Machine Builders Network+ Forums Machine Builders Network Tech Resources/Skills Welding / Cutting Miller Dial Arc - DC ...Dead

My primary stick welder is a old Miller Dial Arc that I got at a school auction for $75. I have had it for about 8 years and it has always worked perfect. I had to weld something last week, I struck an arc and welded for a second, then the welder made a weird noise, kind of a low hum. Then it went away and welded fine, then it did it again and the arc got weird. I turned it off and welded it with my AHP 200X.

The next day I turned it on and it ran for a few seconds then threw the 50 amp breaker. Today I turned it on again and it did the same thing. I unplugged it and took the panels, and top off. Nothing obvious wrong inside. I hooked the meter to the switch and made sure it wasn't single phasing. it was fine.

I turned it on again and it read 120 amps. I turned it off again to walk to the other side to see if anything looked hot, I flipped it from low to high amps and when I turned it back on it sounded normal and was only reading 5 amps. I let it run for a few minutes and it stayed on and sounded normal. I turned it back to low and it still sounded normal.

I plugged the leads back in and tried to weld, the DC side seemed like it was a slow AC frequency weld. The machine seemed like it was truing to vibrate when the arc was struck on DC regardless of polarity. The AC side seemed to weld normally. I haven't used the AC side ever that I remember.

Anyway I don't have a osciliciscope but the DC leads read 290 V on AC (on the meter), and 129 with the meter on DC.

The AC side read OCV of 79.

I seems like my DC has gone out. I haven't tried to look up a wiring diagram yet. The Transformer that is hooked to the DC terminals are reading 1 OHM between the leads, so basically a dead short. I think that is normal for that. I guess I need to figure out how to troubleshoot the rectifier.

Here are some pics.

Any ideas of what to check or if it's repairable would be appreciated. I doubt it would be worth it to fix it. As it sits It is a nice AC only welder. Worst case I guess I got my money out it for $75. The sad thing is that that day I could have bout 5 of them fot $75 each....

The paint above the terminals for the AC are burnt, it has been this way since I have had it.



The transformer on the bottom is the one that hooks directly to the DC terminals. Is reads a dead short between the leads. This is with the wires hooked up, I guess I should unwire it and check it by itself.



Upper Transformers


Lower Terminal this is the one that seems to be shorted




Other side

I went back out there after reading up on this a bit and it seems I have open diodes on all four. Going to trouble shoot some more, but that looks like the problem. Might be a $160 fix to replace all four.

Miss Doc at times like this...

We all miss Doc.! ---- Ya, I am of no help on this electrical stuff. That looks like a well made welder----might be worth fixing IF you can get the parts!

get on the net, millerwelds.com with your model and serial number and you should be able to get a schematic in PDF format...the 4 diodes should be available even if not from Miller..there should be a number/letter combination on the side of the diode/rectifier,like "1N4558" or something like that..a lot of times two are the same and the other two have an inverted polarity for ease of manufacture and mounting/wiring.They usually are wired/configured in a 'bridge' formation(like a square)-with AC(~) in on two corners and DC (+/-) out on the other two. They are mounted right in front of the fan on a strip of aluminum as a heatsink..it draws the heat off the diodes.Sometimes they use two diodes and two SCR's to control the output..Lets start in the basement fIrst and work our way thru it..If you have to, its a good idea to replace all 4 diodes as the one that went bad will fatigue the others when it blew up or shorted. Newark.com or Mouser Electronics may have the diodes you need. Get back to me/us with the diode numbers,maybe I can help. By the way the big chunky transformer (4th photo down) coil you said had a measurement of 1ohm is a choke,it's probably OK..they are bombproof.

If I read your description correctly, the device, in pic 5, is a "choke" or inductor use to reduce the ripple in the DC output. If you look closely there is an approx 1/4" insulating board between the top lamination pack and the bottom "E " lamination pack. It has a single continuous winding, it may have several additional taps depending on the design.

I would examine the entire assembly carefully. IF one of the high current winding failed you should have little difficulty in finding the failure.

Check the control circuitry to determine if the failure was caused by a bad relay/contactor.

These old brute strength units are difficult to break unless you really try.

Many of the manuals are on the net. It is likely you can get a manual from Miller (bring clean money). The old welders in many cases had incremental changes so a before or after vintage manual will assist you in getting a schematic.

Diodes are available on Ebay. If you can find current and voltage ratings for your diodes, more E & I is good. Also Banggood & AliExpress primo chinesium products.

Good Luck

the rectifier module is available from Miller,the module looks to be the same for all serial number ranges..Miller P/N 202580 about $250..you could just replace the diodes on it like oldasdirt said,but price it out.

Looked at your machine pics again and I believe that unit uses a mag amp/saturable reactor to control the output current. Looking at pic #7 it shows a 3 coil transformer. Two end coils are the high current coils, center coil is connected to weld current adjuster.

Need to check that the DC supply for the magamp is working. Magamp saturates the center leg of the weld current transformer. Vary how much current changes the unit produces. This unit was produced before there were commercial semiconductors (read cheap) that would handle current. I have Miller OM 321 pulled up. It is free download and has schematic. Can't say if manual will apply to your unit, but it will help you tell what you looking at.

Your machine is very similar to my Eutectic machine that I bought (2) from Gov surplus and and made one working unit. Mine has HF on it. We used it to cut lids off of transformers using a Airarc torch. It has reactor control too.

Good Luck

I read up on checking these big diodes a little more and tried a different meter, I also disconnected them this time to make sure I was only reading one diode. Three checked out good. One was open in both directions. It had a part number listed on it (037-306B). These and the other ones (037-305B) are readily available from several places, the cheapest that I found being Amazon for $40.

There were a pair of 037-306B diodes used ones on ebay for $20 free shipping. I went ahead and bought those If that fixes it and they burn up later, I will have a spare, and, at least I will know that was the problem and I will only be out $20. I appreciate the help and comments.

BTW the serial on the welder is HH002948, Miller Dial Arc 250 AC/DC. I downloaded the manual from Miller, the manuals for all the year ranges show a big capacitor that mine doesn't have. I looked a little more and that capacitor comes on models that have power facto correction which mine does not have.

I will update when the new Diodes come in and report back on if that fixes it,

The diodes from eBay came in yesterday. I put the welder back together. It is working good again.

While I had everything moved around i decided to rearrange my welding area. I have a steel weld table that ends up being my main work surface. I have a shroud around it to catch grindings and weld sparks. Today I welded 2" angles under the feet so that the welder would tuck under the corner. This gives me enough space to get the other weld cabinet in there also and not just floating around the shop.

Scrap pile under the table,

Glad you got it fixed! ----I love seeing other guys nice shops!!! mine is mostly outdoors! lol!