I am trying to fix a 1991 Syncrowave 250. It has no amperage control. The welder was working and suddenly, went full output. I am quite competent in electronics and repairs and need a few hints as to the operation of the control board (135254). I am aware of the fact that the PCB common is NOT at chassis ground potential and the associated dangers with this.
As it is an older welder, it uses the shunt to detect current flow. It has the problem whether it is pedal or panel control of amperage. The output control works as releasing the pedal will stop the output. The usual things check out such as +15 V, - 15, +10V. Panel controls seem to have proper voltages, non symmetrical -15VDC and +6VDC on the AC balance potentiometer. I disconnected the shunt and applied a small voltage to the feedback pins to simulate between 100 - 200 amps (1.3VDC per 100 amps feedback). This would trick the board into thinking it was supplying 100 amps of welding current. By changing the panel control, it should have changed the firing angle of the SCR's thereby reducing the average output voltage. This had no effect. Knowing what systems are working on the control PCB, I need to figure out specifically where the panel setting and feedback control firing of the SCRs. I could probably change out OP amps and the dreaded Tantalum capacitors which are notorious for shorting out but I would rather troubleshoot this and specifically replace the damaged components. Any help would be appreciated.
PS anyone have a better digital copy of the schematic than what is in the online manual? Quite difficult to make out pin numbers, etc.
As it is an older welder, it uses the shunt to detect current flow. It has the problem whether it is pedal or panel control of amperage. The output control works as releasing the pedal will stop the output. The usual things check out such as +15 V, - 15, +10V. Panel controls seem to have proper voltages, non symmetrical -15VDC and +6VDC on the AC balance potentiometer. I disconnected the shunt and applied a small voltage to the feedback pins to simulate between 100 - 200 amps (1.3VDC per 100 amps feedback). This would trick the board into thinking it was supplying 100 amps of welding current. By changing the panel control, it should have changed the firing angle of the SCR's thereby reducing the average output voltage. This had no effect. Knowing what systems are working on the control PCB, I need to figure out specifically where the panel setting and feedback control firing of the SCRs. I could probably change out OP amps and the dreaded Tantalum capacitors which are notorious for shorting out but I would rather troubleshoot this and specifically replace the damaged components. Any help would be appreciated.
PS anyone have a better digital copy of the schematic than what is in the online manual? Quite difficult to make out pin numbers, etc.
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