AGA 300 mig solenoid problem

[Gary Huston]

Hi guys, I'm new here but need some help. My AGA 300 mig has developed a problem where the gas runs immediately the power is switched on. I have taken the magnetic coil off the solenoid and powered it up and it is working, well it holds a screwdriver! I have the main cover off but can't see any relay or contactor that isn't pulling over when I pull the trigger. I'm at a bit of a loss. Could the coil have it's own control built in?
Gary

 

[Robotstar5]

Do you mean the coil switches on & off OK with the screwdriver test?, if so the valve could be mechanically stuck, try a sharp tap on the body to free it.

 

[Gary Huston]

The valve seems free, as I turn the power on and off I can feel it moving up and down. Besides, surly if it was stuck open gas would flow even with the power off.

 

[mike 109444]

Gary H it could be the coil of the valve is driven by a transistor not a relay? I take it that if the coil is unpluged then when powered on no gas flows? If so thenTrace the two wires of the coil bact to a/the pcb. one wire will prob go to the common ground of the pcb and the other through some resistors/diodes ect will go to a chunky transistor (metal tag on back or type that looks like a flying saucer!! It could be that that's duff. pics of pcb ?

 

[Gary Huston]

Mike, I have tried to trace the wire back but it's tricky, they are all the same grey colour with tiny numbers on. I found one with the same number on back from the wire feed to the main unit but when I removed it nothing changed. I then put a meter on both ends and they are different cables with the same number. It appears that the cables are numbered in the wire feed unit and then again with the same numbers in the main unit but don't have anything in common!
It's a very small pcb but I will see if I can get some pics tomorrow.

 

[Gary Huston]

I have tried again to trace the wires but neither of them seem to go to the PCB!

I have attached a few photos for those who know and a video here http://youtu.be/TFdSeQz_BUk for those who prefer video......

 

[mike 109444]

Gary see if attached pics make sense. It may not be a faulty transistor but could be a component driving it (resistor, diode ect) but if you can find the pin on the pcb plug or if the 24v comes from the main box then we maybe able to work out what could be causing fault.

 

[Gary Huston]

Thanks, I'll give it a try and get back to you....

 

[Gary Huston]

Finally got time to trace the cable. It goes to the terminal coloured red. That goes to one leg of the transistor and one leg of what looks like a diode. The Yellow circle is a break in the copper trace but I don't think it has anything to do with the problem.

 

[mike 109444]

sus transistors

Gary I think one of the two transistors T1 or T2 maybe short causing the valve to be turned on all the time. You need to make a note of the orintation of the transistors (they have a small tab around the edge to show which way round they need to be fitted) Once you have that info you can remove them and make a note of any numbers and letters on the devices. Will be somthing like "BC108" or "2N304" or similar. Then with a meter set on resistance, meter between two of the pins working your way round the pins till you have checked all combinations. You should find that one combination will give a short 00.00 or there abouts on a digital meter. I would look to also replace the two diodes D3 and D4. clean up the track (with a dremmel?) and repair track. Headach will be figuring what value the diodes are. They to will have numbers on but you will need a very good magnifing glass to see them!! D 1 could be duff but don't think it would cause the fault. It could be tested by lifting one leg of D1 and metering for a short (try meter leads both ways round) one way will give a reading 400 to 600 ohms but other way will give no reading, this is correct. If you get 00.00 then it is duff. Let us know how it goes

 

[Gary Huston]

Ok, so, No short on either T1 or T2. These are BFY50 and BC177B respectively. The Diode D1 is a 1N4004 and the two diodes D3 & D4 are also labelled 1N4004! D3 & D4 fell apart when removed but I haven't tried D1 yet.
I'm getting rather out of my depth now as there are so many diodes listed with the same numbers but very different specs... I think it's time to scrap the welder and fork out for a newer old machine!! Thanks very much for your help though...

 

[shenion]

Looks like D3 and D4 have been replaced before as they are different than D1 (glass vs plastic case.)

The glass ones are very fragile.

 

[keith19]

What's the problem with replacing the glass 1N4004s with standard ones? They are certainly cheap enough to warrant a try.

 

[mike 109444]

Gary befor you chuck in the towell on this welder if you don't have the time to trace the 24v across the pcb then a dirty trick would be to meter around the pcb plug for a 24v pin that's active when the trigger is pressed and disconnect the 24v connector on the gas valve, tape it up and then tag on a wire from the "found" 24v pin and feed it across to th egas valve. may or may not work depending what the 24v is also feeding. If you take it from the feed motor 24v this will vary with wire feed speed so you may not have enough voltage at lower feed rate (would have to go via an added in relay ect)

 

[Gary Huston]

Well I've replaced the diodes D3 & D4, fixed the broken track with a piece of wire, put it back together and it is still exactly the same. Don't know what the diodes do but can't see any difference!
Have no Idea how to trace the 24v across the pcb so might try your trick if I get time.. It might work from the motor as at the solenoid there is only 10v and it works on that even though it's marked 24v.
It doesn't owe me anything, I've had it years and it only cost me £75 when I bought it from a firm closing down. I might just look for another one... there are still a few old machines floating about..

 

[Gary Huston]

If anyone is interested I'm selling my Trafimet Spool Gun + Control Box on ebay, it's set up for ali and comes with three and a half spools of 1.2mm ali wire. Cost me a fortune a while ago so should be a bargain for someone!