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  • #61
    Geeky stuff ahead...

    Could be that some seleniums will outlast their owners-I would never say that some wouldn't, and would not say you should replace them if you're OK with the risk. There is absolutely no way to say when a particular part will fail--some may survive for more decades--and some for a few more days. In all seriousness, I'm glad yours continue to work just fine. My only thought is to just state the facts and let each one make their own decision. I would never try to convince anyone to do something they don't want to do unless I felt it was a risk to life--like leaving FPE Stab-Lok circuit breakers/boxes in occupied houses, or refusing to deal with high blood pressure before the very-statistically-probable stroke happens. House fires and strokes are very ugly, but that's a way different issue than an old welder. But, the statistical principles apply equally. Not everyone who has FPE breakers has their house burn down, but far, far more have than those with other brands. Not everyone with untreated high blood pressure has a stroke, but an awful lot do. And not everyone's selenium rectifier is going to burn up. I was talking today with my 90 year old uncle who smoked at least 2 packs a day for 50-60 years and his lungs are perfect. Doctors can't understand it. Statistics do not and cannot address individual cases. I just don't like the statistical probabilities related to any of those things--so I have a reliable brand of circuit breaker box in my house and shop, don't smoke, and I take my blood pressure pills. Been up close and personal with folks that had strokes, and it's not pretty.

    One of the fundamentals of reliability engineering is the "bathtub curve" that says failures will be high at first due to infant mortality, then drop into the "bathtub" area where the failure rate remains low for a period of time. As 'end of life' of components approaches, the failure curve starts back up. There is a lot of math and statistical analysis behind the shape and depth of the curve for a given piece of equipment, but it is as sure as life and death. When doing the math for such design evaluations, selenium rectifiers are one of the "bad actors" that tend to make the bathtub area ("the good times") shorter when analyzing an overall machine. But, as I said earlier, that does not address an individual case--it is a statistical analysis that is all about mathematical probability, not anecdotal individual cases, and it does not predict that your individual component aaa, whatever it is, is going to fail in the next 236.5 days or whatever.

    What it does say is, if you have 1,000 of component aaa, and they have been running for yyy years under some specified conditions, you have an x% probability that z% of them will fail in the next year, provided you have enough long-term failure experience data to know where you are on the curve to support doing the analysis. So, there is no math that can assess Sberry's Sync 300 and say when the rectifiers will fail--It is simply not possible to know that--and they might well outlast all of us--or fail next week--who could tell? (Frankly, my bet is they will last well beyond next week, but who knows?) It's just like gambling at the casino--it's all about the statistical odds, assuming the casino is legit. In fact, one of the common mathematical tools used in reliability engineering is actually called Monte Carlo simulation.

    For those who "beat the odds", it's great. If you don't it's not so good if there is secondary damage. Fixing electrical and electronic things for 60+ years has allowed me to see numerous rectifiers that have burnt up, and many more whose evolving internal failure process reduced efficiency to where secondary components around them failed before the rectifiers themselves actually burned up. Granted, I only got to see the ones that failed--the ones that kept working never made it to me. If it is just the rectifier, and it doesn't take anything irreplaceable along with it, the owner wins. Granted, most of the replacements I've done were in equipment where the parts were 20 bucks or less (including collateral damage). I just feel like it is only fair to warn of possible consequences and let the individual make the decision. I totally get the line of thought that says, hey, if it goes, I'll part it out and move on. Nothing in the world wrong with that.

    While this talk about math and curves and statistics might all sound very "ivory tower", it is a really big deal in developing a design for a piece of equipment, choosing design approaches and the resulting parts required and parts count, quality of parts you choose (cheaper or better?), warranty period, "value" of your company's reputation, cost of maintaining your factory repair service, length of time you plan to provide support for the product, how much to invest in spares stock of what will eventually be obsolete components and which ones to choose....and on and on. For example, selenium rectifiers were the hands-down choice when all of these things were considered "back in the day"--nothing else stacked up even close cost-wise or logically when you looked at all the variables.

    So-keeping or replacing selenium rectifiers is a personal decision. Individual choice. No harm, no foul--it's a free country (so far....). I agree with Miller's guidance--I change them.

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    • #62
      I like the analysis. I am used to people dealing with the risk reward all skewed. The things they think fail don't and ones do they don't think will. All cheap China wrenches break and strip bolts
      it's actually hard to find a true junk wrench anymore and that snap screwdriver isn't much if any better than the new HF.
      I like that you point out that you see the failed ones and not the ones don't.
      this is really a value question for sure, how much or what extent we going to insure a machine with relatively little value,,, that's different than real use of course but its actual value. My car might be more or longer useful with a 5k engine but it doesn't change its value much.
      I asked a car the other day, worth 4k in avg or poor condition and 4600 in excellent so the guy gets a 600$ bonus for fussing with it for 10 years.
      . A 330 welder is probably in the 500$ range, another couple if it's fixed and beautiful.

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      • #63
        I had a neighbor stop a while back looking for or how to get some parts for a machine he paid 500 for 5 years ago and ran the living snot out of it,, , way beyond what any hobby type could or would. Some circuit board type stuff as I recall. Was complaining about price for part, wondering if that was what was wrong and I finally had to nearly poke him in the eye to say,,,, they have every part in stock for immediate delivery, all of them, 500$ and all assembled.
        I understand there is a place for a 5k$ Miller but the math here shouldn't be all that difficult, would have had to spend 5 large today and hope it didnt need a nickle for 50 years to break even.
        this is a guy can do complicated calculations, got 20k in broke engine drives could have bought new for 2 or 3.
        I use old machines, most of it and dont always fix according to book value, equipment part of a process slightly different.

