Bzzz
I concluded you knew enough to do the job when you demonstrated you were aware of the audio curve potentiometers. Dont go to mush on us now, buddy, you have come too far.
In theory you can substitute fixed resistors for the potentiometer's variable resistance, as long as it's within the max and min range of the pot. Why bother to run the risk, though. If your tests can be trusted and the intermittancies occur at the same point on the pot, its a slam dunk. You have found the problem and probably can correct it at component level.
That is, if the pot itself funks out.
If you overheat the circuit traces they will lift from the board; you are undoubtedly aware. You will gain tremendous advantage if it is possible to cut off the stubs from the faulty device, preferably with a small shear, and solder the replacement to them. Diagonal cutters or nippers will do, but, by the way, never use them on leads of a good solid state device as the mechanical shock often does internal damage.
It is possible the board is of the multi-layer variety, perhaps unlikely, but possible. Complex circuits such as computer boards today are. They have traces laminated within the board itself, and, of course, its best to play it safe. Moreover, you will put less heat to the board, eliminate the need to remove solder and create bridges or leave drops that can short later, and of course, not be dissappointed to find a multi layer issue at hand. Of course, if its obviously a simple board, the worst that can happen is you ugly it up and shunt accross the lifted traces with bits of wire or wind up buying a new one as they intended anyway.
If the board has been properly identified as a simple one or two sided type and you do choose to remove the component do so as previously advised by Mr Vanderhoof. However, a fixture to hold the board (a vise of some sort) is a big help.
There are techniques which leave the hole clear of solder without removal and not needing much, if any, clean-up. Solder is drawn to the heat, but although I can work NiCd solder tabs and diode or transistor leads effectively, the size I imagine this potentiometer to be is very difficult to work. I presume, and suggest, you will wick the solder first before removal. From decades of experience I prefer to use an appropriate sized drill in a pin vise and a very light touch to drill out any solder clogging the hole or slot starting from the open side of the hole; slots take a couple of drill holes to clear and I use the same bit as a file or reamer... dont try to push it through the hole, let it cut- the solder is generally softer than the glass filling the epoxy or phenolic board-, and if filing with it, cut on the return stroke or push from the trace side, as pushing force from beneath the foil from the component side can lift the foil from the board.
Some boards, particularly those made by Phillips, seem to loose their traces if you look at them while thinking about heat; they can be very sensitive to heat although they are mechanically durable.
There should be no swearing and your comments should fall far short of blasphemy. In fact, anticipate smooth progress.
It is doubtful you will find silver bearing solder in use here; this isnt a test instrument, but I have seen it. I would add to Mr Vanderhoof's excellent advice that the thinnest solder melts with the least heat. 60/40 rosin core .032" diameter solder can be used successfully on nearly all standard electronics repair unless factory specificifications require a different alloy such as silver- I would, however, tend to agree; Bill's alloy is the best choice here if you can get it.
Before you devote too much energy to the repair, find out the cost of the replacement board. I have been impressed by the reasonable cost of a small percentage of Sears repair parts and you may have a pleasant surprise.
Best luck
d
The pot. is out of the board. I just heated it and used desoldering braid to remove the solder. Now I am not so convinced it is faulty. I should have realized that ohmeters themselves are not linear in movement. What appeared to be zero movement was when the needle was going past the high ohm markings. ( they are much closer together). I also spoke with the Century rep and he pointed toward faulty diodes. ($40.00) He said it was not uncommon in a machine of this type and age. Also said it often manisfested iself with erratic arc. (AC leaking into the circut or something) But he ALSO agreed that a faulty wire drive circut card could also cause similar problems. I think I'll fill the airspace inside the machine with concrete, allow to cure, attach rope. What an anchor!! Actually I'll call Sears parts and see what both of them cost and go from there.
new pot. put in circut board- little difference in arc stability. I put in a 50k and I believe the other was 100k. It still tunes in ok, but looses arc for no apparent reason. The tech said look into replacing the rectifier(diodes), but I'm not too thrilled with that idea. $85.00.
Bzzz
That's bad news. On the positive side, nothing can be said to be a total loss... at worst it can be used for educational purposes as a horrible example. You gained a valuable education at a bargain rate.
Did you price Sears parts for the board? What do the diodes cost themselves? Semiconductors are sold all over the internet. I dont have any idea how you can test them; high power diodes from microwave gear required special equipment.
There is no way for us [me] to know if the diodes, or in fact the board, is at fault or not. I cannot advise you on power supply issues; they are beyond my scope. With the closest stuff I work with the symptom tends towards bad capacitors, but they aint nuthin like welders. Does your tech test the circuitry or does he let you pay and pray?
I can do no more than wish you luck and perhaps recommend the power of prayer.
You might focus on one of the other things that affect your arc and eliminate IT as the trouble if you have not already. I am not aware that you have verified the gas delivery at the nozzle.
Non-invasive procedures should be attempted before surgery.
Many regulators put out a high pressure burst which tapers off after a second or three, sometimes outputs are simply irregular because they are faulty... I feel terrible about the trouble you have already had and would hate to feel worse if your guess that it was electronic-based was also wrong and winds up traced to your gas choking up.
