[machinewbie]

Hi Jim,

Thanks for responding. I have followed your threads for some time now.

I took a leap of faith and made the connection of the extra wire to the only terminal left on the PC board. I now have control of the amperage. However, it is of very low output. On the low and medium ranges it is unable to lay a puddle in 1/8" mild steel. On the high setting, the pedal responds but for a 300 amp machine it seems very weak. I think there is something miss connected on the output, but going from the "block" diagram I cannot proceed any further. I was really happy that I did not let the smoke out of the newly repaired board!

I could still use any help by means of documentation or pictures or advice. I agree with you, it is probably a very simple fix, a five minute modification. I am just at the end of my capabilities with my current resources. There is absolutely no rush. I use the welders to help friends and family. The TIG is learning vehicle. It is not a business, mostly for my own pleasure!

Thanks,
Lou

[tadream]

Hi Jim,

Thanks for responding. I have followed your threads for some time now.

I took a leap of faith and made the connection of the extra wire to the only terminal left on the PC board. I now have control of the amperage. However, it is of very low output. On the low and medium ranges it is unable to lay a puddle in 1/8" mild steel. On the high setting, the pedal responds but for a 300 amp machine it seems very weak. I think there is something miss connected on the output, but going from the "block" diagram I cannot proceed any further. I was really happy that I did not let the smoke out of the newly repaired board!

I could still use any help by means of documentation or pictures or advice. I agree with you, it is probably a very simple fix, a five minute modification. I am just at the end of my capabilities with my current resources. There is absolutely no rush. I use the welders to help friends and family. The TIG is learning vehicle. It is not a business, mostly for my own pleasure!

Thanks,
Lou

I'll try to locate that book this week, but no guarantee I'll find it. I'm sure you probably got the right things on that board, but the thyrister you mentioned is actually a Triac, not just an SCR, so therefore will pass both halves of the AC waveform on the control voltage. (It's kinda an oddball circuit compared to other welders). I do recall from memory that the AC voltage from the windings to that triac is approx. 80VAC. Once it passes through the board circuit, it is fed to a bridge rectifier and then to the control winding. This might be a good initial test. The machine I fought with all those years ago initially had a bad bridge and there was where I screwed up. I stuck a new one in, and ASSUMED that the red wire was positive and the black negative. Ended up that this was backwards, but I was blind to it. The board monitors this voltage (but the polarity must be correct). The voltage from this circuit, as well as the control voltage from the potentiometer on the panel/pedal are fed into different parts of the LM324 OP-AMP on the board which in turn, controls the firing circuit for the triac. Maybe some on that will give you a good starting point while I search for that manual. LMK if you make any progress in the meantime.
Later,
Jim