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        • #64
          We got a problem with executive types in this country. Business adm I think they call them. They do not understand maintenance despite being in charge of budgets.
          The old saying was a stitch in time saved 9 but we know that's wrong. Minimum number being 15, avg being 40 and goes up from there with liability, clean up, lost production etc.
          But the exec thinks more in line with the compulsive junk collector, thinks its 3 or 4 to one or sometimes even less.
          As Aero pointed out, this PM may not be simple, may not be all that cheap, so many other things beside this that stops these machines, mainly circuit boards si we dumping in as one said in earlier post,,, almost 1/2 the cost of the machine to protect SOME of its parts, does not include the rest of it so we coming up on 1 to 2 IF it happens and if that's what goes wrong with it.
          That's parts, I understand we might not pay labor but it does have value if it's not hobby.
          This would also be different if part of a critical operation, had long hard service life ahead of it and was expensive to replace.
          . While those are not "facts" in a true statistical sense they are in the sense or as accurate as any the rest of it especially since we cant put a number to how many will actually fail and when.

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          • #65
            I see some things described as "cheap insurance",,, usually those are really a poor bet simply makes people feel better, makes them feel like they did something. "I am doing my 2x a year 30 hr oil change to tractor cause "I dont want the engine to blow up". While it won't hurt if it's done without error it not going to significantly add to life and user will never approach the 6k or 10k hr life of the unit.

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            • #66
              One of my old buds was a consultant for Ross Perot. He had his issues but when he put the cap on always went right to it and would say,,, that's a good idea but won't make or save them any money.
              while we might not follow that all the way all the time it's worth noting or considering up front. We not saying dont take care of it, actually the opposite, while the oil change makes one feel good it doesn't fix the brakes.

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              • #67
                We see board after board posts on this forum. When was the last time we see a blown up rectifier problem? I am sure it has happened but I cant recall or never had it happen or know anyone had it happen. Not saying it won't or cant but what's the realath,,, or the "facts"?

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                • #68
                  Yep--agree with all above. I have seen people with beat-up old vehicles who change oil every 3000 miles or so, buying only the best synthetic oil and premium filters--there is just no return on that. Sad. On the other hand if you change the brake pads before they are down to metal and avoid having to replace the rotors, too, you really did save yourself some bucks.

                  In formal risk management programs, each risk is assessed for probability of occurrence and potential impact, which must be reduced to dollars or impact to mission success, and evaluated accordingly. You can have a high risk factor by either a very high probability of occurrence and medium to high impact, or very high cost or mission impact, with lower probability of occurrence, and it's all relative. Several smart people sit in a room, evaluate the technical data, costs, and missions, do a bunch of math, and assign probability and impact numbers. There are usually a huge number of variables and it ultimately comes down to a "well, what do you think?" consensus conclusion. I have seen big bucks spent for risk mitigation even with a very low probability of occurrence, simply because risk to mission success was so incredibly high--loss of life and/or loss of a multimillion dollar piece of equipment that could be mitigated by a one-time expenditure to select a different design or implementation approach.


                  With the old 330A/BP, the transformers and magamps will still be running another hundred years from now if the rectifiers don't short out and toast them. If a guy needs that kind of welding capability, or just has a sentimental attachment to the thing, or for any number of other reasons, he may consider a couple of hundred bucks for prevention cheap compared to buying a more modern machine with similar capabilities, or going through the effort of finding another old one. May be a case of low probability but massive impact, resulting in high risk factor. Small investment to prevent a total destruction event. For the hobbyist, it's a risk-reward calculation for each individual. As an old retired guy, if I had one, I'd change them just to avoid having to move another machine in even if someone gave it to me. I'm too old for all that heavy work these days. Unimaginable a few years ago, but true. And then I'd have a machine I know nothing about and might find something else wrong with it. It's all a gamble, and you have to make your own decisions. To each his own. Lots of variables in almost everything in life. Assess, make your choice, and don't look back. Much happier that way.


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                  • #69
                    I am not sure I would replace mine if it let the smoke out tomorrow. I have other ways of doing the same thing. Only reason I dont sell it is it's so cheap to keep.

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                    • #70
                      --and there is a perfect example of risk assessment and selection of a risk mitigation plan....or not, in this case! :-)

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                      • #71
                        I really agree with the brakes and oil change . Had a guy work for me I had to say enough.

                        A guy named David Todd Geaslin has a good take on this and should be required for exec's.
                        I spend most of my day on this. So many possible moves, so many mistakes.
                        Last edited by Sberry; 06-11-2021, 04:09 AM.

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                        • #72
                          We done this both ways. 1 operation we decide to hit the obvious and accept the time risk. Way different for 30, 300, 3000 and 30k.

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                          • #73
                            Retired and scrap it all before any majors. Doesn't mean we ignore a bearing or u joint only that it been running like that for years and not likely to be an issue.
                            I was working on combine a while back, they gonna stop and drive 2 hrs each way for a deal. I had to ask,,,, how long you had it? 20 years. You ever had to add oil?

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                            • #74
                              Yes, I keep some welders in case I go in the biz. The loot I get for selling would be rather minor and having to gear up would be costly. I should have sold a couple while back when the market was high for them. I haven't checked lately.

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                              • #75
                                They made a lot of these I think. I used them under Airco sheet and seems maybe under something else too but its been a while. They were reliable and worked on single phase where business was a concern. (this is a single model correct? )

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