Remember that your weld parameters will vary according to the ionization potential and associated characteristics of the atmosphere surrounding the arc; if it is inconsistant, so will be the arc.
Good luck
d
I believe the diodes are part of an assembly. The tech. advised me per phone so it's definitely pay and pray. It seems to work ok on the high setting with thin(.024) wire though. I would myself not feel at ease substituting parts in the power supply itself. I may just put some flux core in and see if it runs that ok. If so it may just be sold as is. For a reasonable price it's an ok welder. Really did teach me a few things about mig weldiing. And I do mean a few! I may look at a small 220v unit such as 175 amp. As for the gas flow, I would be surprised if it is erratic. I have a flowmeter and it seems to work just fine. I keep it set aroung 15 - 20 cu.ft./hr. How much could the liner effect this situation. I took it out and cleaned it at work with high pressure and solvent. But it is old looking. It was longer than the manual recommends so I cut it and reinstalled it. I just don't want to doctor a dead horse here. Thank you for the advice, this will be my last post on this matter unless a true revelation occurs.
Bzzz
You need to talk to a real technician, not a salesman with a soldering iron. Today they may be called engineers. A technician who has no means of testing prior to a repair, even if by some means of substitution, will get no respect from me.
YOU do not substitute the part, the manufacturers cross reference and component distributors make the substitution according to the characteristics of the component... the specs are matched.
If it works OK on high why not use it. Go ahead and try a backing if necessary.
The little "T" handle on your regulator is very easy to turn: double the flow to 40 and see what happens... stop back 1/2 way between the two and note what happens. It's gotta be a $1.00 investment in gas to experiment. Whaddayagot to loose?
In new welders, hold out for 250 amp or larger; you will benefit.
My final word, Bzzzz, and speaking as friendly as possible, I believe the most serious problem you are facing is interpreting your own observations. There is probably little, other than experience, that can help. You have to persist and critique your own mistakes to learn from them. Also, it is necessary to build upon known quantities... if that welder does FC OK what's different between that, and this application... it seems you need to establish a base-line which is known good, be it gas, control-pot characteristics, or performance in low or perhaps high range. To complicate things, filler wire has been known to be bad as have gas mixes as well. If you have a buddy who welds perhaps you can try to swap some consumables to see on his rig at similar parameters.
My, in fact our, problem with advice for you is I really dont know what all might be going on. I may have mentioned my new Miller was making me go nuts in a similar way and I know your pain. Good luck.
sincerely
d
well things have taken a turn for the worse now. the thing won't hold an arc at all with gas shielded wire. Lots of sparks ,but just blobs of weldment laying on top of the surface with zero fusion. High or low power it just does not arc. It acts as if no gas is getting to it. I have an adjustable flowmeter now so I can see what the flow is doing at the tank end. I feel plenty of gas coming out the gun nozzle. I have tried all kinds of flow rates. The unit works perfect with flux core on high or low. good steady arc. decent penetration. no intermitent arc. i am wondering if there is some truth to the bad diode conclusion. Yes I am switching the polarity from fluxcore to gas wire. Maybe the arc with fluxcore is more tolerant to less than perfect rectifying of the ac voltage than with gas shielded wire? I did stress to the tech at century that fluxcore worked just fine. That's when he talked of diode failure. I will go to the gas supplier Mon. and have him check the gas tank for problems. I'll just bring my welder with me and check if right there. Could a bad tank work ok for awhile, then start delivering faulty mix? The tank is still almost full ( 3/4 or so). The mix is C-25.
Bzzz
Sorry but at this point I cannot be of much help. Odds are that the odds are against any solution over the telephone or internet. That is not to cast any doubt on the Century tech's opinion. He may very well be right, but I cannot repair equipment under his terms and I have a hard time recommending them; my alternative would be to shop around for another repair facility.
Also, be advised yet again that I have no expertise on repair of weld power supplies.
If your supplier has the facility and inclination to try it out himself it should be a tremendous asset. Bringing a piece of steel?
I did not realize you had a fixed flow regulator.
Bzzz, please, dont just do what I or others suggest but think about it and see if it fits your situation. I have various nozzles which work best at different flow rates; I also have a different welder and am unfamiliar with yours. I forgot there were fixed types in the world.
If you find yourself stuck with that flowmeter email me and I may be able to give it a home. I feel responsible.
Hope all goes well
d
not a fixed flow regulator. a victor flowmeter.(ball type) You're right though I need to go to the supplier and discuss it with them. I'll bet he's gonna show me a welder before I leave. thanks for the suggestions though!
The diode is probably dead now- that is a good thing- a diode should have a very small resistance in one direction and a very large resistance in the other you can test with a normal ohmmeter- usually they blow to open- infinite resistance both ways.
Perhaps you can get a service manual from century. It should have debugging procedures that will get you right to the problem.
Luck to you
Bill
The rectifiers are probably stud mounted things on heat sinks with one wire attached to the top. Could be two or four of them. Might just be one device with 4 wires on it. Get the numbers (could be two different ones, one anode to case, one cathode to case). Look them up on digikey site. If you get hits then century hasn't put on there own numbers and you are in. Guessing at what you need in a super conservative way came to less than 20 bucks.
Do you have circuit diagram?
Bill