Millermatic 250 Problem, No Arc

[Big Wrench]

I have a MM250 analog machine that I bought new in 1995 and have used it all these years without any problems till now. I was welding up a project and lost the arc, I pull the trigger and have wire feed just no magic. I got my volt meter out and checked for voltage on the posts by the feed assembly and didn't get any voltage. I'd say this is a relatively "low miles" machine because I've probably only ran 3-4 30# spools through it since new. I odd thing and it's done it since new is after about 20 minutes of the machine being on I get a buzz noise, it's done it since new and I just put up with it for all these years.
I have years of auto electric experience so with maybe a little help and some guidance I'd sure like to try and diagnose this machine. I did a search on this site and didn't find anything that fit my problem, I did find a thread about "burn back" which I have noticed I have, I have been dealing with the burn back and if there is a fix for that I'll do that after the machine is fixed.

[Aeronca41]

Need your serial number to find the right diagrams. Do you have the manual?

[MAC702]

Make sure you are on V-DC on your meter. You won't get anything on V-AC for the welding voltage.

Buzzing could be a bad contactor. Are you able to identify that part? It might be on the AC side, so you have to remember to use the right settings on the meter for what you are testing.

[Big Wrench]

The serial number is KF842921
I do have the original book that came with the machine.
When I checked the voltage on the output posts I was on DCV

When I bought this machine I actually bought 2, the other was for a buddy of mine. I called him today and asked him if his ever buzzed and he said no. So mine has had that buzz for 25 years and it wasn't problem, just annoying.

[Aeronca41]

Do you have the version with the little LCD displays on the front, or two knobs? Does it have the 14-pin connector on the front?

[Aeronca41]

You've probably already done it, but check all the connections between the output of the SCRs and the output terminals of the machine. Could just be a bad connection somewhere.

I guess no matter which version you have, there isn't really much I can tell from the schematic in the book. Everything is on a circuit card, which is just a box. Going to need someone with access to the Miller Technical Manual for your machine unless you are a component-level PC board troubleshooter/fixer who can work through it by the seat of your pants. . You could try reseating the connectors on PC1. If I were guessing, it looks like there may be separate relays on the board for gas and power; see if you can find them and feel/hear them click when you pull the trigger. Look at the board and see if you see anything that shows signs of having been overheated or burned up.

With power off, check pins RC4-9 and RC4-2 with an ohmmeter and see if you get continuity when you pull the trigger on the gun.

Is your gas valve opening when you pull the trigger?

I guess I probably wouldn't worry a whole lot about the buzzing--could just be a loose piece of transformer core material or something else vibrating at the line frequency rate. I would think it was anything really serious, it would have shown up long before now. You can never say for sure, but not very likely.

You could also check for continuity between wire 44 where it enters the reactor, and the positive output stud.

You don't by any chance have and know how to use a general purpose oscilloscope, do you? First thing I would do is see if there are gating pulses getting to the SCRs; if present, you probably have bad SCRs. If not, likely a problem on the board.

[ryanjones2150]

My machine is a 1996 model, so if there’s something I can do to help you help this guy, Wayne, let me know.

Mine has two big knobs and does not have the LCD display.

A very harsh machine you have there, Big Wremch, but I saved mine out from under some redneck’s tree and it was in very sad shape when I got it. It doesn’t buzz either. It has a very mild hum when I flip the power switch.

[Aeronca41]

Good idea, Ryan. I was pretty sure that's what you had, and maybe a voltage measurement or two might be a help. Let's see what his configuration is and maybe with some pictures of the board we can figure something out. Jjohn may have the tech manual; when he sees this, he may be able to help. Or Cruizer--good to see him back while we're locked up. Pretty hard when you're flying blind and guessing.

[Big Wrench]

This machine has the two knobs, back when I bought it all appliances with digital control panels seemed to always have problems with them so that was my reason for buying the "stripped" down model, lol.

I was using my 215 to do a project the last couple days to build a grader that will work on a skidsteer for a good friend. Up to now I've only used it a couple times. I really like the digital control panel that automatically make your settings, WOW I was impressed with that little thing, it did the heavy steel no problem. I bought it thinking I would only use for body work, I though it was just a "light weight" machine.

[Big Wrench]

I found the wiring diagrams for my machine, I'm going to open it up and start checking it out. I understand some of the diagram, I'm by no means any kind of expert but I know how to use a DVM. If anybody has any suggestions of what I should check let me know and I'll report back what I find.

[Aeronca41]

I found the wiring diagrams for my machine, I'm going to open it up and start checking it out. I understand some of the diagram, I'm by no means any kind of expert but I know how to use a DVM. If anybody has any suggestions of what I should check let me know and I'll report back what I find.

I put some preliminary things to look at in post #6 of this thread. Post a picture of the circuit board.

[Big Wrench]

Thanks Aeronco41, I printed off what you want me to check and I'm going to check those items tomorrow and I'll post what I find.

[Big Wrench]

The power light comes on and when I pull the trigger the gas valve opens and the wire feed rolls work.
The transformer (T1) is now buzzing as soon as I turn the machine on.
I checked voltage from the transformer (T1) to the diode pack (SR1) and didn't have any voltage.

[Aeronca41]

Just to be sure I understand where you're measuring...you went on the anodes of the diodes--measured between wires 6 and 7 and got no voltage?
that is not a good sign. Some failed component is either shorting the transformer secondary to ground (thus very loud buzzing) or the transformer secondary itself has gone bad--that is not common, but it could happen. If the winding was not properly glued or otherwise held in place, the buzzing you heard over the years could have been the secondary winding vibrating in relation to the core material, and it has finally worn through some insulation somewhere, causing a shorted secondary. Didn't think of that before. Again, very unlikely, but possible. The transformer is at least partially working--the 24 volt windings are obviously providing power to the circuit card to enable the things you see working.

How loud is this buzzing? Is it any different now than it has always been?

Power Off


Check the resistance from your work (ground) clamp to the chassis of the welder--should be no connection.

Check resistance from wire 45 or 46 at the wire drive housing to the chassis--should be no connection.

Try disconnecting wires 6 and 7 from whatever they're hooked to--probably the diodes/SCRs. Make sure they can't touch anything--tie them in mid-air if necessary--Power on, and check the voltage between them (AC volts). Use at least one alligator clip lead on your meter--don't hold a test lead in each hand--not good for your heart health if something goes wrong. Only one hand in live equipment, ever. Don't get zapped.

After you measure that voltage, power down and disconnect wire 43 from wherever it connects to C7 and the work clamp. You now have all three transformer secondary leads disconnected from the rest of the circuit. Check the resistance to the transformer core material/chassis (I assume the transformer core is connected to the chassis both mechanically and electrically) from each of the three wires. There should be no continuity. Check resistance from wire 7 to 43, and then from wire 6 to 43. You should get a relatively low resistance reading on each of those two tests, and they should be identical.

[Big Wrench]

I'm thinking that's SR1 circled??, and the cables on the left side are coming from T1??.

I'm thinking I should see VAC going into SR1 ?? and VDC coming out of the studs on the other side of SR1??

Is my thinking correct?

So, I'm not reading any voltage on any of those posts.

I'm not seeing post 6&7 in my diagram