Old oxford 161 mig playing up!!

[Pete busby]

Right. Late update. I disconnected what I'm diodes ( the two red one in the photo? )
And it's still buzzed, drew alot of power before blowing the fuse again!
So what next.? What are the 2 things that the diodes fix to that in turn are fixed to the large alloy heat sink ( I guess that what it's for?) Could these be the problem? ??
It does seem that the main transformer is pulling to much power as it's such a drew when you activity the trigger it makes the work shop lights dim.. and it's never done that before.. well maybe a little when the power setting is up high. But never as much as it does now.! Can I isolate some of the windings in the transformer to see if that is the problem? HELP.... its starting to get to me now.. it must be somthing simple.. but what !

 

[Seadog]

They are the diodes The red components you've disconnected are capacitors

 

[Pete busby]

They are the diodes The red components you've disconnected are capacitors

Ah. Right .. blond moment there then! So the diodes are the 2 that are bolted to the alloy heat sink?? So when mike says to disconnect them to test the main transformer I take it he means to unbolt them from and leave them disconnected then try unit again.???

 

[Seadog]

That's correct. They carry a high current and dissipate a lot of heat, hence the need for cooling. Good luck with the test.

 

[Pete busby]

So i just disconnect them from the main wires that come out of the transformer, and leave them disconnected. Then turn on unit and see if it still buzzes? What should it do ?

 

[Seadog]

There will always be a bit of buzzing with a large transformer, however, if you do disconnect both wires you are removing the load from the secondary, so there should be noticeably less noise. The usual failure mode with diodes is a short circuit. If you have access to a multimeter they can be checked quite easily on 'diode' mode. For a working diode you will get approximately 0.6V in one direction and open circuit in the other.

 

[Pete busby]

Right just tested the diodes.
The top one is showing .0L in one direction and .475 in the other. Which I guess is correct.
But the lower of the 2 diodes is showing
.001 in one direction and .001 in the other which I think ( following the guide on how to test a diode ) means it's a short curcit.
And this would explain the massive drew on the transformer, blowing fuses and the over heated relay? !
is this diodes the cause of all my problems?

 

[mike 109444]

Were getting there !!! Looks to be a duff diode
You should find that with the diodes disconnected as said by seadog there should be a slight hum from transformer and no fuse blowing
There may be a number printed on the side of the duff diode. Bit like this pic

 

[Pete busby]

Were getting there !!! Looks to be a duff diode
You should find that with the diodes disconnected as said by seadog there should be a slight hum from transformer and no fuse blowing
There may be a number printed on the side of the duff diode. Bit like this pic

Right I'll try that tomorrow now. And report back again. .

 

[Seadog]

Sounds very likely.

 

[Pete busby]

So I disconnected the diodes and sure enough there was a slight hum from the transformer but no blown fuse.
So I guess the shorted diode is the problem? Need to find a replacement ebay?and do I uprate it at the same time?

 

[Pete busby]

One like this??
http://pages.ebay.com/link/?nav=item.view&id=171870571277&alt=web

 

[mike 109444]

The problem is trying to find a diode that will match the characteristics of the original with a "similar" device.
If you look at the original you will see an arrow on it. This shows the direction of positive flow through the diode. They tend to be fitted as a matched pair they are chosen to "work" together. That is not to say another "similar" diode won't work but it is taking a chance. If it was being used in a precise, critical or low noise (audio amp?) then it would mater a lot! If you wanted to "up grade" to a higher rating from say the 70amp to 100 then you would have to change both. This would not increase the power output of the welder but would make it a little more robust. You would be better investing in an additional fan to blow onto the heat sink.
You could try contacting technical arc They advertise on ebay under mig pcb repairs if you want there details. One of there staff is a member on this forum by the name of Rosepud. Check them out for price first! and if a bit to much then maybe we can look up the full specs of the original diode and try to match it with a similar (cheaper!) number device.

 

[Pete busby]

Thanks again mike.
I did look for an arrow on it, but I couldn't see one. I'll have to have a look again.
So what if I was to replace both of them with say the item in my eBay link? Would that work ?

 

[mike 109444]

Best bet speak to Tec Arc and see what they say, quoting the number on the device and make + model of machine.

 

[eddie49]

According to the markings, the original diodes are International Rectifier 70HF40, 70 Amps 400 volts. The one you found on eBay is 70HF80 - which is 800 volts. More is better, but the transformer secondary peak voltage probably does not go over 50v anyway, and at £20 that one is not exactly a bargain. The eBay items 201169846810, 200984854404, or 262315860092 are cheaper.

 

[Wedg1e]

You could try contacting technical arc They advertise on ebay under mig pcb repairs if you want there details. One of there staff is a member on this forum by the name of Rosepud. Check them out for price first! and if a bit to much then maybe we can look up the full specs of the original diode and try to match it with a similar (cheaper!) number device.

The last rectifier I bought from them was £38. Mind you that was still cheaper than the US Ebay clown who wanted 10 bucks for the diode... and 95 to post it!

Incidentally the '9416' on the diode shown above is its date of manufacture: week 16, 1994. It's a useful way to date equipment of uncertain history, a lot of components have similar date codes on them.

 

[Seadog]

There's a pair of 100a diodes on the bay at the moment - 182045711689

 

[rikrobson]

you could with a little bit of rewiring convert it to a full wave rectifier

http://www.ebay.co.uk/itm/RECTIFIER...114827?hash=item41a4e17b4b:g:oQcAAOSwZVhWTvrW

 

[Pete busby]

So if I was to go with one of the above diodes from eBay. Would I need to replace both of mine with new ones of the same value?
Or can I just replace the faulty one?