Help with faulty mig welder

[Stevehall]

Hi, I have been given a Migatronic Rally 166, I started with a good clean as it had been in a shed for years, when I removed the side panels, I noticed a damage part, I haven't a clue what its called, or if it can be repaired or replaced. The machine may work as it is but I haven't tried it, and don't want to cause more problems than there is already.
The first pic is the same part thats intact (there are 2 mounted on the alloy plate) the other pics are where the broken one was mounted. Look a bit like some sort of heatsink idea to me?
Anyone got ant ideas?

Thanks Steve

 

[Barking Mat]

Has that been removed by someone?

Looks like a nut and bolt, and washers, holding a load of wires.

But then I could be sooooo wrong...

Welcome to the forum, by the way...

 

[a111r]

That's the rectifier which converts the AC welding voltage to DC welding voltage, and yes, the Aluminium is a heat sink.
One set of diodes have popped, I think. It will now not work at all. Needs a new rectifier.
Replace it with something like this:
https://www.ebay.co.uk/itm/1-DQFI20...488462?hash=item3b452a92ce:g:jHAAAOSwuQFejTzQ

 

[Seadog]

They are diodes. There are two groups of three providing half-wave rectification. Since there is one missing from each plate I'd guess that three is what was fitted originally.

 

[Stevehall]

Ah I see, I removed the bolt that holds the three eyes of what I know now is the diodes together, to get a closer look. The three strips from each diode have a hole in to create a weak spot (like a fuse I guess) this is where they have become detached. Thought I would try and solder them back together...but that didnt work! If I could re-attach them, would that work?
Thanks all very much for your help.

 

[eddie49]

The main welding transformer on the Migatronic Rally 166 has a centre-tapped secondary, which means that it only needs two effective diodes for full-wave rectification. The "effective" diodes each consist of three press-fit diodes wired in parallel, to share the high current. All six diodes are pressed into one aluminium plate, which is the positive DC output. The anode connections on each diode are welded to tags made from thin brass shim. They are pierced with a hole, and seem to be designed to act as fuses, so if a diode goes short-circuit the tag melts.
We can assume that three of the diodes have shorted, and three are still OK. You could test this with the "Diode Test" range of a digital voltmeter.

Usually the easiest way to fix this type of problem would be to replace the entire rectifier with a modern equivalent, as a111r suggested. However, in this case the rectifier is a structural module which also supports and connects the bolted terminals of the capacitor bank. It may be easier to start by identifying the diodes, and checking if equivalents are available. To do that, mark all the connections, make a careful note of where they go, remove the rectifier/capacitor assembly, and look for part numbers on the diodes.
Note - there appears to be a loose thin red wire with a push-on tag. It may be supposed to connect to the brass tag bolted to the heatsink.

 

[Stevehall]

Thats a great explanation Eddie. If there are no part numbers, can't it be worked out by the amount of diodes used & the amps of the welder? sorry if thats a stupid question, but I don't understand electrical workings!
Thank you

 

[Stevehall]

The diodes on the heatsink say DIE 25/03 is that a part number or a date?... I'll do some googling then!

Thanks

 

[eddie49]

Re-attaching the brass strips to the "bad" diodes probably will not work, since it is almost certain that those diodes are short. Normally a diode passes current in one direction only ( near zero resistance ), blocks it ( infinite resistance ) the other way. If you have disconnected and removed the rectifier/capacitor assembly, a check with a DVM on Ohms or Continuity will probably confirm that three diodes are short-circuit. They will show continuity with the meter leads connected in both directions ( i.e. from the aluminium plate to the centre post of each diode, then reverse the leads ).

For the other 3 diodes which might still be OK, set the meter to Diode Test, as indicated by a diode symbol. With the red meter probe on the brass tag of each diode in turn, and the black lead on the plate, you should see forward conduction, indicated by a value from 400 to 700. Zero would mean the diode has failed with a short, and "1" or "OL" means the diode has failed open-circuit. Reversing the leads should show "1" or "OL" on all 3 diodes, i.e. they are correctly blocking current in the reverse direction.

The "DIE 25/03" numbers don't mean anything to me. This type of rectifier assembly, with the diodes pressed in, is not easy to repair, so they are usually just replaced as a whole. However, as mentioned, the new rectifier would have to be mounted and connected to the capacitor bank somehow.

You could guess that each diode needs to stand a third of the current, and about 50 volts or more. These diodes are usually rated from 30 to 50 amps each. They are often found in alternators, so if you have any old ones lying around, there may be suitable diodes inside.

