Welder wiring question , please help bail out a mechanical engineer

[MTPTurbo]

http://www.mtpturbo.com/fairmont/dta250schematic.jpg

I am hoping that those of you who are more electrically inclined can give me a hand with this one. I recently bought an old ESAB (DTA 250 UM) tig welder and pulled the covers off to clean it out and discovered some butchered wiring. I purchased a schematic and have since figured out the bulk of it but have a couple of questions. (btw, there should be a link to the schematic above if I am doing this correctly.) As received the machine had two the triacs (nodes 64-10 and 7-10) removed and the current control pot on the front panel connected across nodes 64 and 10 and the "surge suppressor" (between nodes 64 and 7) disconnected.

Anyway, in this configuration, the machine will run but there is no current adjustment via either the foot control or the front panel (it is 100% current all of the time as best as I can tell). I have since replaced and properly wired the triacs and reconnected the surge suppressor but am not sure where the current control pot (measures approx 0-800 ohms) needs to be connected.

There is a symbol on the remote/panel control board on the schematic but I believe that is in reference to a smaller pot (only adjustable with the side panel removed) which resides on the remote/panel board. That board is all 20 ga wiring while the big current contol pot is 14-16 ga and everything appears to be accounted for (no obvious connection point for the pot wires). I am at a loss here. Upon reviewing the schematic, do any of you have any ideas?

Also, the surge suppressor was disconnected but doesn't appear to be cooked or anything. Is there any way to verify its function before reconnecting it? Can I test the machine with it disconnected (it ran fine before, but I had no current control, or triacs...)



Any help would be greatly appreciated.

-Chris (MTPTurbo)

http://www.mtpturbo.com

 

[Comcokid]

The schematic is not 100% readable, but I'll give it a shot.

I assume the "foot control" you refer to is just a foot switch. This is probably the "Remote Switch" nodes 38(?) and 37(?). If so, the "Remote Control" nodes 30(?) & 31(?) is probably the control pot - it looks to be indicated as "1K" which is close to the 800 Ohms you measured on the pot. If so, your schematic is probably more drawn for a welder model that had a control on the welder handle, and for your welder the control was mounted on the welder front panel. If so, your front panel welder control should connect as if it were the "Remote Control" to nodes 30 and 31.

These are just assumptions and guesses based on limited information. Check it the schematic against your welder.

 

[itsmoked]

I'm in agreement with the 'kid'.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[MTPTurbo]

Thanks for taking a minute to look it over, I guess I better add some info... there is a foot pedal current control (it measures 82k ohms at rest, and approaches zero ohms when fully depressed) which is wired across nodes 30 and 31. The "remote switch" activates the main contactors to make the torch live, and i wired correctly. The foot pedal is supposed to allow the current to be varied fron zero up to whatever level the front panel knob (0-800ohms, 0 ohms = min current, 800 ohms = max current) is adjusted to. Also, the main current control potentiometer appears to be a larger, higher current device than what leads 30 and 31 wil support. I am guessing it should be wired across 12-10, 66-67, 64-7 or some combination of 2 of those 6 nodes. If it helps, I have a copy of the manual (including a few bad photos and the schematic) at:

http://www.mtpturbo.com/fairmont/ESAB

DTA 250 UM.pdf

I sort of drew a schematic of the small control board connected nodes 30,31, etc.. and I just don't see any place where the main current control pot (0-800 ohms, on the front panel) would tie in.. I am questioning whether the foot control is even the correct one at this point... I know I am not giving you all much to go on, just post any questions and i'll do my best to fill in the blanks..

Thanks again
Chris S.

http://www.mtpturbo.com

 

[MTPTurbo]

http://www.mtpturbo.com/fairmont/ESABDTA250UM.pdf

Looks like it won't link with spaces in the filename, I have renamed the document (removed spaces) link above.

MTPTurbo

 

[MTPTurbo]

***UPDATE***

I spoke with Dennis at ARCO welder repair who seems to know these machines better than anyone and think I have it figured out... It turns out that removing the pot currently mounted on the front panel and rotating the PC board with the panel/remote switch etc lines up the board mounted pot with the current control pot mounting hole on the front panel..someone did some creative engineering and added a pot which wasn't needed and re-oriented the board..hence the confusion. It looks like putting a 1K 2 watt pot back on the PC board (the original has been removed along with the current control SCRs) will get me back up & running..


Chris S.

http://www.mtpturbo.com

 

[itsmoked]

Hey that's some good sleuthing there. Good luck!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com