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

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

    Hey guys, I've been digging thru the forum, but couldn't seem to find anything that is quite like the issue I am having with my machine. I've got an old Econotig [Serial: KK162705] that I seem to have no amperage control for the first half or so of pedal travel. With the front dial set to 100% amperage, moving the pedal from 0 to half, the amperage output seems to be constant at 50% amperage (arc starts and is stable, just not adjustable by pedal), but after that (half to full pedal travel) I can vary amperage just as you should be able to. If I turn the front control knob to 50% or lower, then no matter how far the pedal is pressed, there is no difference in amperage out. I checked the resistance between pins 'C' and 'E' of the pedal when pressed and it goes from 1000 to 0 ohm linearly with pedal travel, so that appears OK. Also checked with known working pedal from other machine with same results. All positions of the range/polarity switch do the same (ie AC High, AC Low, DCEP, DCEN). Is there possibly a pot on the board that needs to be adjusted/trimmed? Did I lose high voltage/low amperage somehow? Any thoughts? Thanks!

  • #2
    Took another look at things tonight and it looks like the 2 trim pots (R56 and R57) on the circuit board may have been "adjusted" by the previous owner as there was no glue on either of them to lock their values. It looks like these are in line with the front amperage control knob and pedal, so could it be that these are not set correctly and are causing my issue (one is set at ~500 ohm and the other ~750 ohm)? Does anyone have a procedure for how these need to be set/checked?


    • #3
      I don't have access to that board's schematic, but a couple of things to check up front before figuring out which signals to follow from the remote socket through the board. What kind of equipment are you using to check components and power up the board?

      Does it otherwise work without the pedal connected at those lower panel amperage settings?
      What is the voltage between pins C and K at the remote socket when you adjust the panel amperage control knob throughout its range?


      • #4
        Found this schematic on another forum ( Not great image quality unfortunately, but the area circled in red shows the variable resistors R56 and R57.
        Last edited by osmu0011; 11-06-2019, 10:27 AM.


        • #5
          Ok, it may take a little while to figure out what connects where, but should be doable. Here are a couple of photos I found online to connect schematic to PCB.
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          • #6
            Thanks for digging for those. That looks similar, but maybe an older revision of the PC1 board (KG-51)? I've got a couple snapshots of the board in the machine that I will attach. Also cleaned up a small section of the schematic to relate to the physical board layout, so hopefully that will help a little.

            Also, to answer your earlier questions:
            - The board is still in the machine, so it is powered from it and I'm just using a DMM for checking components out.
            - Voltage between 'C' and 'K' on remote socket measures 16.0 VDC at 0% and decreases linearly to 15.0 VDC at 100% amperage setting.
            - Varying the amperage setting using the front knob only (without using the pedal) it acts the same way.

            Click image for larger version  Name:	IMG_1987.jpg Views:	0 Size:	86.9 KB ID:	604189Click image for larger version  Name:	IMG_1989.jpg Views:	0 Size:	71.2 KB ID:	604190Click image for larger version  Name:	Trim Pots Schematic.JPG Views:	0 Size:	56.7 KB ID:	604191
            Last edited by osmu0011; 11-06-2019, 09:01 PM.


            • #7
              How are you clearing up the schematic? I think your issue is in the lower left of the schematic, where the Tig potentiometer feeds. That seems like a very narrow voltage range for your panel amperage, so that could be an issue too. I don't think those two circled board potentiometers do much other than bias the max voltage for your amperage dials. As I read the schematics (refer to schematic), the mode switch signal comes in left middle (just above the split connector that receives the panel amperage on RC1-9 and connects it to the pedal amperage input at RC1-10) to the transistor just above and to the left of your potentiometer, which pulls the emitter to ground and disables the NAND gates at the top, which control your primary input contactor. When the mode switch is low (stick), that transistor is open, which doesn't disable the NAND gates at the top (overvoltage circuit in the upper left can still disable). The trigger input signal on the left RC1-7) when the pedal is pressed pulls the lower input on the first NAND high, which negates the mode switch high on Tig setting.

              That mode switch also turns what looks like Q44 (middle-ish lower left) on, which bypasses the pedal by connecting your panel amperage straight to the pedal potentiometer output. Hopefully that all makes sense. Q44 also controls which of the two potentiometers in the red circle controls max amperage - the left one for Tig, the right one for stick.

              Anyways, all of your final feedback output goes to the third connector from the bottom on the left side of the schematic. It's interesting, but the schematic splits up the locations of the optocouplers. The capacitor-resistor-inductor network around L4 seems to be the affected area, so long as that narrow 15-16V for the panel amperage is right (doesn't seem right). Something tells me it's worth checking your panel potentiometer first, as I would expect the voltage across C and K to vary from close to 0V to close to 15V. If it's not the potentiometer, it might be the ground.

              Hope this helps,
              Last edited by jjohn76; 11-06-2019, 10:41 PM.


              • #8
                What voltages do you measure across the C and K socket when you have it in stick mode and adjust the panel amperage? It should be the same voltage you see across E and K in stick mode. How does it perform in stick mode?


