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Hobart Handler 120 Welder - No Arc

dalmatiangirl61

Subscriber
Last Subscription Date
07/10/2019
I picked up this little mig welder last week for $20, figured if nothing else it was worth that for the gun as the 15 yo HF mig could use a better gun. When turned on the fan comes on, when trigger is pulled the contactor engages and wire feeder works, but I get no arc at the gun, not even a faint spark.

There is no circuit board in this machine, there is a resettable circuit breaker, it is not tripped. The "button" on capacitor has not raised, no idea if that is a guarantee cap is not bad. No loose wires, and nothing looks burned or cooked on transformer. First thing I need to check is the contactor, it looks like the same style as in the home AC system that I have replaced multiple times, so on that basis it is suspect. Beyond that there are 2 diodes, the capacitor, and an SCR, not sure what scr looks like and have not positively identified it yet.

Any suggestions on testing diodes, cap, and scr would be appreciated.

Hmm, trying to upload pics, but keep getting message that it failed because server reset?
 

dalmatiangirl61

Subscriber
Last Subscription Date
07/10/2019
Pulled the top cover off to get a better view and spotted the first problem, wires to back diode had melted thru the diodes ring terminal, not sure if this is indicative of a bad diode, or a problem up/down stream of the diode. Diode markings are "Italy 9507" and below that "I*R 405170-3", not finding a replacement searching those numbers.

Googled SCR and not seeing anything in here that looks like one, unless it is the part with 3 wires to right of the Potter Brumfield relay?
 

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Wayne 440

Registered
That decal was probably stuck on many different Hobart welders- yours may or may not contain SCRs.

I searched for a diagram for "Hobart Handler 120" and the one I found does not show an SCR, looks easy enough
to troubleshoot, but they have made that model for a long time, so there are probably variations.
Do you have a schematic diagram for your machine?

IF that was my machine, I would try an 85HF60 or similar to replace the damaged diode, they are about $10-15 each.
 
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dalmatiangirl61

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Last Subscription Date
07/10/2019
Yeah, I was wondering if that might be a one size fits all sticker. 2 new diodes ordered, $15.95 for 2 American Semiconductor 85HF60 diodes. Any thoughts on what would cause it to burn out terminal on diode? Wires were just inserted thru eye and soldered, maybe just got hot enough to melt out solder, next trigger pull arced and burned out eye?

Probably found same schematic you did here https://www.hobartwelders.com/files/owners-manuals/O210088D_HOB.pdf I think its right, but have to study these things before the info really sinks in. No schematic inside the actual unit.
 

Wayne 440

Registered
My guess is that someone abused the machine. There are a surprising number of people who can weld, but have no idea what "duty cycle" means.
 

Tim0477

Registered
Do you have a meter with a diode test function? It should conduct in one direction and show open in the other. If it shows open both ways or conducts both ways, its bad.

Tim

---------- Post added at 10:54:17 PM ---------- Previous post was at 10:50:25 PM ----------

P.S. may not hurt to ensure you are getting voltage out of the transformer secondary itself when the trigger is pressed. It will be AC.
 

cobbadog

Registered
I just read the link posted for the Hobart welder and inside it has some valuable information on setting up and using your MIG welder which is very helpful for me being a learner driver. Even techniques for welding and setting feed speeds and vaolts. Very helpful and thanks for the link I hope that your new toy can be sorted soon.
 

dalmatiangirl61

Subscriber
Last Subscription Date
07/10/2019
Do you have a meter with a diode test function? It should conduct in one direction and show open in the other. If it shows open both ways or conducts both ways, its bad.

Tim

---------- Post added at 10:54:17 PM ---------- Previous post was at 10:50:25 PM ----------

P.S. may not hurt to ensure you are getting voltage out of the transformer secondary itself when the trigger is pressed. It will be AC.
I don't think my meter has that function, but know I've worked thru this before with ohm meter, or maybe applying voltage and figuring out which way its going, I just don't do sparky work enough to keep it at forefront of my brain.

Wayne
I think they know about duty cycle, but with these small welding machines its just too easy to run up on the limit. Using the HF welder last month I ran up on the rev limiter several times trying to get a 4" bead in 1 pass:shrug:. I think I can make it, I think i can make it, click, no power, 1/2" short:rant:

Big bonus for me in getting this thing operational will be having a 110v unit for easy portability on light duty jobs, the HF unit is same size, but 220V.
 

cobbadog

Registered
We would call that "Murpies Law". If it can go wrong, it will.

Loosing power with only 4" to go, bugga !

