Murex BOC Transtig 350 - Unstable Problem

[MBConfuse]

Hello all,

My tig machine has all of a sudden developed an annoying issue rendering it unusable. The symptoms are - when welding on DC having struck an arc, it is stable for a few seconds, but then the ark starts to waver and breaks. Then it makes and breaks continuously. The HF seems to be doing its job correctly.

There is a series of diagnostic LED's on the circuit boards and i cant see anything amiss (I have the original diagram to decipher them), but it is near impossible for me to look at them whilst actually striking an arc so I'm not sure what happens then.

Does anyone have any ideas of where to start looking on the circuit boards for potential causes?

It has recently had the main current selecting potentiometer changed (10k ohm 2W) because the welding current would shoot through the roof for no good reason - it turned out to be this pot that was breaking down. I'm wondering if this could have caused more damage.....

As usual and comments, help, grunts gratefully accepted.

Cheers

 

[MBConfuse]

No one had anything similar? Could it be the torch breaking down?

 

[malcolm]

If it starts OK then goes bad then it could be a bad connection heating up. Try measuring resistance from the torch and earth back to the transformer when it's gone bad to see if you can find a bad connection.

Is it a watercooled torch being used with no water by any chance?

 

[MBConfuse]

Is it a watercooled torch being used with no water by any chance?

Lol, yeah i wouldn't put it past me to try something like that, but not this time.

I'll try some resistance measuring, thanks.

 

[Jake SS]

Are you positive that all your connections are right. Do you have hf set to "start"
If you are welding DC try turning off HF and scratch start to see if it welds better.
Are your consumables in good nick, too much gas etc etc
Cheers

 

[MBConfuse]

Yeah absolutely positive. I did try scratch starting (I'm not at all practised at it) with the same result as before. Consumables i tried swapping last night and initially i though it helped lessen the frequency of cutting in and out, but its virtually the same - perhaps once or twice a second. I need to try a new torch and cable next i think.

 

[MBConfuse]

This weekend i'm supposed to be revising for exams, so obviously i've been in the garage playing with this d*mn welder. And I think i've fixed it!

Because I'm loathed to spend any money on a new torch, i decided to clean absolutely everything again, this time more carefully. I got right down to the toggle switches on the front and measured the resistance in each position; they all functioned fine. I then took each and every one out and opened them up to find the toggle mechanism inside. Most looked ok but i cleaned the contacts anyway. The one that selects high / low current range had a broken plastic seal and was full of grinding dust...... Having cleaned them all out ans scraped the contacts the machine now seems to be back to its old self. Yay!

So it seams that testing a high current switch under no load with a multimeter doesn't always give a true indication of its function. Which i guess is obvious really, but I didn't think of it.

 

[MBConfuse]

Now i need another excuse to put off revision. The car overheated last week, but managed to fix that yesterday. Damn!