Miller Thunderbolt welder

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I posted this question in the Miller forum below but got no responce so I thought I'd try here. I have a Miller Thunderbolt AC 225- DC 150 for about 10 years now and for the 2nd time I had either 1 or 2 diodes go out on it. Does anyone know if there is a reason for this or is it normal? I am retired and it is just a hobby for me and it's not used as much as if it were in a business. Do these diodes always go out in DC machines? It gets kind of disgusting after having  an AC machine for 39 years and it would always work. Don't get me wrong, I still love my AC-DC machine, but I was surprised to see such a weak link in a welder. I would like to hear from some af you that know about this. Does this welder have too small of a diode in it or what is the problem?   MPKWelder- Miller Thunderbolt AC 225-DC 150 Plasma cutter, Miller Spectrum 300O/P torch- Victor 100
Reply:NO, it doesn't make sense.Can you provide more details about exactly what was happening prior to the diodes crapping out?  Have you measured the input voltage to the machine?Are you experiencing short life of lightbulbs too?Is the machine in a corrosive atmousphere?Were you exceding the duty cycle of the machine?Is the cooling obstructed?Appreciation Gains You Recognition-
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Reply:sorry about that, wrong forum
Reply:Franz---- No I don't believe I have exceeded the duty cycle.I am not doing that much big stuff now. Also everything was built new and wired heavy duty 8 years ago. We just moved to town then. It is nice and dry in the shop, Only the normal dust from grinding and plasma cutting. We have very good voltage here in town, much better than on the farm. Since we left the farm they upped the voltage out there too. Have you ever had any diodes go out on your welders? Thanks for your response.Welder- Miller Thunderbolt AC 225-DC 150 Plasma cutter, Miller Spectrum 300O/P torch- Victor 100
Reply:Can you post the numbers for the machine, so I can pull up the roadmap on the Miller site, both model & serial.Since you say it's the second time they went, I'm wondering if the machine was properly repaired the first time.  Excessive heat soldering the diodes in might have damaged them, or improper fitup to the heatsink.Were all of the diodes replaced the first time, or just the blown ones?  That might be the problem if some clown only replaced part of the set.I've never had a diode blow in a welder, but that could well be because I don't own a machine with diodes in it, other than a plasma.Appreciation Gains You Recognition-
Reply:Franz--- I have no trouble with light bulbs, the power is good, I just checked it now and it's 118-236 and if I remember right it goes up to 120 in the day time which I believe is good. The diodes that were replaced the first time are not the same as now. I had the dealer send it in to fix the first time and this time I just took them out and had my dealer order them. They should be here tomorrow. They are not that hard to replace once you see where they are and if you know how to test them. They are not soldered in. They just bolt in with small bolts. One end has a pigtail on it. The worst part is they only break down when I am using them. Here is the serial no. KD545847----stock no.902775. I see no model no. other than the name Thunderbolt. It's 225 AC-150 DC. If I would keep some diodes on hand I could replace all 4 of them in an hour or less. Thank you for your response again.Welder- Miller Thunderbolt AC 225-DC 150 Plasma cutter, Miller Spectrum 300O/P torch- Victor 100
Reply:Having looked at the diagram, the diodes burning out makes no sense, assuming the fan is running, and cooling the diodes.The one thing that might be worth checking, although it seems impossible, the lead that feeds the diode array should be connected to the Lo AC output.  If that's on the Hi output, all hell would be driving the diode array, and your DC currents would be way above that indicated on the pointer.Are you running near the top of the output most of the time, or near middle range?Also, have you contacted Miller directly about this?Appreciation Gains You Recognition-
Reply:Franz, MPKAnother possibility is when the diodes were bolted back into the heat sinks , they weren't tightened enough or maybe the heat sink grease was left off.  Also if the fan is running slow or not at all, that would shorten the life of them.
Reply:Franz, Paychk,     The fan is running fine. I did notice one thing though. When I took one of the diodes out, it was not tight and also it had a little white powder in and around the hole where it was bolted.  I think maybe this is my problem. If this metal where it is bolted to is supposed to act as a heat sink, then being the diodes were not tight enough, this could very easily be causing them to get hot. I did not see any grease of any type on the diodes. I think now I will recheck all of them for tightness.Welder- Miller Thunderbolt AC 225-DC 150 Plasma cutter, Miller Spectrum 300O/P torch- Victor 100
Reply:Lack of proper heat sinking is what took the diode out, when the first one went, the second one followed cause it couldn't handle the current.The white powder is aluminum oxide (rust) and was caused cause the installer who put it in did NOT use the dielectric grease he was supposed to.Appreciation Gains You Recognition-
Reply:Franz, Paychk, I got the new diodes today and put them in.  there are 4 diodes in there. 2 went out the first time and now the other 2. Actually only one was out this time but I put them both in. I noticed that the original factory installed ones did not have any of the heat sink grease on them but when the service center fixed it the first time they did put some of the grease on them. Also these new ones did come with the grease and also some anti oxidant to put on the connection on the other end of the pigtail. I think the guy that installed them in the factory just didn't take the time to put the grease on and tighten them properly. I call that poor workmanship. I am now hopeful that I have found the cause of this problem. I want to again thank you guys that have helped me with this. You guys have a wealth of knowledge and with the help of this great board it is just great. Thanks again. MPKWelder- Miller Thunderbolt AC 225-DC 150 Plasma cutter, Miller Spectrum 300O/P torch- Victor 100
Reply:The 9 year old Miller thunderbolt I have did the same thing. Lost 2 diodes and one was not tight in the heat sink when I removed it.
Reply:Hello, I have a Thunderbolt welder just like the ones you have discribed. The last time I welded with it, it worked fine. Today, I can't even get it to draw an arc. Is this the diode problem you are referring to in your posts?The only thing I know to be wrong with mine is one of the plastic button heads around the port were you plug the A/C lead into, ( I only use the DC) is broken, and falls off from time to time... No arcing has occured around it I can see. My machine is around the same vintage as yours was discribed. About 10 years old. Used very little. Maybe 25 lbs. of rod use with it so far.Any suggetstions?
Reply:When my diodes went out when I struck the arc the welder would just sit there and buzz a loud noise and it kept on doing it till I flicked the power switch off. If I put it back on it would buzz again. That's all  it would do. It sounds like yours is acting different. I think my diodes were shorted like a dead short,( for lack of a better explanation). I suppose they could also be open so it would be like an open circuit or a bad connection, or you could have a bad connection somewhere else. That is all the help I can think of now.MPKWelder- Miller Thunderbolt AC 225-DC 150 Plasma cutter, Miller Spectrum 300O/P torch- Victor 100
Reply:i'm looking at a thunderbolt ac/dc machine but it's a 460 volt machine single phase, what do i need to do to get 460 service in my garage?
Reply:I don't really know anything about 460 service but I would guess you would have to have the power company bring in a whole new line to your place.Welder- Miller Thunderbolt AC 225-DC 150 Plasma cutter, Miller Spectrum 300O/P torch- Victor 100
Reply:Assuming it's even available, you have to give a lot of money to your utility service to bring in the lines and give you your own transformer.