 

[Stevehall]

I measured the diodes with my meter on the Diode test and wrote the values next to them (pic attached) the tolerances were out of the range you said, but that could be down to me or the cheap meter?
Ive also spent a long time looking for stud diodes on eBay etc, I can find plenty of 50 amp versions, but they tend to be much higher voltages, ill keep searching tomorrow,
Thanks again Steve

 

[Seadog]

Don't worry about the voltage, other than the minimum. If they're higher it's irrelevant.

 

[Stevehall]

Thank you so much, for helping me with this.
I have found some 70 amp stud diodes on eBay, and was thinking of using just 2 instead of the three diodes (so changing the rectifier to using 4 in all rather than the 6 used as it was originally.
These are the diodes I'm thinking of using, is there any negatives to trying this?
https://www.ebay.co.uk/itm/1-x-70HF...117622?hash=item595c080076:g:e~QAAOSwBg5curVD
Thanks again steve

 

[eddie49]

Steve, from your measurements it looks like 4 of the diodes are still OK, but you need six. When wired in parallel, diodes have to have the same forward resistance to ensure that they share the current equally, so they should all be of the same type ( and preferably even from the same batch ). Since we probably are not going to find identical parts to replace the failed ones, you need a new set of diodes.
Using larger stud-mounted diodes to replace the original press-fit ones will be fine, and the ones that you have found look good, and at a good price. Two of the 70A diodes in parallel at each location is adequate. Is there enough space behind the large aluminium plate for the diodes studs and nuts to fit, without fouling the case?
On assembly, the stud bases of the diodes need a light coating of silicone paste to ensure good conduction of heat to the heatsink:
https://www.ebay.co.uk/itm/Heat-con...Thermo-conductive-grease-7g-tube/303588378398

The two metal-cased electrolytic smoothing capacitors that are bolted between the large and the small aluminium plates are polarised. Can you confirm that the positive ends are connected to the larger ( rear ) plate, the one that the six diodes are pressed into?

 

[Stevehall]

Ive had a look & yes, there is an earth sign on the stud of the capacitor at the small plate end so I'm presuming the positive is the other end connected to the larger diode plate end.
Regarding the silicone paste, I have some computer heatsink thermal paste, would that be suitable?

Thank you Steve

 

[Stevehall]

Oh, and there is plenty of space for the larger diodes, thanks for pointing that out!

 

[eddie49]

Regarding the silicone paste, I have some computer heatsink thermal paste, would that be suitable?

Yes, that is the right stuff.
Regarding connection to the anode ( tag ) end of the new diodes when you get them: they can be soldered, but the wire used needs to be thick and semiconductors do not like heat, so the joint has to be done quickly. I would suggest ring tags, crimped or soldered to a short thick flex, with a large ring at the other end to bolt to the solid flat copper strip winding from the transformer. Fasten the ring tags to the diodes with screws, maybe M4 size?

 

[Stevehall]

I will wait for the diodes to arrive to see exactly what I'm dealing with, then hope to bolt/screw all connections together.
can't wait to see the outcome... will report back when I get to the build stage.
Thanks to all for your input & help so far

 

[eddie49]

For completeness, I'm adding links to the circuit diagrams:
https://www.migatronic.com/media/3376/50111550_spare-parts-list_rally-166.pdf
https://www.migatronic.com/media/1923/50111890_spare-parts-list_rallymig-166-188-202-266.pdf

Earlier Migatronic threads on this Forum:
https://www.mig-welding.co.uk/forum/threads/new-capacitors-for-migatronic-rally-166.6064/
https://www.mig-welding.co.uk/forum/threads/migatronic-rectifier.57920/
https://www.mig-welding.co.uk/forum/threads/migatronic-rally-166-issues.31823/

In post #10 of that last thread, there is a picture showing a spiral wire resistor with a spade tag, near the capacitors. This may be a bleeder resistor across the capacitor bank, i.e. between the two aluminium plates, to discharge them at the end of the welding process. If this is not done, there will be a small arc the next time the MIG wire touches the workpiece, caused by residual charge in the capacitors. It is not enough to melt the wire, but can be annoying. I hope this resistor is present on your machine.

 

[Stevehall]

SHE'S ALIVE!
Thank you to everyone involved for all your help and advice, I would not have done this without you.

I mounted the stud diodes through the aluminium plate, then bolted everything together using thick strips of brass coated steel, put the machine back together and it works a treat!

I have posted some pictures of the rebuild, if you have any comments please let me know, and once again thank you all for you help with this fix.
Cheers Steve

 

[Seadog]

Great news, Steve. It's usually worth persevering