BOC Transmig 140

[Tech2]

So as to keep things simple for me, I have the following:

101/18 PCB pin has a fluctuating max of 31.9v AC going to it and 101/17 has a fluctuating max of 9.8v AC

These have been taken by putting a mono copper wire in the block and taking the meter Com lead to earth on the welder. The meter set at 200v AC.

101/17 seems too low?

The connection on the blue electrolyte capacitor should now be cleaned and sorted.

 

[Tech2]

Were exactly is the small rectangular bridge rectifier please?
The blue cylinder to the right of the PCB has two terminals, I'm getting a max of 31v DC here.

 

[eddie49]

The voltages that we are checking, both AC and DC, have nothing to do with AC mains. They are derived from power supplies inside the welder and come from the secondaries of transformers. A transformer isolates the output on the secondary from the original AC mains which feeds the primary. Therefore, connecting one of the voltmeter test leads to Earth on the welder ( i.e. the case ) is not relevant.
These voltages, such as 24v AC or 24-ish volts DC or 3v DC at the contactor are always measured across the stated test points, such as "Pin 17" and "Pin 18" on connector PL101, or "A1" and "A2" at the contactor. One test lead from the meter is on one pin and the other test lead is on the other pin, with nothing on the welder Earth.

 

[eddie49]

The small bridge rectifier on the PCB is on the lower edge, just to the right of the left-hand white cable connector PL102, and next to the finned heatsink of transistor TR107. You can just make it out in the left-hand picture of Post #11 on this thread:

murex transmig 140 fault

Hi, I hope someone with a bit more knowhow of these machines can help. The machine will not power up... fan comes on, but no wire feed. I have taken the cover off and found the contactor does not click when the trigger pressed. I manually pressed contactor and there is power at the torch...

www.mig-welding.co.uk

However, any voltage measurements there would involve getting to the back of the PCB whilst powered up. That is why I suggested checking the DC power supply of the PCB at the wire leads of the 220uF capacitor on that board, which is much easier to access. On the picture referred to above, that component is a pale purple coloured cylinder, above connector PL102, with positive on the left wire, negative on the right wire.

[ The blue cylinder mounted on the chassis of the welder to the right of the PCB is an electrolytic capacitor. I do not know what part of the circuit it connects to, but I don't think that it is relevant to the present issue of the contactor not closing.]

 

[Tech2]

Ok, I did wonder!

Across 18 & 17 I vave24.2V AC across 16 & 114 I have 28.5v DC (trigger pulled).

 

[Tech2]

I think I may have done other tests incorrectly, so going to earth on the cabinet!

 

[Tech2]

I've tested the horizontal purple/blue cylinder to the left of the heat sink above 102/ it's 30v DC.

 

[Tech2]

I've tested this and I have 30.2v DC

 

[eddie49]

Good news, those are good results!
The PCB is being supplied with 24.2v AC and after rectification and smoothing that provides 30v DC.
With the trigger pressed, transistor TR107 is sending 28.5v DC to the contactor and gas valve. This implies that the PCB is OK.
With the contactor having a 24v coil, that 28.5v should be more than enough to make it pull in. However, it is not doing so, but the gas valve is operating. That really does sort-of confirm that the normally-closed contacts 21 and 22 are not closed...

 

[Tech2]

Yes, I could see that the PCB is doing it's job after following your test, and I guessed the the coil pulling the contactor in, isn't! My thinking was that ether I have to buy a complete contactor unit or I look to replace the coil? My guess is that the coil can be found at RS?
I can see that two clips hold the contactor to the coil and spring base unit, I did peep inside a little a few days ago, sounds like time to open it properly?

 

[eddie49]

With a coil rated at 24v, to pull in the contactor coil it does need something like the full 28.5 volts DC that is being sent from the PCB. However, it looks like that full voltage is not reaching the contactor coil. The current is routed through the normally-closed ( NC ) contacts 21 and 22. If they were closed, the gas valve would not be opening, because those contacts short across the coil of the valve. However, the gas valve does open, before the contactor moves, or before you try to operate it manually, so that implies that the NC contacts 21 and 22 are not closed. To check those contacts, please do the continuity test that I described in Post #17:

Start by measuring the continuity across the normally-closed contacts at the top and bottom right of the contactor. I think they will be labelled "NC" and numbered 21 and 22. With the machine powered off, set your meter to the lowest Ohms range ( probably 200 ) and check across those two contacts. The meter should indicate about zero, or less than 1 Ohm.

