Introduction: MIG Welder Repair and Mods
The torch has a cold tip, in that its only live/hot when the torch trigger is pulled. It also came with a good quality ground clamp. .
It only gets used a few times a year, but on my last project it wouldn't spark or wirefeed at all.
On attempting to repair this welder, I learnt a lot about the internals of the welder.I also made a few modifications which could be useful to others.
Hence this Instructable, on how it was diagnosed, repaired, and the mods carried out.
Step 1: Fault Diagnosis
Inside the welder, the electricals comprise of
The building blocks of this welder are actually relatively simple (in comparison to modern day inverters). It may be heavy and bulky but it comprises of simple analogue electronic components, which makes it fairly easy to repair.
The control PCB is split into two functional modules on one board
On this MIG welder, mains power enters the control PCB. The control PCB has a miniature 12v transformer which feeds ac voltage through the torch trigger switch.
When the trigger is pulled, the 12v ac is rectifed by a mini bridge rectifier into 12v dc, which then powers a "big daddy relay", also known as a contactor. This contactor is basically a magnetic switch which uses the low voltage from the trigger switch to control the heavy current high voltage feed to the primary coil of the big daddy transformer.
The primary coil of the big daddy transformer has multiple primary windings.4 primary coils for the 4 settings via the 2 power control switches (Min/Max, I/II). Depending on the switch settings, this controls the operating voltage and current of the torch.
The secondary coil of the big daddy transformer is a single fat copper wire coil. The step down AC output (typically 15-30V) of the big daddy transformer, is then fed into a heavy-current full bridge rectifier. This is capable of rectifying up to 130 amps on my welder.
On my welder, this DC rectified output feeds the torch, and also goes back in to the control PCB.
The control PCB uses this DC power to control the wire feed speed of the motor. The speed control is done using a miniature relay, a potentiometer and a TIP35 transistor.
The fault on mines was pretty simple, the big daddy contactor was faulty, and not switching power to the "big daddy" transformer. Both on board fuses tested ok, as well as the overheat thermal cutout fuse.
On desoldering the contactor and measuring the resistance of the coil, it was 87Kohms, far too high, this should be a few hundred ohms at max.
See Wiring Diagram above. The diagram is missing the 3 fuses for AC Input, thermal overheat cutout and wirefeed speed controller..
WARNING: Welders work on high voltages and currents. You should not meddle unless you know what you are doing and taking the necessary safety precautions.
Step 2: Source It
Easier said than done!
Finding an exact replacement contactor, was not going to be easy to source. In todays throwaway society, its often cheaper to replace than repair. I refuse to do that, where theres a will theres a way!
I found an alternative contactor/relay which I can connect "off-board" using wires rather than mount it on board, as the physical pinout for the legs will not match.
My original contactor was rated for 240V at 40Amps, operated by a 12v coil.
40 amps is an overkill, as this welder has a 13amp fuse, so uses 13 Amps at max, lets say 20A at an instantaneuos spike.
I found an alternative contactor on eBay which was rated at 240V at 30 Amps. The item description mentioned that it had been tested to work between 6V-15V, although it was rated at 9 volts. At only £3 including postage, it was worth a try, so I ordered it.
Step 3: Fix It
You must ensure that the heavy current wire connected between the board and relay is rated for at least 13 amps. The contactor should also be mechanically affixed in position using tie/wraps etc to ensure it is securely fixed.
.
Step 4: Erratic Wirefeed?
Once the contactor fault was fixed, I noticed that the wirefeed speed was a bit erratic.
It was definitely the knob, as putting pressure on it at certain angles made it behav mis-behave.
At first I thought it was the pcb solder joints, so ran wires across the tracks to the potentiometer pins. That didn't help.
Then I took apart the potentiometer knob, thinking it was a dirty track or loose wiper, but that looked ok too.
After that I gave up and just replaced the 10Kohm potentiometer. I suspect it had a hairline crack on the resistive track inside the pot.
Wirefeed Speed problem fixed.
I added hot glue to secure it further!
Step 5: PIMP UP YOUR MIG!
Step 6: Add Castor Oil
Relatively simple mod.
Remove the plastic feets, enlarge the holes using an 8mm drill bit for the M8 holes..
Bolt on the castor wheels. I added double locknuts to prevent them from working loose.
Toolstation do a set of 4 of these castor wheels for a couple of quid.
Step 7: Notch in the Hood !
Typically, you need to open the hinged lid to fix mechanical wirefeed problems. But when you open the hinge lid fully, it cramps the torch feed cable, putting a further strain on the wirefeed, conduit. This exacerbates the issue you are trying to fix, not to mention weakening the heavy current cable. Pathetic crap design. Why couldnt they hinge it at the rear, so it opens like a normal car bonnet, and totally avoiding this issue?
Swapping the hinge mechanism from front to rear is possible but too much work, so I just notched out the pinch point on the lid, using a grinder. Doesnt look great, but function takes over form when it comes to welders.
I also drilled and tap screwed a max open restraint to prevent the lid leaning over 90 degrees.
Step 8: Crackle On!
My welder is working once again, and it was repaired for a few quid.
The castor wheels are really useful for rolling this around the garage
The wirefeed speed is smooth
The lid restraint works a treat
Now, after being side tracked, back to the sound of sizzling bacon. Laters!