Century Speed Control - mod 117-034

[eldergeek]

This is out of a Century Quick Fix mig welder. The speed control has died. The wire speed is very irratic. Runs smooth for a few seconds then starts 'stuttering' then smooths out again. It happened at all speed settings. I bypassed the speed control and the motor runs fine. The liner is good as well as the tip. I did some reading, cleaned the rheostat & because they are cheap, I decided to change out a couple of Transistors. One was no problem as I was able to find an exact replacement. The other is no longer made. Can someone confirm I crossed this part correctly. The original is a:

S1055M which crosses to a NTE-5539 then to a BTW69-600RG I believe.

All these are found here after a bunch of clicking. (scroll down)

I replaced them and now nothing out of the speed control at all. I did sumpin wrong.

[ccawgc]

Go back and make sure you put the right pins in the correct holes.
Also make sure you did not reverse the wires to the speed control.
One of the two below has a web site you can down load manuals.
If not PM me your e-mail address.
I do not have the diagram for your speed control. WCTA might be willing to send it to you.
880-094-666 is the part number of the speed control. cost is around $80.00
Depending of shipping and store mark up. Can be ordered from any LWS that sells Lincoln Electric welders.
You can call Lincoln Electric (WCTA Tech Service) 1-866-236-0044, and ask for help.
Two other sources of parts is A & A Hydraulic Repair Co. 1-800-992-9898
and Star Hydraulics 1-800-925-7741
If you need to cross a transistor, type into google NTE cross then select the NTE site and use the cross reference there. Once you get a cross you can check stock in a lot of stores that might still have it is stock.
The S1055M SCR is a 100 volt 55 amp unit, The NTE is a 400 volt 55 amp and the digi-key is a 600 volt 50 amp. The 600 volt SCRs use a little more power to turn on and may not work. The century engineers some times cut things real close so only a part of the original spec will work. Good luck and do give the guys at WCTA a call.

[CharleyL]

The Century MIG that I once owned had a problem with the tensioner holding the reel of wire. When in use the wing nut would gradually turn, tightening and slowing the wire feed. I fixed it by putting a grade 8 washer under the wing nut. No more wire feed speed problems.


Charley

[eldergeek]

Thanks gentlemen.

Charlie, I did suspect that early on and that is not the issue. The speed control board is the issue right now.

ccawgc, I was hoping going to the 600V would not be an issue. I am in the process of tracing the control board to draw a diagram. I am 99% sure it is in the correct holes on the board but I need to confirm. I will call WTCA tomorrow to see if they will send me one. I have found drawings & parts list for everything else. I also found the PN & control for $80, but as usual with these home units, its just not worth $80 to fix. But I will dink with it for a few months to try to bring it back to life..... LOL My time is cheap....

I wonder how many of these units are sitting in the trash with a dead speed control. I see posts for them once in a awhile. If they priced them at $40 they might sell a few more speed controls. LOL

[Bluewelders]

Could you post a picture of the board ?

[eldergeek]

No problem. Here are a couple. And a drawing of the Speed Control I made up. Lincoln PN: 880-094-666








The drawing is how the board is traced. I have checked it many, many times in an effort for it to be correct. But I have been staring at it for a while.

The pics are after I replaced both Q1 & Q2. I am positive Q2 is installed correctly. There is a very, very slight chance I have Q1 in backards. A pic of a working board is one way to tell us which way it should be facing, the drawing is the other. On the drawing I may have both of the Q1/Q2 SCR symbols wrong, along with their pinout labeling. The diodes are in the correct polarization I believe, based on their stripping. Feel free to correct me on any of it. The two diodes, D1 is black ceramic, D2 is glass w/copper color. You can see them pretty clear in one of the shots. I may have to pull them to read their numbers completely.



I know just enough about electronics to be dangerous. I am assuming this is a simple voltage regulating circuit. Seeing as how it is used to vary the voltage the the drive motor. I would bet someone with strong electronics skills can look at this and be able to see if I have anything wrong. If anyone can explain in some detail how this circuit works I would appreciate that too.

