Millermatic 250 (1992) troubleshooting

[ZAROS]

Good day to you all,

I’m tempting my luck today on behalf of a friend.
I wish to state first that I’m not a welder (nor do I have a machine to play around with yet) nor is my friend. I have a decent base in both electrical knowledge and electronic components. I’m looking out to helping him find and solve the issue/s of one of the welding machines he’s got to repair.
His knowledge of both electronic components and electricity might be far ahead of mine, although he’s doing business in such field and I am not.

Long story short, one of his customers asked him to repair a millermatic 250 machine as some of their welders noticed some issues with it, whenever the amp would be cranked up, it’d begin to stop welding either partially (intermittently) or completely and the said issue mainly occurred at higher amperage.

The feeding gears are new, the wire is of the adequate thickness, no apparent damage to the motherboard and all capacitors were replaced and he even added 2 extras (for a total of 10).
I believe he could do some R.E. and follow up all the pcb circuits and flow of the machine to eventually get to the issue but as of many of you might be well aware, that’s both a pain and a non profitable process.
He had the chance to get his hands on some interesting tech manuals over the years but this one is missing and wherever there seems to be some interesting info, we get to hit a wall with incorrect data.
He’s tried to contact Miller for the right manual but when you aren’t a certified repair tech with thousands spent on training of their machines...

So here’s some specs and more specific info;
MILLERMATIC 250
Serial# KC239955 (1992)
Assembly# 151987
Manual Required; TM-1308 (Not OM, nor 1308A+, the original 1308)

The original manual doesn’t cover the electrical and circuit diagrams, and the info we got our hands on weren’t in-depth enough or simply not of the right revision, meaning some components weren’t the same.

The schematics for KC227524 through KC289995 were D-151-990 (ours is 151-987 but I’m certain the data would be accurate)
And the schematics he has are for a newer revision of the TM-1308 which are not accurate at all for our model (SD-184-318) which is for KH313867 and up.

Ideas and suggestions are appreciated although I’m limited in what I can achieve as I’m simply a friend trying to help one and I don’t have experience with such machines unlike him. Although the puzzle is interesting.
I believe he wouldn’t be closed off to the idea of possibly paying for access to those schematics (both the circuit diagram and electrical one)


Any info would be of huge help!
Thank you and have a nice day,
-Mike

[G-ManBart]

People with proper access to Miller service manuals risk losing that access if they get caught sharing it. It's hard to imagine anybody taking that risk for someone they don't know...

[ZAROS]

People with proper access to Miller service manuals risk losing that access if they get caught sharing it. It's hard to imagine anybody taking that risk for someone they don't know...

Kind of figured as much. Sounds like Coca Cola’s recipe hidden in a vault.
That being said, event though I don’t have access to the machine anytime I want, I’m still open to ideas, possible tests to be done and whatnot.

Whenever they were using the welder, they’d usually do about 1in welds every few minutes at a lower amp setting and never really noticed any issue, but when some other welders came to use it for more time (still within the duty cycle range), they’d crank up the amperage and would notice issues with it. iirc it’d stop welding altogether although the wire would keep feeding.
As I mentioned, some caps were bad and all were changed with 2 extra caps added to the bank.
There’s no apparent wiring issue or failed component. We’re able to turn on the welder and do some testing with it but since we can’t replicate the welding process to troubleshoot the issue, it makes the repair somewhat troublesome and takes much longer to figure out.
The SCR appeared to be good from what my friend told me.

EDIT;
I’d have thought it could be a problem with the rectifiers because of the instability in the issue.
The problem only happens, or mainly happen, when the current is increased. If for some reasons the SCR didn’t do its job correctly of the adjusting the electrical frequencies, it could potentially cause inconsistent welding and intermittent problems. But then again, the SCR seemed fine (iirc he told me that when he tested it, it was fine) so are there any other components that could have a similar effect?

If you have any possible explanation, input, far stretched possibilities, I’ll be open to all.
Thanks,
-Mike

[ccawgc]

May need an O-scope and a load bank the see what is going on.
That way you can look at the gate pulses going to the SCRs then how the SCRs are
performing under load. But look real hard at the miggun. Could just be a bad liner or a wrong size or damaged tip. Most of the time if the pcb starts to fail it bad all the time, not just at high current. The other thing to look for is burnt or loose connections in the weld conductors. They make work ok at low amps and fail at higher amps.

[ZAROS]

May need an O-scope and a load bank the see what is going on.
That way you can look at the gate pulses going to the SCRs then how the SCRs are
performing under load. But look real hard at the miggun. Could just be a bad liner or a wrong size or damaged tip. Most of the time if the pcb starts to fail it bad all the time, not just at high current. The other thing to look for is burnt or loose connections in the weld conductors. They make work ok at low amps and fail at higher amps.

Thanks a lot !
I’ll look into this whenever I get the chance and give some updates!

-Mike