Snap-On Pro-MiG 160 - power fading?

[Mike_H]

I bought a used one of these a few years ago. It's never welded completely consistently... starts off OK but then seems to struggle to strike a decent weld.
I replaced the torch with a EURO one from Tec Arc (I think) but it didn't really solve the problem. I thought it was the wire sticking, as it struggles to feed, and loses power at the same time. I also took a slight skim off of the wire feed roller in the lathe, in case the groove was worn too deep to grip the wire properly... bloomin hard material, those rollers, so probably wasn't that!

It's the older type with the square sides.... probably 25 years old. I could just retire it, but I hate throwing 'good' stuff away.

I'm not very confident with AC power, but I do have a multi-meter, and prepared to have a go with a bit of instruction. Any suggestions where to start?

As I haven't got a pic of mine to hand, it's this type - pics pinched from another thread on here by Banstead2021.

 

[a111r]

Put the meter on DC and check the OCV voltages to the spec.

The symptoms you have could point to some rectifier diodes failed.
Works OK, but then after a few mins they die - and the welding wire doesn't, or glows red...

 

[eddie49]

Another potential cause of a machine losing power after some usage is a high-resistance, loose, corroded or burnt joint in the high-current welding path. Resistance mostly increases with temperature, so after the flow of high current causes a bad joint to heat up, it will then drop even more voltage. Check all the bolted joints at the rectifier, the inductor ( choke ), the torch socket, and both ends of the work return cable and clamp. Even if joints look good externally, they may be corroded or crumbling inside.

 

[Mike_H]

Put the meter on DC and check the OCV voltages to the spec.

The symptoms you have could point to some rectifier diodes failed.
Works OK, but then after a few mins they die - and the welding wire doesn't, or glows red...

Thanks for the suggestion. It does sound a bit like the symptoms I have. I'll have a proper look at the weekend.

Sorry for not understanding, but what are OCV voltages and where do I measure them? Output Current(?) Voltage?

 

[Mike_H]

Another potential cause of a machine losing power after some usage is a high-resistance, loose, corroded or burnt joint in the high-current welding path. Resistance mostly increases with temperature, so after the flow of high current causes a bad joint to heat up, it will then drop even more voltage. Check all the bolted joints at the rectifier, the inductor ( choke ), the torch socket, and both ends of the work return cable and clamp. Even if joints look good externally, they may be corroded or crumbling inside.

Thanks for the suggestion. I'll check it out. It would be great if the solution was that simple.

 

[Domdom]

Thanks for the suggestion. It does sound a bit like the symptoms I have. I'll have a proper look at the weekend.

Sorry for not understanding, but what are OCV voltages and where do I measure them? Output Current(?) Voltage?

OCV = Open Circuit Voltages, i.e when the machine is in ready to weld mode but not actually welding. So trigger pulled but not striking an arc. No wire unless you want a birds nest on the floor.

 

[a111r]

Yep, open circuit voltage, as opposed to the 'short circuit voltage' (which is less, when welding)
Just flip the wire tensioner 'off' when doing this testing, so the wire doesn't feed.

Multi-meter set to the DC voltage range (100v or 200v), measured between the torch tip and return clamp.

 

[Mike_H]

So, I did some testing last night. These are baseline numbers without any clean up of contacts or other work. No wire reel on the machine.

The voltage at the highest power setting was consistently 39.9-40.0 volts, and 23.5 on the lowest power setting. I did several tests, of about 10-30 seconds each, although I wasn't timing it, and the voltages were consistent, until.....

... the welder started to trip the fuse box in the barn, as soon as I pulled the trigger, which it did consistently 3 times after that. I'll go back and test it tonight or tomorrow after it's had plenty of cooling time. It was getting late so I had to leave anyway.

The welder is on a 13A plug. I think it should really be on a 16A, but it's never been a problem in the past.

The barn has a modern circuit breaker type fuse box, about 2 years old, so if a component in the welder is breaking down, I guess that could be causing it?


The pic of the table on the front of the welder shows a voltage range of 26-44v. Is this the spec for the open circuit voltages? If that's the case then it's 10% below, but I'm guessing that wouldn't be a massive issue in itself, and might be improved by cleaning up the contacts in the weld path. I think this machine is 20+ years old.


What do you think?

 

[a111r]

Try a few other tools on the power supply, just to rule out the breaker first?

The voltage range you measured should be fine. It does sound like diodes failing when getting warm.
But get the case off first and have a good look around at all the joints, as Eddie said.

You will probably have to remove the rectifier to access and test all 12 or 16 diodes.
That's OK, as you can then heat them up with a hairdryer / heat gun to see if they stop working.

 

[Hutcho]

Probably worth pulling the contactor out too - they don’t last long in damp conditions

 

[Mike_H]

Thanks for the feedback. Went back tonight and re-tested. Trigger worked 5 times then tripped the power again. I’ve been using my portamig through the same sockets with no issues. The snap on is the backup, these days. One is meant to be in the workshop, the other in the garage at home.

What do I do to test diodes? Is it a resistance check?

I opened the machine tonight and took some pics. Looks a bit tired inside. Are the last two the main transformer? Looks like it might be breaking up? Or maybe they all look like that?

 

[a111r]

That's all fine, it just needs a good clean out.

Your multi-meter probably has a diode test function.
On good diodes you'll get a reading of 0.5 to 0.6, then when reversed it will be zero.

Try these measurements cold, then hot. If found faulty here is the easiest repair option.
Gone up a bit, like everything. They were about £40.

1×DQFI200A silicon controlled rectifiers MIG Welder bridge rectifier 200*100*2P | eBay

Model: DQFI200A. Product size: 200 100 2 pieces. Working junction temperature: ≤80 (℃). Weight: about 0.5kg.

www.ebay.co.uk

 

[Mike_H]

Thanks... 0.5-0.6 Ohms? kOhms?

Those chinese rectifiers make me nervous, but there are UK sourced ones for a bit more. I wonder if it's possible just to source and replace the diodes?

 

[a111r]

Using the meter's diode setting, I think the value is one of 'forward voltage'?
UK sourced just come from the same place.
Repair is not viable for the pressed in diodes. They are also best matched, or others will fail.

 

[Hutcho]

Looks like the fan is catching the rectifier

 

[Mike_H]

Looks like the fan is catching the rectifier

Thanks for the shout, but it’s a good 10mm away. May just be the angle of the pic

 

[Mike_H]

i got the rectifier out tonight and started testing. My multimeter gave readings in the 470-482 range (on diode test - assume it’s resistance but not sure - the meter doesn’t specify the unit of measure) except one which is open circuit both ways. The connection to it was broken as well, so I reckon that’s blown, properly!

so it looks like I need a new rectifier - the diodes look easily serviceable, and there’s a very similar one on ebay for £70, from York Welding Supplies. the only question in my mind is that it’s 150 amp rated (peak 180 @ 35% duty cycle). My welder is a 160 amp.

my instinct is to avoid the cheaper Chinese alternatives (even though a lot of the components probably come from there anyway) unless people tell me they’re all using the Chinese ones for years with no issues.

 

[Mike_H]

The offending diode…

 

[Mike_H]

Probably worth pulling the contactor out too - they don’t last long in damp conditions

Thanks for this suggestion, earlier in the thread. Now that it’s got a faulty rectifier diagnosed, do you think it’s still worth doing this? If so, what am I looking for once it’s out?

 

[Mike_H]

Using the meter's diode setting, I think the value is one of 'forward voltage'?
UK sourced just come from the same place.
Repair is not viable for the pressed in diodes. They are also best matched, or others will fail.

Sorry, I see you already answered the question about the Chinese rectifiers