                • #9
                  OK, finally got a few minutes to take a look at this today. Checking the voltages you mentioned while in stick mode resulted in some interesting numbers...
                  with the Range/Polarity Switch set to DCEP I got readings of:
                  C-K: 8-11.5 vdc
                  E-K: 7-8.5 vdc
                  and set to DCEN, both were -70vdc !!!

                  After getting that reading, I went back to tig mode to double check both DCEN and DCEP there too, but both were the same for C-K voltage range (16-15vdc) while adjusting panel amperage knob from 0-100%.

                  After that, I pulled the board from the machine to have a look and see that there is a little bit of funky-ness (pics below) happening around the polarity relay, which just so happens to be where +70 vdc comes in on the schematic. Hard to see from the photo, but there is a good sized area that has a different sheen to the conformal coating, almost like something dripped on this area and flashed or the area just got very hot and re-melted the coating... That's as far as I got so far, will have to come back to the schematic to try and understand what is active on the circuit with DCEN in stick mode and try to figure out what is failing.
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                  • #10
                    Looks like you'll get plenty of practice replacing multi-pin components. That Electrolytic cap is toast given its heat signature through the ground plane. If that 70V leaked through to your remote socket, it likely made it through a lot of your board. Do you have a bench power supply?

                    ​​​​​​The initial problem sounded like your phase control circuit (bottom left of schematic) was only controlling the SCR firing in the first 90 degrees (also 180-270 degrees for the other SCR) of input phase (~half power to full power), and would just fire at 90/270 degrees (peak on the sinewave input voltage) for any SCR firing timings that should have occurred in the 90-180 or 270-360 windows. Hopefully that made sense.


                    Last edited by jjohn76; 11-08-2019, 10:21 AM.


                    • #11
                      Yeah, exactly what I was hoping to for... No, I don't have a power supply, that would certainly come in handy for this.

                      On a plus note, I did come across a Technical Manual for the machine today which has all the schematics, and you can actually read them. It also has test points for voltages and resistances listed which should come in handy.


                      • #12
                        After a few hours of probing, I've determined by pulling the voltage sense line (Vfb on RC1-4), the odd stray voltage goes away and most of the test points as laid out in the manual check out. It really helps being able to read the schematic labels now (new file attached and snapshot shown below), but still not sure what the issue is. Is looks like with the sense lines attached, the stray voltage happens for AC High, AC Low, and DCEN (DCEP appears ok). All 3 of these settings put negative polarity on RC1-4 (very top left of schematic) and positive on RC1-5, so I'm thinking something in the "voltage level sense" network is not happy. Quickly checking voltage across R4, the polarity is consistent for both DCEN and DCEP which you'd expect if the set of diodes were functioning, but the magnitude is about 7 volts less for DCEP (72 vs 65).

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                        • #13
                          Just trying to catch up, but to feed back what I understand so far, you measured R4 in DCEN and DCEP, and both are showing the correct polarity according to the schematic, but DCEP is lower. Something tells me it is in the upper right of your schematic instead of the upper left, as CR1 is the first component influenced by the polarity switch (CR1 coil is energized directly from the polarity switch in the bottom left).


                          • #14
                            Looking further at the schematic, that entire top portion from the left of the page to the right just looks like it controls the HF impulse start. The voltage level sense is rectified (you measured it across R4), divided down (R5, R6) and clamped (R14, zener D10) to Keep A1 alive, then compared with Whatever voltage is divided between R7 and R17, then compensated (R10,C3,R19) before fed into the pair of NAND triggers that turn on the pulse generator that switches totem pole gate driver Q5/Q6, which switches Q7 on and off. CR1 has a bunch of normally open and normally closed connections that gets the correct +70V to the collector on Q7.
                            Hopefully this makes sense. Either way, some things to check on RC7, which appears to have a lot of the test points you need. I bet those voltages aren't in the manual, which would have made troubleshooting much easier...
                            RC7-1: Should be a constant voltage that measures the same as the voltage across R17
                            RC7-2: should be some smaller scaled value of whatever is across R4
                            RC 7-4: should be +15V since you're at open circuit voltage
                            RC7-5: should be +15V if you're at open circuit voltage and Tig mode and pedal pressed, 0V otherwise.
                            RC7-6: should be ~7.5V if you're at open circuit voltage, Tig mode, pedal pressed; 0V otherwise.

                            Also, with power off, it's worth pulling the part numbers and schematics for the polarity relay and Q7 igbt. Check continuity on the polarity relay pins to see if any are stuck and different than their normally open or closed setting. Also, check resistance across the Q7 collector and emitter, both directions. Something tells me that IGBT is fried, and if it is, it may be the component backfeeding 70V through a lot of other components. Did your HF start circuit work before?


                            • #15
                              Also, sorry for steering you in the wrong direction on the phase control. This welder doesn't use phase control, it has a shunt winding on the main transformer to limit current like the older Dialarc and AP-B, but uses PWM (lower left of the schematic) instead of a 120W rheostat to control current in the winding.
                              Last edited by jjohn76; 11-09-2019, 10:20 AM.