Help trouble shooting a HF 125

[mickri]

Hi,
I was using my HF 125 today and all of a sudden it stopped working. My original thought was that maybe it had over heated some how. I had done 4 tacks and then about an inch long bead. Power on max and wire feed on four.

I have done the bridge rectifier/capacitor modification. The welder powers on. The fan runs. When I push the switch I hear a click and a buzzing sound. The wire does not feed. When I touch the wire to what I am welding there is a tiny spark.

Where do I start to trouble shoot what has gone wrong with my welder?

 

[markba633csi]

Blown rectifier? Bad connection? Check for dc voltage across the capacitor if you have a multimeter. Use caution of course

 

[mickri]

I checked the DC voltage at the rectifier using the 200 volt setting. Didn't get a reading. Any idea on how many volts I should see coming out of the rectifier?

 

[markba633csi]

You should have something there, probably around 50 volts DC on the + and - terminals. Check the input to the rectifier for similar AC voltage, if you have that then the rectifier is smoked. What are the ratings for the rectifier you bought? It should be at least 100 amps (200 is better) at about 1000 volts or more
Is there a heat sink on it?

 

[mickri]

The rectifier is rated at 150 amps at I believe 1600 volts with a heat sink. I'll be out of town for several days and will check again when I get back. Thanks for the suggestions.

 

[markba633csi]

It's easy enough to check it out of the circuit with an ohmmeter or battery/bulb continuity tester. We can show you that when you get back
-mark
ps the ratings are fine, may have just been an infant mortality

 

[homebrewed]

Looking at the schematic, it appears to me that the wire feeder is run with a separate 24V transformer. So even if the rectifier is open it should push wire out. I'm wondering if the power switch is defective, with resistive contacts. I could be wrong, but I THINK you'd know if the rectifier was shorted -- smoke coming from it, or the main transformer overheating. If the rectifier failed by outputting AC, AC across the big electrolytic capacitor (part of the welder mod) will make it very unhappy as well. Electrolytics don't like to have reverse voltage applied to their terminals and will rapidly fail if that condition lasts very long. In this case "failure" could include exploding. I've seen it happen

I did the same mod to an HF welder. Before turning it on for the first time I checked the output side by applying an external voltage (with the correct polarity of course) to see if I had wired everything correctly. If you've got a battery charger that would probably work. Naturally, you don't want to apply any voltage that's higher than the working voltage of the electrolytic capacitor. If the rectifier + electrolytic are OK, you should see a brief inrush of current as the capacitor charges, then it should decline to a very low current. If you've got a bleeder resistor across the capacitor that will increase the standing current a little bit. If the current starts to go up, either you've got the external power supply connected backwards or the electrolytic capacitor is defective (or wired wrong).

For checking a DCEN setup the gun should be negative and the ground terminal is connected to the positive output of the power supply.

Good Luck!

 

[mickri]

Got home this afternoon and have a full day of errands tomorrow. Won't be able to dive into this until Friday.

The welder has been working property for months. Mind you it doesn't get used a lot. But it does get used. I had just used up the HF crap wire and am now using the NR211 that I had recently bought. No smoke and nothing looks burnt or discolored. The welder is plugged into its own 20 amp outlet.

I am going to take the switch apart. It has been sticking sometimes recently. Might find something there. Or maybe not. Then I will check voltage at the ac and dc side of the rectifier. I'll report back with what I find.

Thanks for the suggestions.

Attached is a picture of how everything is connected. For the connections I made lugs out of 3/16 copper tubing. It was an exact fit on the wire. I flattened the copper tubing and covered it with electrical tape to connect the rectifier to the capacitor.

 

[mickri]

I checked out the switch on the gun and it is working ok. No problems there. Checked voltage on both the ac and dc side of the rectifier and only got nominal voltage. 1 to 2 volts. Took the rectifier, capacitor and resistor out of the system and the welder works. Did a couple of welds on my current project. I am back to having a fair amount of splatter even with the NR211 Lincoln wire.

I now need some help to determine which component failed. How do I test a rectifier and a capacitor? The rectifier is not expensive but the capacitor was over $50.

 

[mickri]

In looking at rectifiers some are described as "SQL" and others are "MDQ" The SQL rectifiers have a heat sink and are 3 phase. The MDQ rectifiers do not have a heat sink and come in either single phase or 3 phase. What is the difference between SQL and MDQ rectifiers.