Migatronic Automig 273 repair

[cs.dk]

One of my smaller weldingmachines refused to work suddently.
Swapped the PCB from another similar machine, and it ran. So I guess we can leave the "power-stage" out of the troubleshooting.

The schematics are here (but the PCB is not shown) - https://www.migatronic.com/Files/Files/Downloads/ServiceSite/Manuals/50111805.pdf (Site 72)

The symptoms are; Pressing the trigger, it drives the contactor for a short duration (about 1 sec), and then it switches off. The wire-motor does not run. Weird as it should be, when releasing the trigger, the contactor switches again for ~1 sec.

I've measured, after swapping boards - The first big 5W 33R resistor is open. But i'm really in doubt that would be the only failure? Nothing seems burned, or otherwise damaged. Pics here (many are the same, just for compensating my focus, and a steady hand) https://photos.google.com/share/AF1QipM2NdUqkmed0LkO-4m4Ps07JSIcs3Ue_QIC_dliZPDppX_0a2f3z6EXNxR2dk7wxQ?key=aGhURDZLcHhmMlA1ajBnYm1hVjJLcnVqM1ZvbEln

Am I lucky, that the resistor is the only broken component? - The 4 other devices are CD4093, a couple of LM7915, an opto, and a triac IIRC.

[tautech]

Which model exactly ?

I bought a broken Migatronic Automig 180MX and easily found the schematics for it on their site and fixed it with just a 20c transistor.
For motor control mine has paralleled 33 ohm 4W resistors with the current sourced through paralleled PNP transistors.

See attached schematic.

[cs.dk]

It's a 273 model. Btw. not to confuse with the "i" models, those are newer inverter machines.

I can't figure out what the resistors actually do there.

The two (on the working board) 33 ohms resistors measures ~1 ohm in circiut, but i'm probably measuring series/parallel with other components.

I'm impressed, that a small Danish welder find it's way to NZ. But they used to be strong and reliable machines.
EDIT: Holy, that schematic is old - From the day, where they documented their machines.. My newer Sigma2 400 aren't documented at all. At least not for public. Sad, but the machine is still awesome.

[cs.dk]

Okay, now i'm really lost.
Removed both power resistors from the PCB. One is a dead short, the other is complete open.. How can that happen? No visual sign of burning, etc.

[james_s]

I've never seen a resistor go shorted, but I've had them open lots of times without any visible damage.

[wictor]

Well, the resistors are 0.33 ohm, so they are almost shorts. R33 means 0.33 ohms, not 33ohms.
Wictor

[wictor]

And most probably those resistors are used to measure welding current.

[tautech]

Well, the resistors are 0.33 ohm, so they are almost shorts. R33 means 0.33 ohms, not 33ohms.
Wictor

Yes of course, 'twas late last night when I posted and R33 was staring me in the face too ! 

And most probably those resistors are used to measure welding current.

No, they're current limiting for the motor.

I'm impressed, that a small Danish welder find it's way to NZ. But they used to be strong and reliable machines.
EDIT: Holy, that schematic is old - From the day, where they documented their machines..

I'd never seen one before I saw this one advertised as broken however when I went online and found manuals, parts list and the prized schematic I had to have it. 
The main welding smoothing caps were stuffed too but I had a 50,000uF cap so I put that in it.
This welder doesn't get much work however there's always little jobs when a MIG is just the right tool.
It still has the original handpiece which is a bit of a mongrel compare to a modern one.

[cs.dk]

You're right.. It is .33 ohms R33 and 33R is not the same thing.

Is there anything wrong in soldering 2x0,5m cable in place of the resistors, for a short test? Just to see if there are other failures, before i order new resistors.

[tautech]

You're right.. It is .33 ohms R33 and 33R is not the same thing.

Is there anything wrong in soldering 2x0,5m cable in place of the resistors, for a short test? Just to see if there are other failures, before i order new resistors.

Probably no problem but I would just check the wire motor is working first with another power source.

Can you post a picture of the PCB ?

[wictor]

First post has the info already. Mig is working with other board and there is link to pictures.

[wictor]

Have you already checked those transistors that are attached to heat sink?

Wictor

[cs.dk]

I haven't checked those. At least one of them is a LM7915 regulator. I'll have a further look after work today.

[cs.dk]

Had time today to go further - Soldered a "short" in place of the resistors (4m 0,5 mm2 wire for each ~200 mohm). That sort a works. The wire got hot really quick, but the wirefeed motor did run, slowly though. I could not adjust it on the pot, so something is still wrong.

The wirefeed motor can now run with the old PCB. But it still does the same as mentioned in OP. It runs for a sec, and then turns off.

@wictor, components on the heatsinks are marked up in the attached photo.

Hope someone can help me on the right track.
Thanks

[cs.dk]

Confused now..
From pin 2 of each regulator to the others input is 65V. From pin 2 to the same regulators input, there is 30-32V.. How can that happen? Is it an elevated/virtual ground?