[ The faulty component is ringed in a red circle, centre left, just by the shaft of the larger Variac.]
It is a "Wima"-type capacitor, used for noise filtering, across the output of the bridge rectifier that it is attached to.
Probably a metallised polyester or polypropylene film capacitor, 0.1uF and 200v.
As minimutly said, they often go bang. Has it's failure had any consequence for the functionality of the welder?
Les on eBay tells me this failed capacitor wont stop it working, I have removed it via its attached wires to get the capacitor replaced (so its off the machine at present).
Les has advised I check the fuses, but I cant find any! There is CB1 & CB2 but I judge these to be trips? I've tested for continuity and both seem fine.
When I first powered up the welder (Sunday) the gas flowed without pulling the torch trigger. My fist step was clean the trigger contacts.
Next time I powered it up the gas solenoid must have returned to the closed position, no gas flowing. I've gone over all (I think) spade connectors, all seems good.
I've put a loop wire in the torch lead Euro output socket and no change, still only the fan running.
The solenoid on the PCB remains inoperative.
What can I check now, before I take a punt and send the PCB off to Les.
The photo I sent of the guts of the machine is from Google Images so not my machine, But to my eye it is representative of mine's condition.
When I first got the welder (untested) the torch and wire were all in place, like it was switched off and that was it. It came from a closed down body shop I understand, probably unused for 30 years I would guess?
Even when they were new, the biggest problem with BOC and Murex welders was poor contact at connections, screw terminals, dry solder joints, and loose or corroded spade tags!
For your welder, I suggest you will have to go through every joint, spade tag, and connector. This includes removing the PCB and inspecting all the solder joints, especially the many pins for the push-on connectors.
I would guess that the gas flow at first power-up was due to a mechanical or seal fault in the valve, rather than an electronic problem.
The relay on the PCB is only for connecting power to the motor, or braking it by shorting it. It does not drive the operating coil of the contactor, that is done by transistors.
The control circuit for the contactor is unusual in that it includes, in series, a pair of normally-closed contacts on the contactor itself. After an initial 24v DC is applied to the coil, these NC contacts open and place the 12v coil of the gas valve in series with the contactor coil, so the two coils "share" 24 volts to hold them active.
There are quite a lot of forum threads about the Transmig 140; this one may be helpful:
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
It explains that on the contactor, A1 and A2 are the coil terminals, and A2 and A3 are the normally-closed contacts.
I would suggest checking that A2 and A3 are fully closed and show zero resistance across them. If in doubt, jumper across them. Then press the torch trigger switch and check for 24v DC across the contactor coil at A1 and A2.
Thank for all the help, this morning I've been looking at and testing PCB input current, using mono copper wire in each black female PCB pin terminal on both connector blocks. I've only tested the black because only black leave the busbar. Then putting both terminals back on the pins I find I now have wire feed and gas, but no welding current. I will do as advised and check more connections, but it's progress!
I've tested the weld feed and return bullet connectors in the unit for continuity (trigger taped closed), and the earth clamp return back to just-inside the unit, both are good. So it seems the weld power out is missing. what voltage should this be? I see there is a gold block under the PCB what voltage input should there be please?
As Hutcho said, the key to the welder starting up is the contactor.
When the torch trigger is pressed, you should be able to hear the contactor close. There is a grey plastic button in the centre which allows the contactor to be closed manually, instead of relying on the solenoid coil. If you press this button, e.g. using an insulated screwdriver, the arc and gas should be enabled. You may also need to press the torch switch to get wirefeed as well.
If this manual test works, then you need to check the contactor coil voltage as per Post #9. The coil terminals ( A1 and A2 ) are towards the back of the contactor, top and bottom. The normally-closed terminals ( A2 and A3, or 21 and 22 ) may be marked "NC". The actual switched power contacts will be L1,L2,L3 in and T1,T2,T3 out.
At the bottom left of the PCB there is a small square bridge rectifier. Above it there is a 220uF electrolytic capacitor, and you could check the PCB voltage across that capacitor, positive on the left wire.
The DC welding voltage of a MIG welder is typically 15 to 30 volts. On this machine, that is varied steplessly by the larger of the two Variacs.
The gold block is the finned case of a high-wattage wirewound resistor. I do not have a circuit diagram for the Transmig, so I do not know what part of the circuit this connects to.
Ok, so I have the weld current when the contactor is pushed in. So that's wire, gas and with the contactor manually pushed in weld-current.
I'm no electrician so my meter use isn't fluent, but testing across A1 and A2 (hard to reach!) and with my meter set at 20v DC I'm getting 3v (trigger in). So I could be measuring incorrectly and loosing the zero? Alternatively I could have narrowed down the fault?
This sounds like you are making good progress and narrowing down the problem!
Normally a contactor that doesn't pull in would be a fairly simple diagnosis - either the operating coil, the driver component on the PCB, or the source of power is to blame. However in this case the strange series circuit with the gas valve makes it more complicated to decide where the fault lies.
When it is pressed, the torch trigger switches on the output transistor TR107 ( a TIP41A, 60 volt 10 Amp NPN power transistor ) on the PCB. This feeds 24 volts DC to the contactor operating coil, and in theory it pulls in. However, as it does so the normally-closed contacts ( labelled NC, 21 and 22 ) open, and add the solenoid coil of the gas valve in series with the contactor coil.
