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Cruizer, I will tomorrow. Thanks, and thanks to all replies. Correct, working on surface mount technology is a far cry from basic semiconductors.Comment
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Thanks to all for your help. I received the schematic from an Ebay source thanks for this forum. Will let you know what I find on this control PCB when fixed.Comment
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Super! Anxious to hear how it goes.Comment
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Well I got the service manual, with PCB schematic, but now there are no component values nor part numbers for any components. Unreal. Trying again...Comment
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Found Q5 2N6388 Darlington pair transistor defective. Replaced. Motor speed now works but only with voltage control, not the WFS control. Will be looking at Zener diode D6 next out of circuit along with a few other discrete components as its likely Q5 degraded other components as well.Comment
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Progress is good! Thanks for posting.Comment
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To the senior members of this group and any Miller technicians, as I posted last week, I have fixed the issue with the drive motor not working at all, which was the Q5 Darlington transistor. WFS speed control STILL does not work at all. Here is what I found so far: Voltage control pot R1 (pin RC2_pin12) ranges from (1-6 vdc), which is normal according to the test procedure in the tech manual. The voltage control DOES alter the drive motor speed ranging from (16-26.5 vdc), so it appears the voltage control circuit is operating OK. WFS speed pot (RC2_pin14) voltage ranges from (3.68-16.08 vdc). Connector (RC2_pin5/VOUT) measures (16.07-26.57 vdc) with trigger pressed. The output of comparator A2_pin7 ranges from (3.67-16.03 vdc/tracks with the WFS pot adjust voltage). Q5 base voltage changes from (11.03-15.03 vdc) using the WFS pot.I have lifted TVS zener diode D6_anode along with diode D7_anode and both appear to measure correctly with the diode function on the DMM and using ohms function in forward and reverse polarities. Even after lifting these two diodes the WFS still does not adjust. For some reason, the base voltage on Q5 does not appear to be adjusting the turn on/off states of this transistor, even though it's brand new. I have ordered a different 2N6388 to try along with these two diodes. The only other possibility would be if the incorrect voltage at (RC2_pin14/3.68-16.08 vdc) is causing this problem. The tech manual states the voltage at this pin should range from (6-26.5 vdc). If anyone knows this to be true, this could be the issue and maybe the WFS pot is BAD when under voltage (ohms out to 50K each side OK). I will continue on until I find the issue and at least replace Q5 again to make sure that is not the problem. Thanks for the help!Last edited by hemi-1; 02-13-2017, 02:41 PM.Comment
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I hope one of the real welder experts chimes in here. I guess I'm a bit surprised the voltage control varies the wire speed; I would expect it to only vary output voltage, but I'm no welder expert. Any chance you could get a readable copy of the schematic up on photobucket or some other site? Not likely to get a readable copy posted here, at least in my experience.
From your description, it doesn't sound like a bad WFS pot since the voltage at the comparator is varying nicely; seems unlikely the pot would be the source of the funky voltage range at RC2 pin 14, but hard to tell without a diagram.Comment
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D6 lifted (TVS diode) has no effect, as I've inserted D7_anode so a path to drive the motor is there. Q5 base resistor (R32_9.95K) and A2 input resistor (R18_997ohms) match their color codes, so no issues there. PTC1 is shorted and the motor drives all the time so that's not the issue.There is another diode on the PCB not shown on the schematic: (D3) which connects Q5_base_cathode to Q5_emitter_anode, which I've lifted as well with no effect either. I think the issue has to do with the voltage range being fed to Q5 base. If that range is supposed to vary all the way up to 26.5 vdc and it's only reaching 16 vdc, this is most likely the issue. I will remove the WFS pot from the circuit and use a decade box to insert varying resistances to see if I can get the voltage up to 26 vdc. I tried once swapping the pot connections to see if the volts adjust pot would make the WFS work, but there was no change (will try again to verify). Thanks for the input.Comment
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Aeronika, yes the voltage control is designed to vary the motor drive speed, in concert with the WFS control. Called wire-speed tracking according to what I've read, and you can see this in the circuit as the VOUT voltage directly connects to both the WFS circuitry supply voltage to the WFS pot and also is the same voltage present at Q5 collector which then feeds the drive motor.Comment
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Thanks for the info. Never dug into that; good to know. As I said, I'm not a welder expert, just a retired Systems Engineer who spent his life outside of design work fixing stuff from toasters to electrical wiring, cars and trucks, airplanes, helicopters, flight simulators, computers, and lots of other stuff. Still learning about welders and you have helped! Thanks. One of the best things about this site is you can always learn something from someone else.Originally posted by hemi-1 View PostComment
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Update. Installing a new Q5 Darlington fixed the WFS speed control issue (different manufacturer). Both the Voltage and WFS controls worked at this point.Only problem now is after I reinserted D3 and D6, electrolytic C24 decided to spark with a small hole in the side of the cap (top did not expand). I shut it down after that and have ordered new parts for this cap and the diodes. I think the cap (which is tied to relay CR3), had too many surges put on it (stress) and it blew by coincidence. Diodes D3 and D6 really have no bearing on this cap and are nowhere near the circuit except voltage wise. I will report back once I replace this cap and hopefully resolves all issues.Comment
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Thanks for the update. Hope you are now at the end of this trail of problems!Comment
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Well now I fully understand the entire schematic so any future issues that arise should be fixed in short order.Comment
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