Faulty UniMig 240 - suspect contactor or diode pack

[StevieD]

Hi Folks,

1st post here but have always used the forum as a really usfull resource and project inspiration.

I've recent 'aquired' an old unimig 240 pro series machine. Came out of an old bodyshop so it took a bit of scrapping of overspray to actually find out what it was - the bonus on my part was it came with a full size boc argoshield light bottle foc

Initially nothing worked - don't think it's been used for 10 years, and was full - I mean full of dust & red overspray - red oxide colour lol....

I've read quite a few posts here and now have the thing powering up, the wire feed working, gas valve opening etc.

Put a new euro torch, got the bottle filled, and a new coil of wire on at this stage.

And pop - just a fizzle, like a light bulb fillament with 0.8mm wire. Same on all settings until I turn it up to full power then it blows the trip.

Tried my dc clamp meter over the ambilical cord but it just blinks?

I suspect it may be the contactor as it took alot of messing to get it to work initially, so will source a new one of those in the next few days,clean upthe connections - they're all tight, I'll put an new earth lead and clamp on it too.
I've gleaned from info on here that it may be the diode pack - how does one test it - my meter has a diode test setting but i've never used it?

Other question is what polarity should the torch and earth be for welding mild steel primarily? There's also 2 negative conections - I beleive for spray or deposition with what looks like a diesel glow plug symbol above - which should I use?

Last question for now is what's the second main rheostat / pot for on the front of the control panel, I think it's probably a spot timer (the print is gone here)? There's two smaller 'screwdriver' style pots for burnback and ramp settings.

I'll get a few photos later.

Thanks Steve
PS I also got an older spotmatic M2 in the deal for an extra £20 so know doubt I'll have some questions about that too.

 

[R Kraft]

It does sound like a diode problem.
How you would test them depends on the type of diode assembly you have .
A photo of the insides would be helpful.
Robert

 

[eddie49]

Using normal MIG wire and a gas cylinder, the torch should be positive and the earth negative. The "coil" symbols offer a choice of inductance values in series with the earth circuit - see an earlier posting on the Unimig: http://www.mig-welding.co.uk/forum/showthread.php?t=25530
The problem of the breaker tripping could be a shorted diode. You will probably have to disconnect the rectifier to check each diode. Make a note of where all the connections go first. A DVM on "diode test" should show infinity ( no conduction ) one way, and, by reversing the probes on the diode, the forward voltage drop across the diode ( about 0.5 to 0.6 ) the other way. Measure the AC going into the rectifier - about 20 to 40 VAC, and the DC coming out. With no load, that should be about 16 to 35 VDC, and it should increase as you click up the power settings.

 

[StevieD]

Cheers folks....

Took a look this eve - wrong polarity on the gun / earth, that's now set up correctly.

A look inside the dirty machine....

The cleaned rectifier

There's a capacitor and what appers to be a possibly a restistor or some other component on the ends of the 2nd and 4th heat sink plates? It's blue , but hard to see with the paint.

Took a few photos and took the rectifier out to check the diodes, gave it a good clean and tested with the digital multi meter - all read 0.425 ish V in one direction and more or less infinity on the probe swap.

Interestly the bottom two rows/ heat sink plates all the diodes ar the opposite polarity of the top two?

I'll put the rectifier back in and see what the ac in and dc out voltages are.

The trip may be my fault - it's only a 16 amp breaker - I'll get a bigger one once I get the welder working - it was my only spare

Update later!

Thanks again.

Steve

 

[StevieD]

All put back together and just the same

Tried to measure the AC in, not much luck -- only seem to be getting 2-5 volts and that looks like it's diminishing / fading, I measured accross the two heavy gauge wires from the bigger transformer on the left of this photo with all the different tappings for the current control feeding into it, wires marked a & b assuming these to be the input?

as the other connections onto the rectifier on the oppposite go to the +ve connector and to the other transformer via a mini copper busbar - I couldn't get a sensible DC reading off these?

Next step seems likely to be the contactor, in case that's giving me the stange input to the AC, didn't seem to make a big difference ramping up the power knob?

Steve

 

[eddie49]

The blue component is probably a spike suppressor ( voltage dependent resistor -VDR / metal-oxide varistor -MOV ).
Regarding the diode polarity: a bridge rectifier is sometimes made with alternate stud-anode and stud-cathode diodes, so they can share
a common heatsink without needing to be insulated. In this case, rather than beefy studs, the diodes are those nasty press-fit ones, so there are multiple diodes in parallel to share the current.
A 16A mains breaker should not trip unless you are welding at high power for a long time, and you're getting "just a fizzle". The breaker tripping could indicate a short somewhere. With the low AC reading you are getting on the secondary, it could be worth checking the resistance of the main transformer primary ( but it is very rare for them to fail ). Expect 40 to 20 Ohms, as you step up the power switch settings.
Yes, the "a" and "b" wires are the AC input to the rectifier. The DC connections go to the +ve Dinse socket and to the inductor or "choke" ( looks like a transformer, has a single winding of heavy copper wire or strip).
Try measuring AC out from the main transformer secondary with it not connected to the rectifier at all.
To rule out the contactor, you could disconnect it and jumper around the power connections, so that the main transformer primary gets AC power direct from the mains.
Another item to consider is the power range switch: faulty connections or burned contacts. Again, you could disconnect it, and jumper one mid-range power setting to test.

 

[StevieD]

Cheers Eddie - I'll try that overth weekend and see if I can get any positive results one way or the other.

Thnaks, Steve

 

[StevieD]

It's the contactor, disconected everything again and check out the main transformer primary windings ok,nothing evident on a short to the chassis etc.

By-passed the contactor with a 240v supply and checked the secondary ac output - 26VAC.

Connected the AC output back onto the rectifier, good dc out!

Tried to strike an arc and

Brilliant - thanks Eddie your a star!

I've contacter out now with everything labeled up- looks like a standard 3 phase jobie. It's an ABB A16 30 10 so I'll get one of those or its equivilent somewhere this week.

Thanks agaian folks.

Steve

 

[eddie49]

That's really good news Steve! It is always great to fix a good-quality machine and give it a new lease of life.

 

[StevieD]

Yes I'm pleased Eddie - couple of local repair places wanted silly money to look at it - or just weren't interested.

Just bought 2 new abb a16 30 10 contactors off the 'e' bay for £8 delivered.

Be back in business by the end of the week.

Now to focus on geting my old sealy 150 working properly again as a spare machine (actually I have two that i have canibalised over the years to keep working lol)with the knowledge if gained on this one, & try out the spot welder.

 

[Andy wag]

Thread resurrection time!

I have the exact same issue on the same machine but it looks like I have a different type of contactor in there.
Can I ask why you went for an abb a16 30 10
Or if anyone has any recomendations for an alternative contactor for the same machine.