Pro 90 MIG Feed motor not working

[Digitalo]

Hi all,
This is my first post. I signed up so I could ask you guys some advice as I've seen some very helpful threads on here.
A friend has asked me to take a look at his Pro 90-MIG. The problem is that the feed motor is not kicking in when activated. There is just a low hum. I have been able so far to ascertain that the machine is getting voltage up to the solder joints in the below picture taken from a post by
mike 109444 from this thread http://www.mig-welding.co.uk/forum/threads/mig-pro-90-sudenlly-stopped-working.41552/page-2
I have also been able to ascertain that the feed motor is working by giving it an independent power supply to test.
I thought the problem might be the relay as there is a lot of sparking happening when the feed is activated so I opened it up and carefully cleaned it with an emery cloth but the problem persists.
Perhaps the voltage reaching the relay is too low ?
I'm afraid I'm far from an electronics expert but I do have some experience building circuits with micro controllers etc. I don't normally work with higher voltage equipment.
Any advice or help would be very much appreciated. I would really like to get to the bottom of this one.
Thank You.

 

[mike 109444]

So are you getting BOTH the 240 AC in AND the 12 ish AC out ?
And welcome BTW

 

[Digitalo]

Hi,
Thanks for your reply and for the welcome.
Yes I am. The 12ish v is reading at about 15.8v and the 240v at about 238v

 

[mike 109444]

Now you need to check if you have 240 V AC out from the relay to the main transformer (as that is what is being switched by the relay)
If you meter between the two points on pcb marked "X" you should get 240 v when trigger is pulled and relay operates. Note pic of pcb has been rotated !
This pic is of a 130 pcb but they are all much the same for the smaller Clarke machines. They did change the ID's of some of the connections though.
Have also shown a schematic for the 90 (poor image) and also one for the 160 which is much clearer, but note the different labels for the connections but basically they are much the same.

 

[mike 109444]

Just remember a mig is just a DC power supply ! 240V AC in to T1 gives 20 to 40V AC out and the diodes (2 on a 90 and 4 on a 160) turn it into DC 15 to 20 ish. They then "pinch" some of that DC voltage to feed down to the feed motor "M" in the diagram. So if there is no 240 in then there is no AC out and no DC to feed it.
If the above is ok then the "control" of the DC for the motor could be stuffed. The "control" (wire feed knob!) controls the transistors of the pcb, so they could be stuffed. Not to difficult to get replacements.

 

[Digitalo]

Hi Mike. This is really great thank you. I've been finding the schematic a little bit confusing. I have been away for the morning but now plan on spending the rest of the afternoon tackling this.
I will check the relay now and report back.
Thanks again.

 

[Digitalo]

Hi again. So I have measured the two points marked x and I get 245v AC when the trigger is pulled. You mention that the pic is of a 130 pcb. It is identical to the one I'm testing. Maybe a different or earlier model ?
I presume I now need to start narrowing down what components might be at fault. Are there any other tests that could help me here ?
Thanks again !

 

[mike 109444]

You would now need to check you have output from the other side of the main transformer (AC 15 to 30 ish volts that go into the diode pack), but you can jump ahead a bit by checking to see if you have any output at the torch tip to ground clamp. remove the shroud from the torch and put one meter probe on the copper tip and the other meter probe in the work ground clamp (this will be DC 12 to 20 ish volts) if when you pull the trigger you have that DC voltage then skip the AC out check.
Once you know you have the DC output then fault should be on the pcb.
I have included the two pic to show the circuit (on a 160) one with out and one with the DC (the thick black and red) supply to the feed motor. The variable on the pcb are various components on there. Main ones to fail are the transistors. Can't recall what they are but they may still have there numbers on them. De solder and meter to check. Either dead short or open circuit (will need diode or transistor check facility on meter for proper check)

 

[Digitalo]

Okay. Have just tested the voltage between torch tip and ground clamp. 7.8v before triggering and 28.9v when torch is triggered, so it looks like the problem definitely lies with a component on the the pcb.
I'll start with taking the transistors out of circuit and testing there. Have my trusty fluke 179 meter ready !

 

[Digitalo]

Does anybody happen to know the specs on the smaller TO-92 package transistor (second transistor in the above photo) ? The writing on the back says it is an MPS AG2 M738 but I can't find any datasheet for such a transistor online.
Thanks

 

[mike 109444]

Looking at this post http://www.mig-welding.co.uk/forum/threads/clarke-110en-turbo-mig-welder-problem.43626/#post-483528
Looks like they also may use one with a different number. Avi ebay http://www.ebay.co.uk/sch/i.html?_f...70.l1313.TR0.TRC0.H0.TRS0&_nkw=KSP42&_sacat=0
Other bigger one also may have different numbers http://www.ebay.co.uk/itm/TYN608-TR...038?pt=LH_DefaultDomain_3&hash=item2edb4b49e6

 

[Digitalo]

Thanks again. I should have thought to check the forums rather than just trawling the net. Have removed the T0220 package TYN406 thyristor first, and set up a breadboard circuit for testing.
Going to hit the bed now and continue tomorrow. Will update any findings.

 

[Digitalo]

Hi again. Have just finished testing both the T0-220 thyristor and the T0-92 unnamed transistor. Both check out okay.
I wasn't able to positively identify the T0-92 transistor so I treated it like BJT NPN transistor like the KSP42 that seems to be used in similar boards. based on it's behavior during testing it appears to be a properly working transistor.
Any idea what the next most likely component to fail might be ? Perhaps the schottky diodes ?
Thanks

 

[Digitalo]

Okay. I think I've found the culprit. I took the large 7 Watt 2R2 resistor out of circuit and it's reading way too high at 1.09 K ohms.

 

[Digitalo]

Hi all. Just an update on this. I finally received a replacment 7W 2r2 resistor and put it in circuit. I used a Welwyn W22 Series Axial Wirewound Resistor from Radionics.
The welder is working perfectly now.
Many thanks for all the help !

 

[Robotstar5]

Thanks for letting us know you got it sorted.