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Syncrowave 250 full amperage output KB057067

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  • #16
    I like the RV water heaters for load banks...

    Mark,

    I am in the middle of moving, so am working from my phone for a little while. I can hopefully provide more help once I can look at the schematic on a larger screen or paper. I haven't worked on your model, but can say of the 80's and 90's sync/square wave models I fixed, most all control voltage/signals in the feedback are static until you get up to the timing signal from your transformer (Pin 5, plug 55). If you track down signal A (page 39 and 40), it is your control output to the SCR timing circuit. A4 (pins 8,9,10) opamp is you feedback opamp, all signals are summed on the inverting pin 9, so it is calibrated at steady state when the sum of your amperage control and your amperage output match. So if signal A doesn't change when you adjust the knob, it's worth checking the components connected to pins 8,9,10, as well as isolating all other inputs from the start and crater circuits.

    The output voltage sense is there for your arc force - when output voltage goes below 20V, you get an added (negative) amperage. I didn't track this in the schematic yet, but you should see a pair of opamps in the output voltage return circuit - one is basically a comparator that determines if the output voltage is below 20V, the second one adjusts the amperage addition based on the difference between the output voltage and 20V and your arc force adjustment setting.

    If you look at page 39, signal A goes into the inverting inputs of two opamps, which fire your SCR drivers. Those opamps are AND'D with your disable circuit (signal B) and the BJTs Q7 (one diagonal of SCR) and what looks like Q8 (the other diagonal of SCRs). If you trace the gates back from Q7 and Q8, basically they are 180 degrees out of phase, and are timed with the input voltage so only the correct SCR IGBT is activated (hopefully that makes sense). The timing circuit signal is also phase shifted ~90 degrees (opamp A6 pins 8,9,10), which basically gives you full 180 degrees of firing control. It is biased by the balance signal, and split (one side is inverted so you have positive ramps) and fed into the non-inverting pins where they're compared against signal A. Now hopefully that made sense...

    All that for context, you can scope the timing waveforms from the transformer, phase shifts, and bias from the balance knob to make sure those look right (confirm no ground loop between COM and chassis ground). A multimeter should be enough to work through the feedback and error voltages.

    I didn't see the potentiometers you referenced above, where are they on the schematic?

    Hope this helps,
    Jon

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    • #17
      Edit, looking further into the schematic, the ARC control uses opamp A3 (pins 5,6,7), to determine if the voltage is below 20V, and Q13 provides the appropriate voltage to pin 10 (max voltage available to the arc control potentiometer). The Arc Control potentiometer is filtered and added directly to the feedback opamp.

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      • #18
        Also, I think your spot on with the potentiometers. I'll see if I can track down in another TM how to calibrate. R4 should control the minimum, and R3 should control the scaling.

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        • #19
          Hi Jon,
          I did scope the drive signals from A4 pin 7 and A5 pin 7 which drive the transistors that drive the primary of the drive transformers for the SCR pairs. (no ground loop as the secondary is completely isolated). As I change the amperage control on the front panel, the pulse width changes, as expected. I can also adjust the relative the pulse width by adjusting the R4 potentiometer, hence amperage calibration I would imagine.

          That being said, the pair of transistors that alternately fire the pair of SCRs 180 degrees out of phase is also working fine.

          I will have a look into the arc control circuit. RC55 put a voltage on one end of the arc control potentiometer. This, in turn, will apply a voltage to pin 4 on RC55 which is proportional to the position of the potentiometer. However, I cannot find that **** RC55 pin 4 on the schematic to see what that voltage does!! (April 1991 FORM OM-353E manual) Nevertheless, I am not using arc control so it should not affect the rest of the circuit.

          I did replace many ICs on the board and conventional removal techniques did not work due to the conformal coating. I ended up doing what I think you may have mentioned in a previous post. That was to cut the legs off the ICs and remove and clean holes one by one. I know I am close to functioning.

          Thanks,

          Mark

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          • #20
            Great news Mark, sounds like you have it about there.

            That RC55 pin 4 feeds into Pin 9 of A4 after a low pass filter and what looks like R109. It is the 3rd signal junction down on the right hand side of page 38 of the OM-353E PDF (page 28 if you go by the page numbers written in the manual).

            It's a toss up as to what is easier when removing the ICs. I am troubleshooting a XMT-350 main board, and received a few junk boards from a buddy to depopulate for easier schematic capture. I removed conformal coating and used a desolder gun on the components I may need to reuse (microcontroller, power module, gate drive transformers) and either cut off the legs of the remaining ICs or ripped off the ICs with pliers. Each method has its merits and presents its own problems...

            Jon

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            • #21
              Thanks Jon,
              I found that RC55 pin 4.

              For those cheap op amps, I decided to cut off legs rather than heat and reheat the pads and risk lifting pads and tracks. Thankfully, there are no microcontrollers on this board.

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              • #22
                After I verified operation of each portion of the board such as AC timing reference, switching, etc, the welder was still exhibiting strange behavior. Using a stick welding, I would set the current very low. Once I struck an arc, the current rose dramatically. After some additional checking through the schematic, something occurred to me. As this is a shunt resistor for feedback, there is a voltage differential on each side of the shunt. This difference is applied the filter then an op amp where the signal is conditioned as it is fed back into the control circuit. When I was doing some testing, I must have reversed the wires going to the filter. The effect this would have is that once an arc is struck and current increased, the feedback would "report" that the current has decreased which in turn, would try to increase the output current to maintain regulation. This, of course, would feedback that even less current was in the welding circuit and the "summing amp" would increase the current even more. Anyhow, I think you get the point. I reversed the lines at the filter, verified that the numbers on the wires correspond to those in the schematic and now I have control of the welding current. I have determined that R4 on the PCB is to adjust and calibrate the output current. R3 is to adjust the amount of feedback voltage is applied to the summing circuit.
                I haven't seen any information on calibration of the unit. Let me know if anyone found calibration for these or any similar topology. Thanks for the help from members.

                Mark

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