I thought that an ohm meter could read a diode too.
 

dkamp

eMail NOT Working
Kris- I had the earliest predecessor to the Handler 120... it was just called the "Handler", case was plastic... and I think it was probably maxed out at 90A with a 20A duty cycle, and I used it, and abused it... and it did fine until one day, it did what YOURS did...

It blew it's rectifier diodes.

The reason the ring terminal is burned, is because the diode shorted internally, and excess current melted the solder, leaving the tab to arc to the wire, which it burned through readily.

The reason why this failure occurs, isn't because of exceeding the duty cycle, or dead-shorting the unit... it's because the snubber capacitors (the blue things) age and fail, and at that point, they stop suppressing the HF AC noise that occurs in the welding primary circuit as a result of the arc's presence (look up "spark gap transmitter"). A diode has ratings for Peak Reverse Voltage, and they're good for line-frequency level switching, but when you go to really high frequencies, the 'gating' action of the diode becomes ineffective.

Imagine the diode like being a screen door that opens when the wind blows through the house one way, and when it changes the other way, the door blows shut.

now imagine to wind is blowing back and forth so fast, that the screen door is trying to slam shut, but it's flipping open at the top while the bottom is trying to close... eventually, the screen door rips itself apart. This is what happens to the diode... excess frequency, combined with the capacitive reactance of the DIODE's gate, results in very high RF voltages beating on the gate while it's in transition between blocking and passing, and the gate shorts.

Replace the stock diode with a diode of substantially higher Peak Reverse Voltage. IIRC my Handler's stock diodes were around 70A (it's a center-tapped full wave secondary, so that's 140A of capacity), and the snubber caps were rated to like... 120vac.

I replaced my diodes with basically same form factor, but 600v or so, and the snubber capacitors were replaced with 1kv mica capacitors. They'll last a LONG time before something can bite them.:salute:
 

dalmatiangirl61

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Last Subscription Date
07/10/2019
Capacitor seems to be an odd, not common, expensive to replace value of 53000uf 30vdc variety, I can find 2 on ebay, one is 35v, other is 70v, but both are large can (2.5" dia x 5-5/8" h) and I don't think they will fit, and price is $85. How much wiggle room do I have on uf rating? I can find some in 52000uf 30v for $15 and 50000uf 70V for $30ish.

I'm thinking the 52000uf should work.
 

dkamp

eMail NOT Working
The snubber capacitors aren't the big one... it's the two small blue ones connected across the diodes... they're usually 100v or left, about the size of a penny. Go up to several hundred volts, or a thousand,if you can. Capacitance is low... and not critical... it's just enough to swallow the HF oscillation that would 'bite' the diodes.
 

dalmatiangirl61

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Last Subscription Date
07/10/2019
Got home a few days ago and found the bottom hinge on front gate had broken off, the perfect job for this little welder. Drug the welder out for a few test beads as it has not been tested since replacing diodes and capacitor, and finding 2 issues.

#1 Power output seems low, on max amp setting I can barely get a decent arc
#2 Wire does not start feeding until 7 on dial, so my range is between 7 and 10, I ran some light sandpaper across backside of rheostat, did not improve things.

Any ideas on what to test, maybe need to replace the little snubber caps?
 

Wayne 440

Registered
My first test would be the transformer secondary windings (X1 to X2 and X2 to X3). Seems like maybe you are getting half voltage. Another thought- are you sure it is a 120V machine?
 

dalmatiangirl61

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Last Subscription Date
07/10/2019
Label on back of unit indicates it is 120v only. Guess I'll drag it in and pull the cover off again, kind of a pia. I'm also wondering about the little contactor/relay, it is same style used in home ac at tx house, and I've replaced it numerous times.

So ohm test transformer windings?
 

Wayne 440

Registered
That is my suggestion. Remove all leads from X1, X2, X3, and test with an ohmmeter between each one. "X2" may actually be two wires coming from the transformer- if so call them X2A, X2B and advise your readings. Feel free to email if you need more specific guidance.
 

dkamp

eMail NOT Working
Disengage the pressure roller in the wire feeder, pull the trigger, noodle the speed control, and see if the drive roller speed changes. If so, you might actually have a liner problem... like... mebbie it ain't connected solid at the machine end... that'd cause the wire to not feed well, and limit output current too...
 

dkamp

eMail NOT Working
Another thing Kris- when you worked on this before, you had it going good? Check-see if your ground lead mebbie wicked some moisture up into it and rotted it in the middle of the cable somewhere... (Don't ask me how I know about this...) It won't affect wire feed, but it sure will sack the power output.
 
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