[ It is also possible that the fault is somewhere in the wiring ( i.e. another loose connection ), but the NC contacts are the first place to check.]

If you wanted to verify that the contactor itself is OK, you could disconnect both wires at A1 and A2 and connect an external source of DC power instead, to see and hear if it moves. A 12v car battery might just do it, but you probably need closer to 24 volts.

 

[Tech2]

Across 21 & 22 I have zero (or OL.) on the meter.

 

[Tech2]

12v car battery worked great, contactor operated quickly and positively.

 

[eddie49]

12v car battery worked great, contactor operated quickly and positively.

That is good, there's nothing wrong with the contactor itself. It just needs some voltage to drive it ....!

Across 21 & 22 I have zero (or OL.) on the meter.

When measuring Ohms, if the meter shows "000", or slightly more, such as "0.5" , that would mean Zero Ohms, or half an Ohm, which is more or less a direct short, and that is what there should be across a pair of contacts that are "Normally Closed".
However, if it's an open circuit, or a higher resistance than that range can measure ( e.g. a 1 MegOhm resistor tested on the 200 Ohm range ), then meters show either "OL" ( overload ) or "1."
So I believe the meter is telling us that the normally-closed contacts, with the contactor at rest, are not closed, which is what I suspected ( since they are supposed to short the gas valve when at rest, but the gas valve is getting power ).
If you press the torch trigger and then briefly touch a wire jumper across the terminals for 21 and 22, the contactor should pull in and the main arc transformer will power up. Taking the wire away, the contactor will stay in, and the gas valve will open.
The NC contacts are not conducting; they don't have a tough job, and should not be burnt, but something is wrong. You may be able to reach the contact points inside just by releasing the top of the contactor. If you do have to dismantle it fully, make a very careful note of where all the wires go first!

 

[Tech2]

I have the back of the contactor in my hand, this houses the coil that pulls the contactor and contacts in, is their a test you would like, it's now completely detached from the welder. As I see it it's function was proven with car battery.
The (forward) contactor part is a problem to detach completely because there are two, one top and one bottom heavy duty (to H/D Coil) unions, these aren't spade and don't come off the clamp screws because I can't (seemingly) retract the screws far enough to fully release them; (they must have been crimped-up in situ). I'd prefer not to cut them unless I have to.

 

[Tech2]

where I'm a little confused is that the car battery proves the contactor works fine. And it is the power supply that's missing to operate it. If I understand correctly 21 & 22 are the power circuit for this? Via closed contacts inside (alongside the other (open) contacts)?
Looking in the back of the contact breaker I can see that the 21 & 22 contacts are made (or should be). But I can also see they don't look that clean! Is this what we are looking to find here? It makes sense to me! Quite how I do clean them I'm yet to figure out.

 

[Tech2]

Bingo! Cleaned the contacts (in the contactor) between 21 & 22 (managed to get a needle file in from ether side) and we now have welding current!

Not taken it through it's paces as the .6 wire isn't great, bit of light rust in places. I'll lower the spool down some and hope it improves?

But apart from this, which I knew would be an issue, it's working, and as you've said elsewhere "another Transmig saved" thank very largely to your and knowledge and 'prove it' approach. So very many thanks.

 

[eddie49]

That is a great result, congratulations and thank you for staying with it and persevering with the remote troubleshooting till you succeeded!
For completeness, I will add that the fixed part of each one of the NC contacts ( and the others ) can be removed from the plastic block, but it is not easy. Each flat brass contact is tapered and has serrated teeth on both sides. The terminal screws have to be set at about half-way open, and using pliers plus those screws for leverage, the contact blades can be pulled out for cleaning.

 

[Tech2]

I did look at that but was short of half way on the screws, I had a peace of 2.5mm bent wire to try to push them trough from the back! However this added info is very helpful as doing it with the two wires in place isn't good! Yes I do stick at things so thankyou for recognising this, tolerating my minimal electrical knowledge, and you sticking at it. I now have the replacement resistor I opened this enquiry with, clearly the welder operates without it!

 

[Tech2]

With reference to my original enquiry, and identification of a capacitor. Which side is the positive please, my guess left? But I'd rather not guess.