When you push the contactor closed manually you get gas flow. Is that due to about 12 volts measured across the coil of the gas valve, or is it perhaps just leaking again? Please check the DC voltage across the gas valve solenoid coil with the contactor manual operating button pressed in.
If there is enough voltage across the gas valve to operate it when the contactor is closed manually, that means the 24v power source, the contactor coil, and the TR107 transistor on the PCB are all O.K., so the fault is not on the PCB.
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.
Then set the meter to the 200 volt DC range and find a way to attach the test leads permanently to the contactor coil connections A1 and A2. Switch on the welder and press the torch trigger switch. The meter should show 24 volts briefly, the contactor should close, the NC contacts 21 and 22 will open, the gas valve will operate, and the contactor should stay shut, with about 12 volts on A1 and A2 to hold it closed.
The problem is that all this happens quickly, and your digital meter may not react fast enough to show the process. If you have one, an old-fashioned analogue voltmeter may respond better.
From what you have already said, that initial 24 volts when pressing the trigger switch is not there, it is only 3 volts? That is not enough to move the contactor at all. If so, try operating the contactor manually by pressing the button. Does the voltage across A1 and A2 change?
Just to clarify, I have gas and wire by pulling the trigger, but no welding current.
I only have the welding current if I press the contactor in.
So if the gas is only supplies via the Contactor coil is it part working (possibly with a 3v input? So not the 24v needed for the contactor to operate?
I've just tested again (in the dark with a head torch) and I've seen a spark (twice) when pushing the contactor in (twice), this on the connections on top of the front left (of three) large blue pots, mounted on the base at the front inside, clearly that not ok! I'll look at this tomorrow. Pushing the contactor in didn't change the 3v on A1 & A2
I have a modern (Parkside) meter, with this on A1 & A2 and I then pull the trigger I get 3v. If I push the contactor in it's still 3v.
Les has asked me to do this test, how do I identify these pins please? Clearly I need to measure the Inputs via the block and the outputs via the PCB pins.
There should be 24vac going in on pins 101/17 and 101/18. Coming out to the relay on pins 101/14 and 101/16 should be 24v dc. If the 24v in is OK but you are not getting 24v dc out when triggered then the PCB should be the fault.
Les ( Les Ford of Tec-Arc in York ) must have a circuit diagram for the Transmig.....!
At the lower edge of the PCB there are two multi-pin connectors. From the left, they are PL102 ( 9-pin, with one being a gap ), then PL101 ( 18-pin, with one gap near the centre ).
At the extreme left-hand end of PL101 ( the RH connector on the PCB ), i.e. the "start" of that line of pins, is Pin 18 ( marked on the board ), and to the right of it is Pin 17. Les is asking you to check that the power supply for the PCB is present, with 24v AC coming in across Pins 17 and 18. That voltage feeds directly into the small rectangular bridge rectifier, and you could equally verify the PCB power supply by checking the DC voltage after the rectifier, across the electrolytic capacitor, positive on the left wire, as I mentioned in Post #13. By checking the DC, you are automatically confirming that the AC is there. Please let me know what this DC voltage is.
The arc that you can see inside is at the three blue cylindrical components, which are high-value electrolytic capacitors. They are probably something like 10,000uF at 50 volts each, and they are all wired in parallel across the DC welding output, to smooth it and provide a stable arc. There should not be any arcing there! There must still be some loose or corroded screw terminals or push-on tags in the machine.
The output from the board when the power transistor TR107 has been switched on by the torch trigger switch is on Pins 13 ( not 14 ) and 16 of that PL101 connector. These pins are located 3 pins and 6 pins down to the right from the LH end pin ( number 18 ). With the trigger pressed, you should see around 24v DC coming out across Pins 13 and 16.
This switched voltage goes on to the series circuit with the contactor, the gas valve, and the NC contacts, which I have already described. Pin 13 goes to terminal A1 on the contactor ( the upper terminal ). The contactor coil resistance is 7 Ohms. From terminal A2 ( the lower terminal ) there is a link to normally-closed contact 22. The other NC contact, number 21, goes back to the PCB at Pin 16.
When they are closed, these two contacts short-circuit the operating solenoid of the gas valve. When open, i.e. the contactor is pulled in by it's coil, or manually with the button, the gas valve coil resistance of 50 Ohms can be seen across those two contacts.
The fact that you get gas flow when the torch trigger is pressed but without the contactor closing, and that the 3 volts measured across the contactor coil does not change when the contactor is operated manually is wrong, and hopefully indicates where the fault may be:
- the gas valve is supposed to be shorted by the normally-closed contacts 21 and 22 until the contactor moves and opens those contacts. If the gas valve operates immediately with the closure of the torch switch, when the contactor has not moved at all, then those contacts are not normally closed!
As suggested in Post#17, please check continuity across those NC contacts with the machine powered off and the contactor at rest. You should see about zero Ohms. If you then press the manual contactor button, the resistance should rise to about 50 Ohms, which represents the coil of the gas valve.