nbc mig 140

[mike 109444]

In "diode" test mode it should not show as dead short if meter is anything like my diode test function (fluke meter) you should see (as some of the diodes have) 0.60 to 0.50.... ish. 0 is a dead short diode which is duff. Asthe four diodes look to be same types you should get same readings from each. Note readings will as you realise be different depending which way round your test leads are.

 

[mike 109444]

ps yes you could just melt solder at one end of diode and lift one leg but much better to use a solder sucker as when you go to replace diodes you will need to clear holes of solder.
If you find one duff just replace all four.

 

[jay1800si]

Thanks for the advice. I didn't get as far as removing the board today but I will try again tomorrow. Im pretty sure 2 of the diodes are rubber ducked so I will replace all four. Where is the best place to get them?

 

[shenion]

Farnell, RS Components and almost any electronics distributor would have them.

They look to be 1N5404. They are the real old "tapered end" package. Looks like some have already been replaced with more modern ones (two to the right of the 4.)

 

[jay1800si]

Thanks for that. I think you are right about some of them being replaced. I had a look at the back of the board today and the soldering on some of the diodes is not as tidy as the rest. Could there be another problem that has caused them to fail again? By the way it is the top 2 of the four that are giving the bad readings.

 

[shenion]

Could be a short somewhere, but it should blow the fuse before damaging the diodes.

Those diodes are probably dying from old age.

 

[jay1800si]

I will replace the four diodes and see what happens. I have no idea how old the welder is!

 

[jay1800si]

I have removed the board to get a closer look at the diodes. They are 1n5408. I will go and order some.

 

[jay1800si]

4 new diodes soldered in. Cant wait to get the welder back together and try it.

 

[jay1800si]

Well that's the welder back together and working again. Thanks for the advice.

 

[shenion]

Great to hear.

Now your good to go for another 30 years

 

[jay1800si]

Well I hope so! Thank you very much for your help.