mightymig 90 feed issues

[you]

Hi i have an old sealey mightymig 90 auto with feed issues, it runs fine for a minute or two then progressively gets worse at feeding the wire, becoming very jerky.
i have replaced the electrolytic caps on the drive board so far with no improvement, also tried new tips, new liner, rebuilt feed gearbox and lubricated gear shafts.
Thinking of trying to replace the motor and or adding seperate drive power supply.
any input welcome

 

[Tom O]

I had a feed problem with my Miller 135 it turns out the nut for the wire spool axle didn’t have a cotter pin allowing it to tighten and bind.

 

[you]

thanks tom, thats a point i do need to put a washer on my spool but im not sure thats the main issue as if i press the trigger without welding it runs smooth again, making me think the issue is electrical, thanks

 

[the snooper]

Try all your connections especially the crimped ones as they can appear to be fine but electrically they can be anything but and if bad they will heat up changing the resistance

 

[you]

thanks will try that also

 

[eddie49]

.... if i press the trigger without welding it runs smooth again, making me think the issue is electrical

If the intermittent wirefeed failure occurs only when actually welding, as opposed to just letting the wire feed out, that implies there is a problem with the main welding power output.
In this small machine, there is no auxiliary power supply for the wirefeed motor, gas valve, and contactor - it all runs from the MIG welding power. Even when there isn't anything wrong, this situation can make smooth wirefeed difficult, and so, as you know, people do add a separate stabilised power supply.
Do you think the welding process itself shows signs of power fluctuation? Might be worth opening, cleaning, and re-tightening all the connections in the high-current path. The transformer secondary ( and the inductor ) is probably wound with aluminium wire, and this can corrode in the joints.

It is also possible that part of the rectifier has failed, and you're only getting half-wave power...

 

[you]

thanks, yep it is alu wire, i will go through all conections and give them a spruce up,
rectifier seems ok using diode test on multimeter