MIG welder fault

[mk1tt16v]

hi everyone i am completely new to the site so please bear with me.

i have a mig welder and i suspect the pcb is at fault could anyone help me identify the pcb and what you think my problem could be

basically the power to the motor(12v) has stopped. the 12v feed should go to the wire arrowed

thanks in advance for any help

i have checked the continuity throughout the pcb so can only assume it is one of the components .
looking at the pcb tracks it looks like it is the big white resistor? on top
soz im not the best with terminology

 

[(*steve*)]

That large resistor looks like a current sense resistor. If it has gone open circuit then you may well get the symptom you describe.

The resistance is printed on it, read this and measure across the leads with a multimeter. Don't be surprised if there is some difference because it is in circuit, but if it reads more than 10% higher than what is printed on the side that's bad.

There are other components which could be the problem, but try this one first.

 

[mk1tt16v]

hi thanks for your reply and help.

just looked on the white resistor and on it it has a circled F and next to that it says 7wj,below that it says 2R2.

so the 2R2 means 2.2ohms?
does the rest mean anything?

once again thanks in advance

 

[Resqueline]

Yes, 2R2 means 2.2 ohms. What value do you measure?
The rest may mean that it can dissipate up to 7W, and that it has a resistance tolerance of j % (guess 10%).
The circled F is probably the make.

 

[mk1tt16v]

hi there is no resistance when measuring across the big white resistor or when i measure across the 2 contacts on the reverse of the pcb.all the other resistors have resistance.
is that white one a normal resistor or does it have to be replaced like for like? (if it is at fault).

here is the wiring diagram from the clarke website( apparently they are not 100% reliable but its a start)
ive arrowed the resistor that i think is the white one. i always thought that that symbol was for a potentiometer

 

[(*steve*)]

When you say there is no resistance, is it like you have touched the multimeter probes together, or is it like the multimeter probes are touching nothing?

And what resistance range are you set to? Suitable ones would be x1, or 200, not x1000 or 20k (you want a small number)

 

[mk1tt16v]

When you say there is no resistance, is it like you have touched the multimeter probes together, or is it like the multimeter probes are touching nothing?

And what resistance range are you set to? Suitable ones would be x1, or 200, not x1000 or 20k (you want a small number)

it is like the probes are not touching anything ie the display stays on 1.

i have the range set to 200

really appreciate your help. did you get my pm about changing the thread title?

 

[mk1tt16v]

is this a direct replacement?

http://www.maplin.co.uk/7-watt-wirewound-resistor-2179

 

[Resqueline]

Ok, that reads as open-circuit, over-range (i.e. over 199.9 ohms) or "infinite" resistance (hardly). That resistor is 35mm long. How long is the original one? 25mm?

 

[mk1tt16v]

mine is 25mm long! didnt even think od the length as being a problem.
do you know where i could get one?

 

[Resqueline]

Just get one that is the same size (though the legs can be bent). The area of it determines the surface temperature. The power level mentioned was just a guess.
The arrowed "potentiometer" in the wiring diagram is probably just a symbolic representation of the whole resistor-transistor-potentiometer circuit.

 

[mk1tt16v]

hi mate i already bought that one by the time i got your reply . do you think it will do the job as all the specs are the same except for the dimensions. if i snip the existing resistor off and solder the new one to the old ones legs will it work ok?
sorry if that sounds stupid but i dont have much experience with soldering circuit boards

 

[(*steve*)]

The circled F is probably the make.

I'm beginning to wonder if that doesn't mean it's a fuseable resistor. However I've not seen one in that type of package before.

It may also be a good idea to measure the resistance across the motor because there's a chance that it is short circuited. (by no means definite, but worth checking before you replace the resistor.)

 

[(*steve*)]

hi mate i already bought that one by the time i got your reply

As long as you maintain clearances, you can remove the old resistor and fit the new one. You'll almost certainly have to bend the leads underneath a little, and you want to be really sure that any pressure on the resistor will not cause a short (those components to the right of the resistor in your first diagram are most likely to be a problem.

If the larger resistor causes problems meaning you can't put the board back inside the housing then you'll need a smaller resistor. You may be able to run tests with it out of the box though...

 

[mk1tt16v]

ah ok thanks mate. with regards to measuring motor resistance, it is a 12 volt motor and i can supply 12v with my power probe and it works fine.

the circuit board it self doesn't have a housing so there is plenty of room for the bigger resistor.

just removed the resistor and measure the resistance and i get nothing so do you think it is almost 100% that this is to blame

if i open it up do you think i will be able to see the open circuit?

 

[mk1tt16v]

success!!

resistor got delivered this morning so i fitted (had to bend the legs) and it now works.

only thing is the new resistor does get hot to touch is this normal?

 

[daddles]

It's probably OK -- that looks like what's called a sand resistor; they are typically intended to dissipate a bit more heat than the smaller resistors.

Note you have your multimeter set to test diodes. The "OL" in the display means there's no current flowing through the device being tested (typical test currents are around 1 mA). Make sure you set your meter to the correct mode to measure (although in this case it probably spotted an open circuit OK).

 

[(*steve*)]

Well, it's normal for the resistor to get quite warm -- that's why they use a high power resistor there. You will also note that the original was mounted clear of the board, another sure indication that it was expected to get rather hot.

If there is room, consider bending the resistor away from the large cylindrical capacitor because they don't like getting too hot.

As long as you can touch the resistor without fear of being burnt (i.e. you can keep your finger resting firmly on it for a second or so) then it's not nearly too hot.

It is possible that the resistor just died, and it is also possible that the motor has work bearings, or a partially seized gearbox (or whatever) that is causing it to draw more current than normal. It may pay to check things like that out too.

 

[Resqueline]

Yes, those resistors can fry your finger and still be at normal dissipation levels.
Just crack the ceramic case carefully with a vise (or a hammer) and you'll see how it's made, and probably be able to spot where the resistance wire has opened.
In the old tube TV sets some of those resistors used to have an (external) overtemperature fuse, consisting of a springy contact leaf and a solder blob.

 

[mk1tt16v]

ok i will check those. thanks very much for your help. saved me a couple of hundred pounds!

i used the welder yesterday for about 4 hours without a fault so im pretty happy now.