[Bluewelders]

Using a PUT or UJT as a relaxation oscillator to drive an SCR is an old enough circuit to be classified as a classic.
The thing that confuses me is using an SCR in a DC powered circuit.
Must be using the inductive kick from the motor for turn off.
http://www.allaboutcircuits.com/worksheets/timers.html

[dumb as a stump]

Just so you know ... A transister WILL be classified as a replacment part BUT it doesn't mean the pins are in the same place. Meaning you may have to mount it backwards or switch 2 pins around etc. So if you have the package it came in and the circuit diagram then you need to confirm e is e, b is b, c is c etc. good luck

[Bluewelders]

I still haven't had time to examine the circuit.
It looks to me like your circuit is one of these.:

[eldergeek]

Dumb as a Stump, I will double check the pinouts. I'm thinking its gotta be something simple.

BW, Thanks for the link, The drawing above is similar, but the speed control is in a series DC circuit. No A/C feed.

[Bluewelders]

Was the top photo the diagram of your machine ?
http://weldingweb.com/showpost.php?p...51&postcount=2

[turbocad6]

hey man, don't know if you want to spend the cash on this but something like this will solve your problem and give you a really nice variable speed control for your wire feeder, I used one to vary the speed of a mig wire feed I converted to a tig wire feed and it works perfect:

http://www.ebay.com/itm/400263776533...ht_4273wt_1263

only complication is you will need a 12v power supply for it.

[eldergeek]

BW, Yes, both diagrams came from Century/Lincoln links. The top drawing is how it is wired now, (after flip-flopping the brown and white wires on the Wire Feed Drive Block. Minie were back were backwards). The ladder diagram does not match the top one. Electrically it makes more sense to me than the top one.

TC6, I am thinking that may be the way to go if I can't get this one fixed. I would need to find a 24VDC model as my drive motor is 24VDC.

[turbocad6]

I'm sure there are others but I see one solution here for 24vdc

http://www.ebay.com/itm/12-24-VDC-30...#ht_951wt_1185

[Bluewelders]

Please correct me if I am wrong.
According to the top diagram.
The White wire of the motor goes through the trigger to the center tap of the output transformer.
The Black wire from the motor goes through the speed control to the bottom tap of the output transformer.
If the diagram is correct,the rectification for the wire drive motor is on the speed control board.

[eldergeek]

This part should probably go in the other thread



The above graphic is the original drawing from Century.

The below graphic I traced out your suspicion in Red-Blue-Green. I'm not sure the flow follows the green line rather it flows through/from the Rectifier. I made changes to the lower (ladder) drawing. Both Purple lines (upper & lower drawings), are how I think it should be terminated.



BW, I thought the rectifier took care of that. (left side of upper drawing) And I don't see any rectifier in the speed control board.

I think I fixed the lower, ladder diagram. Flipped the drive motor and speed control, made polarity change, and termination changes. Double check me. Remember purple is how I think it should be.

[eldergeek]

TC6, Thanks, I'm going to flip around the SCR one time. Then maybe try the NTE5539 version to rule out ccawgc's suspicion on 600v rated SCRs. Its the only thing that I can see is different. Any one know what the ohm rating of the rheostat is? I can't find a spec sheet on it.

245-108-000
R1379244

[Bluewelders]

The SCR PUT type circuit will only work as a rectifier/speed control if fed AC.
To me in your schematic, it looks like the SCR is rotated half a turn (Kathode and Gate swapped).
A SCR type circuit won't work with DC input,four layer devices will turn on with DC,they will not turn off, as long as there is power to provide charge carriers to keep the semi conductor active.

[tprothma]

Your schematic shows pin 3 (G) should connect to the clear diode, instead you have pin 1 (K) connecting to the clear diode. So you're definitely flipped! Do you have a new SCR? You may have damaged it. The pinouts between the original SCR and your new one are the same (NTE5539 and BTW69 are KAG pins 123 respectively).

