Cause or symptom?

[Malky]

The old mig stopped again, got it going the last time with a clean and tighten so not spent anything on it yet this looks like it may involve cash so probably won’t fix it but even after cleaning the contactors on the relay they still rattled a bit when pulling in.
What does the tower of cooling fins mounted on a threaded rod held together with a nut do in this picture, didn’t look like this the last time I looked.
Contactor rattle caused burn or burn caused contactor rattle?
TIA. M.

 

[Malky]

Closeup

 

[Seadog]

It's a selenium rectifier. That dates the welder a bit.

Easily replaced with a modern silicon rectifier.

 

[500e]

As @Seadog says Selenium rectifier normally if they go Futt! you never forget the smell

 

[Malky]

Thanks guys, so would replacing help contactor pull in or do I need a relay as well. £££, have other boxes so if it’s going to be constant hassle I won’t bother.

 

[Seadog]

Replace the rectifier and don't worry about the contactor. If there are issues with it you can then replace it. One thing at a time.

 

[500e]

We are only saying what that item is they normally just stopped working & smelt, it looks U\S

 

[Seadog]

You can see arcing between the top plates. It's faulty so needs to be replaced.

 

[zzr1200]

As @Seadog says Selenium rectifier normally if they go Futt! you never forget the smell

Rotten cabbage..........

 

[Malky]

Ok had a quick look for numbers, will any of these point me in the direction of a suitable silicon rectifier.
best guess-
H 125 LA
9000 or the other way round 0006
6915L, these numbers are difficult to make out and could easy be 0 instead of 6 or 9, 6 instead of 5.

tia M.

 

[a111r]

What's the make and model of the welder?

 

[Malky]

What's the make and model of the welder?

It’s a big old box with MicroMET 400 written on it, weighs a ton and made by an English company called Rockweld. Sat in my shed for ages and probably sat elsewhere for even longer. Stuck a plug on it earlier in the year and stood back, fired up straight away but a bit intermittent, some electric cleaner and a clean/tighten of the terminals and it seemed to settle down with only an occasional rattling contactor as said before, but now nothing and that rectifier didn’t look like that.
Rockweld might have been taken over by gkn after a while but think I’ve seen similar boxes with boc on.
https://www.mig-welding.co.uk/forum/threads/is-this-welder-scrap.104147/#post-1645246

 

[eddie49]

You can get an idea of the voltage involved in this auxiliary power supply by looking at the red electrolytic capacitor, and the octal-base relay.
However, any modern bridge rectifier will do the job:
https://www.ebay.co.uk/itm/KBPC5010...0A-Metal-Case-1000V-Diode-Bridge/254169257903

 

[Malky]

You can get an idea of the voltage involved in this auxiliary power supply by looking at the red electrolytic capacitor, and the octal-base relay.
However, any modern bridge rectifier will do the job:
https://www.ebay.co.uk/itm/KBPC5010...0A-Metal-Case-1000V-Diode-Bridge/254169257903

Ok this has now been delivered including the bent legs, anyway in what order do I go about fixing the wires onto it?

 

[eddie49]

That new rectifier has certainly suffered in shipping! The legs can be gently straightened and it will probably still work. You can make the new connections with push-on tags, or by soldering. A short nut and bolt will fasten the new rectifier down to the metal baseplate.
There's a black wire coming in which goes to a bridge across the two outermost ends of the old selenium rectifier, and then goes to the negative ( the case ) of the red capacitor. That black incoming wire, and the black lead to the capacitor need to be joined together and then connected to the Negative ( - ) tag of the new silicon rectifier.
I am pretty sure that the tag at the exact centre of the selenium rectifier is the positive ( this is taking me back at least 55 years to valve radio days... ), so the red wire gets moved from that centre tag to the Positive ( + ) tag of the new silicon rectifier.
The AC input wires - one is orange ( ? ), one is green with a silver tracer - go to the two AC ( ~ ) tags of the new rectifier. This is alternating current, so it does not matter which way round those two wires go.

 

[Malky]

Thanks Eddie, just had a quick look don't see any marks on the rectified apart from those pictures so +marked with corner off the case the only other terminal marking is ac as you can see in the picture, which would be - and which 2 ac, take it the missing corner must be a standard to avoid plugging in the wrong way thanks also are you speaking about regular spade terminals as used in 12v wiring, they fit ok?

 

[eddie49]

Yes, the "corner off" indicates the positive, and that pin is turned by 90 degrees relative to the other three.
The negative will be diagonally opposite the positive.
One of the AC pins is marked, the second one will be diagonally opposite the first one
Yes, spade terminals as used in cars will work. If you make the joint quickly, soldering the wires would be fine too.

 

[Malky]

Thanks for all your help guys.
Put the new rectifier on and the good news is nothing went pop, the bad news is still no weld so thanks but think I'll just weigh this one in, I have others I can use.

 

[eddie49]

If you take the transparent plastic case off the plug-in relay and close the contacts manually ( i.e. carefully operate the metal armature, using an insulated screwdriver ), does the main contactor then pull in and the welder work?

 

[Malky]

Not at the machine just now will try that later, I know that see through casing is glued on, that relay was flagged up as a problem before, either that or its base but by cleaning and tightening all the terminals I could see it fired into life so didn't replace that relay at the time.