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Econotig: Partial Flame Control (50%-100% only)

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  • #16
    Ok, I will show my sparky ignorance here again, but taking another look after a couple cups of coffee at the upper left portion of the schematic, will the board ground be isolated from the chassis ground due to capacitor C37 and therefore if I measure voltage from chassis ground to board ground expect to see a change (polarity and AC vs DC) depending on what voltage is coming in via Vfb (RC1-4 and RC1-5)? If that is the case, I will need to re-evaluate the measurements I took before and make sure to use board ground rather than chassis ground.

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    • #17
      Capacitor C37 is a bypass capacitor to protect from transient voltage spikes. Be sure to do all of your measurements from board ground.

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      • #18
        Ok sure, so the board ground is just a floating ground. That will probably explain the ~70 volts that showed up across 'C'-'K' when we looked at that in DCEN since K is tied directly to chassis ground. I'll try to make it back out there tonight and start over by taking all the test point values and see how things look.

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        • #19
          Something doesn't seem right. Your remote should be referenced to the board. Is socket D on the remote connected to board ground?

          Also, does your arc starter work?
          Last edited by jjohn76; 11-09-2019, 03:30 PM.

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          • #20
            Yes, seems to be, can hear it lightly buzzing when triggered as they usually do. I did check the voltage across the pins there (RC2 and RC3) with both transformer leads removed and got a very clean +70 and -70 vdc for DCEP and DCEN too.

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            • #21

              No, there is no pin ‘D’ on the machine side. Only A, B, C, E, K have conductors in the socket (see upper right of diagram below)

              Click image for larger version

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              • #22
                That's interesting. I was pretty certain the Miller RFC-14 pedal has reference on the D pin. Does your pedal reference from the K pin? E-K resistance goes from 0-1k ohm?

                ​​Edit: The pedal (C-E) and panel potentiometers are just in series/parallel with the circuit that connect to the board, so the pedal doesn't use pin D. If your arc starter is working, I would say Q7 and the polarity relay are ok. It may be worth putting it back together and checking RC1-9,10,11, and 12 against the board common (RC8-1,2). But those voltages given for the pins don't make the most sense, like how would RC1-9 and RC1-12 be different if they're directly connected?
                Last edited by jjohn76; 11-09-2019, 06:37 PM.

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                • #23

                  You are correct, that is how the pedal is wired. C-E resistance varies from 1k to 0 and D-E varies from 0 to 1k. But as the Miller tech said to me “the Econotig was designed just a bit differently than all the others...” so it has no connection for D, they just use C-E as a rheostat. K pin on machine is for use with a hand control.


                  Click image for larger version

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                  • #24
                    Staring at the schematic some more, it sounds like back to those potentiometers... R57 looks like it controls the GTAW amperage range. At least now you have the prescribed voltage test points so you can get the duty cycle/trigger timing right on Q12.

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                    • #25
                      Ok, so here's an update from last night. I made it thru the list of test points and most checked out fine while using the board ground, so that is good news. There were a couple of oddities from the listed values, for example pin RC1-12 was the same as RC1-9 (as you mentioned in post #22, how could they not be). Another was RC1-10 which was out of the listed range for both modes, but I was able to adjust with R57 for tig and R56 for stick to bring it back into spec. The next was RC1-11 and I'm questioning the listed values again as I was getting ranges from 12.0-5.0vdc with pedal travel which seem to make more sense than the 3.7-5.0 listed in the manual.

                      With all that adjusted, I went to run more arc tests and then immediately ran out of gas... so I'll have to get some supplies this week sometime and see if anything is better. Assuming that it isn't any better, I would think the next logical place to troubleshoot is the shunt driver circuit and coil. Any suggestions on how to start picking that apart without an oscilloscope, or should I just see if I find myself a cheap little USB scope and save some hassle?

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                      • #26
                        I think about as good as you can get without an oscilloscope is the "RMS" reading from your multimeter. There were some AC specs listed for the shunt driver circuit. RC9 has a few test points that may provide info, but those are probably more informative for timing. I still don't quite understand how that circuit works. OC1 seems to check when voltage is reverse biased across the shunt winding and resets Q12's driver through Q10. The amperage control potentiometers all seem to limit the current to the driver circuit, which controls the rise time in Q12's gate circuit. Hopefully that made sense...

                        One word of caution about the oscilloscope is to make sure what you're measuring is isolated from mains and your leads have enough attenuation. The AC voltages through the shunt winding seem to peak around +/- 600V.
                        Last edited by jjohn76; 11-10-2019, 10:31 AM.

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                        • #27
                          Ok, so new bottle of 100% Argon hooked up and new 2% Lanthanated tungsten electrodes installed and the same behavior is still happening... which was expected.

                          I put the DMM on the shunt coil leads to see what the voltage was doing and this is what I saw:
                          - before triggering the arc (no load), voltage was around 60vac
                          - with front control knob set to 100%, when arc is triggered using pedal, voltage climbs to 370vac (spec says should be 430vac)
                          - as pedal is pressed from 0-60% voltage remains constant at 370vac
                          - as pedal is pressed from 60-100% voltage decreases to 80vac
                          I repeated this using the pedal to trigger the arc (pushed pedal to 100% with knob at 0%) and varied the amperage using the front control knob. The same behavior was seen where 0-60% of the knob did nothing, and 60-100% of the knob dropped voltage.

                          As a different test, I then disconnected the leads of the shunt coil just to see if output amperage dropped to minimum with no variable control. The control was completely gone as expected and the amperage was low, but not any lower than when the shunt coil was connected. So, now I think I may have 2 issues as the minimum current is way too hot for the thin materials it should be able to handle and I only have half the control span that I should.

                          I just happened across the patent for this which may contain some further info, will have to read thru that later tonight. If anyone is interested, it is US Patent 5187428.

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                          • #28
                            Todd, I still haven't quite figured out how it shunts both the positive and negative AC cycle, but it seems that at every negative transition, the LED in OC1 is forward biased, which energizes Q11 and Q10, bringing Q12's anode to ground (C22 is discharged). The current through the amperage control circuit (limited by R56, the panel amperage knob, and the pedal resistance) controls how quickly C22 recharges and turns on OC2, which shorts the shunt and increases the output current. If you turn R56 clockwise, it should delay C22's charging, which keep the shunt winding open longer and reducing output current.

                            Edit: I mixed up R56 and R57.
                            Last edited by jjohn76; 11-18-2019, 06:59 AM.

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                            • #29
                              Sorry, I mixed up R56 and R57. What voltage are you getting across the shunt winding when it's disconnected from the board (idle and under arc)? You may be able to get some indication of the shunt driver timing if you measure across C22 (RC9-4 to board common). The average voltage across that capacitor should change with panel and pedal amperage adjustment.

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                              • #30
                                Yeah, I'm trying to sort out how it shunts both sides of the wave as well. Looking at the waveform (chart H), it is a little different than a "normal" phase control which may provide more of a clue. Perhaps triggered by the negative transition with a delay to close, than remains closed after the positive transition with the same delay...

                                I knew you really meant R57 I did try this quickly last night and it did appear to make an effect, so I will have to fine tune it a bit more when time allows. We'll see how far that gets me. I will also check the voltage across the shunt with leads disconnected as you suggested and get back to you.

                                What puzzles me is why the arc is so hot when the shunt coil is disconnected as the power should be minimized. I may have to set up a shunt and try to measure the amperage to see what my range really is right now (should be 30 to 180 amps). Any ideas why the low end power would be so high? Geometry of the transformer, short in a coil, etc.?

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