Ohm out the outer 2 legs of the rheostat (pot) and you should get 64K according to your drawing. Now connect to one of the outer pins to the center pin and measure resistance as you turn the pot. It should smoothly transition from 0ohms to 62kohms without any dead spots (where it goes full open).

You're doing fine, just be careful and double check your work. Get a new SCR if needed.

Good Luck,
Tim

[eldergeek]

Attachment 212001

Could you mark this up? I see 'A' connected to the glass (Zener) diode.

Do I have the symbol labeled correctly?


I just confirmed the S1055M & NTE5539 & BTW69 have the same pinouts. Has to be standard thing.
I do have another BTW69-600RG. if that doesn't work I will try the NTE5539

I do the ohm test with rheo removed from the circuit, right? You are aware R4 is labled 68K ohms, right?

[eldergeek]

Ignore the above post, my edit timed out....




How about this? I made a mistake tracing the board. Looked at it too long that day.
Do I have the symbols labeled correctly?

I just confirmed the S1055M & NTE5539 & BTW69 have the same pinouts and mine is soldered in correctly.

I do have another BTW69-600RG. if that doesn't work I will try the NTE5539

I do the ohm test with rheo removed from the circuit, right? How did you come up with 64 ohms. You are aware R4 is labled 68K ohms, right?


If I understand this right it works like this, in simple laymans terms.

Power is supplied to the circuit. 24VDC(+) flows through D1, R1 & R2 and triggers Q1. This allows flow through Q1 to the Gate of SCR1. Q1 stays in a constant 'on' state.

At the same time 24VDC(+) flows through D1, R1 & R3 to the Rheo through C2 to 24VDC(-)

As The rheo is adjusted, it changes the flow through Q1,

Q1 in turn controls the flow through SCR1 thus the motor speed.

I believe the diodes are for overcurrent/polarity protection??

Don't completely understand what R5, R6 & both caps actually do. Circuit noise, impedance???

[Bluewelders]

That looks pretty reasonable now.
D2 is probably a zener diode to regulate the voltage on the Q1 circuit.

[eldergeek]

Well I tried my second BTW69-600, still no go. I put the original S1055M back in and it does at least get the drive motor turning when cranked way up. Kicks in at '8' and above. (spd 0-10) I did see they make a BTW69-200 for those following along. I checked the specs again and I don't see enough difference between these two, to keep it from working.

Buying a couple more NTE5539 to try will put the repair costs almost equal to a new speed control like TurboCad6 mentioned. Soooooo.......

TC6, What do you think of this one? It's in the states.
http://www.electronickits.com/kit/co...or/CKMX066.htm

I does need to be 30amp rated right? At least that is what the SCR on the original speed control is rated at.

[tprothma]

eldergeek, did you see my post. You definitely have the SCR mounted backwards. When you say you tried the second BTW69-600 did you flip it and make sure that the plastic side of the component faces away from your rheostat (your picture shows it facing toward the rheostat.

To answer your question I doubt the motor pulls more than 2A while loaded pushing wire. You can measure the current by putting a multimeter in series with the +24V (multimeter should be in Amps DC mode with the leads plugged in appropriately). On my Millermatic 135 I just replaced a 2A transitor with a 10A one. I think 30A is a little much, maybe but a 2A or 5A fuse inline with the +24V. You don't want to toast your motor and if your motor is acting up you can replace a fuse, maybe somehow lubricate the motor and such. Otherwise that's a good price and it comes with a pot which is nice also.

But if'n it were me, I'd still be fixing the circuit you have since it's a pretty simple one.
Good Luck,
Tim

[eldergeek]

Looking at the board traces, the SCR spec sheets, (all three) It doesn't look backwards? What is tipping you off that the rest of us are missing?

Not trying to be difficult, just not seeing what you are...