wire feeds but no arc

[dan810]

I have a millermatic 185. It has been working perfect with no issues till yesterday. I was welding 1/4 thick steel channel when I could not strike an arc. The wire feeds but no arc. I even tried at the ground lug and nothing. I thought maybe a duty cycle. I have never had it stop before though. I waited till today but still nothing. All breakers are set and I have full power out of the switch. I thought maybe it was the handset but I get no voltage across the pos and neg lugs in the unit with the trigger pulled. I get wire feed though. What could I try and what steps might I take to test it out. Thanks
Dan

 

[patreusch]

check rusty metal and connections in welder

 

[Robotstar5]

Has it got a contactor / relay that pulls in when you operate the trigger?

 

[malcolm]

The contactor ought to be working as normally switched the power to the wire feed too.

I've realised I don't know how the contactor switches the welding current. However it happens it'll be that bit that's broken, or a bad connection. Other possibility is that 2 diodes in the rectifier have blown open circuit at the same time, but that's really unlikely.

Anyone - how does the contactor switch the welding circuit? On the high volt side with the amps going through the contactor?

 

[Robotstar5]

Anyone - how does the contactor switch the welding circuit? On the high volt side with the amps going through the contactor?

Usually switches the primary side of the transformer.

 

[shenion]

Anyone - how does the contactor switch the welding circuit? On the high volt side with the amps going through the contactor?

Depends, some do AC in to main transformer. Some low end ones do it on the output side.

The Miller will probably feed of the contactor does not work as they ysyally have the feed powered from a separate transformer.

Most likely the contactor or the rectifier diodes.

Edit: Just checked the schematic, the contactor is on the output side. I'd check that and then the diodes.

Hard to tell, but the wire feeder may be off a separate tap on the weld output before the contactor.

 

[R Kraft]

As it looks like they have the contactor on the low voltage side of the transformer.
http://www.millerwelds.com/om/o1313p_mil.pdf
Robert

 

[dan810]

I figured it out. The power buses were very corroded and one contact was blown right out. I repaired it and put it all together and it worked fine for about two minutes and now I have wire feed and only very small amount of current. It barly sparks when I make contact. Ground is good. Also I blow the breaker for my 220 line every time I try to weld.

 

[Jake SS]

Control relay closes when triggered CR1, then W closes which is the "main contactor", which allows gas valve and weld voltage to be available. Main transformer supplies auxiliary 24V also weld voltage via the secondary side.

Where exactly was this "power bus contact" ?
Switch,contactor, where?

Cheers

 

[dan810]

Right next to the fan are two aluminum busses or channels one upper and one lower. There are two large anchors on each. All four were loose and one was completely blown out of the aluminum. I had to redrill the bad one and clean everything. Then I tightened everthing down. I am sure it all was put back exactly as I removed it. It ran fine for about two minutes and now low arc and blowing my 220 breaker on the wall.
thx
Dan

 

[Jake SS]

Can you post some pics?

 

[dan810]

I have posted pics. Still have very low power and blows main breaker.

 

[shenion]

Sounds like a shorted diode.

 

[R Kraft]

If the diode burnt it's way through the aluminum channel, it's pretty sure to be shot, one way or the other.
I would be worried about the companion diode also, a lot of heat was involved.

 

[Jake SS]

If diode or diodes are shorted, which is what it sounds like, then replace all of them.
If the ally plate where diodes and terminals are mounted are "pitted" or have been arcing then you will have to reposition them to a flat, clean smooth surface, if you dont they will "arc" out again, and use a smear of heat sink grease on surfaces when mounting them.

You will have to make sure that you get the correct diodes. There are probably 2 types of diodes in your rectifier, wont go into full on technical details but they should have an arrow with a line across, the direction in which the arrow is pointing tells you that the current will flow in that direction, the line across means that current is blocked in that direction.
Or better yet some numbers. Make sure you install them the same way.

Should be able to see picture on here somewhere, do a search.

Have a good close look to make sure that you have not missed anything that was supposed to be insulated.

 

[shenion]

You can get the diode assembly from Miller:

191 375 - positive half (has wire leading to reactor coil)
191 376 - Negative half (has wire leading to work lead.)

Probably won't be cheap . If you can read the part #s on the diodes you can probably find a supplier (farnell). There should be similar part numbers, one with a 'R' in the #.

I agree, they are likely all bad or soon to be if one has gone out. Replace all four.

 

[dan810]

Well, I'm sure now that is what I must have done wrong. I did not know they were diodes. In fact I didn't even know there were two different part numbers. I had them all out on the bench and cleaned them then randomly installed them again. Murphy's law tells me they are in there wrong. Do you know which one goes where? That is the next thing I will have to figure out. I will look at them tomorow to see if there are part numbers on them. Is there a way to test them? Thank you guys for all the help your awesome.

 

[dan810]

I looked at my parts list and I see where they go. Still need to know if they can be tested .
Thx

 

[shenion]

If they have a diode symbol on them, the ones with the arrow pointing toward the screw stud will go on the positive plate (the one that leads to the reactor/torch.) The one with the arrow pointing toward the wire go on the negative plate (connects to work lead.)

You can test the diodes with an ohmmeter, they will conduct one way and not the other.

 

[R Kraft]

If the diodes don't have a a diode symbol on them.
The threaded end is usually negative, if they have normal part numbers on them (like 1N1234), the ones that have a number ending in R (Like 1N1234R) have a positive threaded part .