and I think I am going to get a good Disc pot installed, along with the 22 pf cap mod and then look to the TX oscillator to shift the disk scale CCW. It isn't much, the red and blue dots in the picture show the amount I am talking about. It was at the yellow dot for nickels before any mods and now it is at the red dot. In fact if I get my disc scale set up an working as well as this Tesoro picture I will be happy.

                          Jerry
                          Attached Files

                          Comment


                          • Dont know if this will shed any light on the matter but after doing the ivonics mod with the preset in the TX stage noticed it did make quite a big shift to the disc settings ie pushed them further up the scale, hence im using both my TGSLs with disc on and disc control fully anticlock wise.
                            Bare in mind this happened with both machines using the TGSL circuit, so certainly has a bearing on it.
                            the advantage of the mod was home made coils it gives that little bit of tweeking for nulling error.
                            If using tesoro manufacture coils best leave as TGSL circuit described.

                            In real use my ground balance on both machines is around 5-10 two in clock format anything from that either falsing or low sense, that with cost is why I didnt go any further with the multiturn Idea

                            Comment


                            • Originally posted by satdaveuk View Post
                              Dont know if this will shed any light on the matter but after doing the ivonics mod with the preset in the TX stage noticed it did make quite a big shift to the disc settings ie pushed them further up the scale, hence im using both my TGSLs with disc on and disc control fully anticlock wise.
                              Bare in mind this happened with both machines using the TGSL circuit, so certainly has a bearing on it.
                              the advantage of the mod was home made coils it gives that little bit of tweeking for nulling error.
                              If using tesoro manufacture coils best leave as TGSL circuit described.

                              In real use my ground balance on both machines is around 5-10 two in clock format anything from that either falsing or low sense, that with cost is why I didnt go any further with the multiturn Idea
                              She told him not to mentally undress her; he said he'd bare that in mind...

                              Comment


                              • Originally posted by satdaveuk View Post
                                Dont know if this will shed any light on the matter but after doing the ivonics mod with the preset in the TX stage noticed it did make quite a big shift to the disc settings ie pushed them further up the scale, hence im using both my TGSLs with disc on and disc control fully anticlock wise.
                                Bare in mind this happened with both machines using the TGSL circuit, so certainly has a bearing on it.
                                the advantage of the mod was home made coils it gives that little bit of tweeking for nulling error.
                                If using tesoro manufacture coils best leave as TGSL circuit described.

                                In real use my ground balance on both machines is around 5-10 two in clock format anything from that either falsing or low sense, that with cost is why I didnt go any further with the multiturn Idea
                                Hi Dave,

                                Yes it does shine some light. It is when I did the TX mod that the shift in the discrimination scale happened. I could have lived with it the way it was because the detector is working very well. Right now I am just fussing with minor tweaks and learning a lot in the process.

                                I hope to have the TGSL mods done and put back together tomorrow. Our club has a seeded hunt on Saturday and I hope to use it for that.

                                Jerry

                                Comment

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