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  • #76
    Finally got around to removing the power selenium rectifier, they must have put that thing in with a shoehorn. Wrestling it out of there was difficult, as you had to be careful not to damage the balancing resistors, and it must weigh about 30 pounds. I got out my meter and measure the resistance across the terminals, and it appears the rectifier was already shot, very low resistance in both directions. From the common terminal 55 to both 54 and 56, and from the common terminal 52 to 51 and 53. See the schematic for the terminal layout.
    I note that on the newer units with silicon rectifers, each diode is paralleled by a capacitor, and there is a thyristor and resistor across the common terminals. It would really help to have values for these, so they can be added to the replacement. If anyone can post a pic of the newer setup, it would be appreciated. Click image for larger version

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    • #77
      Well, apparently I spoke to soon. According to the DVM diode test in resistor mode, the selenium power rectifier is shorted in both directions on all the diode segments, However, hooking up a power supply on a forward biased diode at 30 volts resulted in a current of 5amps, at which point the power supply went into current limiting mode. Hooking it up so as to reverse bias the diode resulted in a reverse current of 200ma at 30 volts. That reverse current seems rather high, but not being an expert on selenium rectifiers, who knows. So to some extent, the diode segments of this power rectifier are acting as a proper diode, but with extremely large reverse currents.
      From some sources on the net, it is said that reverse currents start rising fairly rapidly as the selenium rectifiers age, and seeing as how I am the winner of the oldest Miller 330 A/BP welder at about 60 years or older, that could very well account for the high reverse current. This would obviously have some negative effects on welder operation.

      As well, I am still looking to find the value of the bypass capacitors on the silicon power rectifiers in later units, as I will need these on the replacement. Any help would be really appreciated.
      Last edited by 07wingnut; 06-19-2021, 09:03 PM.

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      • #78
        Those selenium rectifiers are on their way out....high reverse current is the indication of impending failure. I don't think 200ma is horrendous for a selenium, but it will continue to get worse if you keep using them. What you can't predict is how long it will take--could be any time, could be months/years. However, they are significantly adding to the heating of the transformer and mag amps due to the leakage current, and obviously drawing more power from the line than they should.
        Once they short completely, you have to worry about the transformer getting toasted, which takes us back to the original premise of changing them out before they do collateral damage.

        No time now--will look at the bypass caps tonight or tomorrow.

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        • #79
          Here is a comparison between the new silicon rectifiers and the old selenium rectifiers. Input is 70VAC, thru the diode, into a 100ohm load with the oscilloscope hooked across the load. Guess which one is which. Click image for larger version

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          • #80
            That is the proverbial picture worth a thousand words. I assume the input AC is 70 volts RMS? If so, that means the selenium is STILL not cutting off at 99 volts instantaneous, at least at that low a current. Even worse than I expected it to look with the reverse current you measured. Most graphic reason yet to get rid of old dead/dying rectifiers!

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            • #81
              I don't have one of those machines to take pictures of the snubbing capacitors for you, but looking at the schematics, they used only the caps from the early seventies until 1983, when they added the varistor and resistor in place of C9. Couldn't find the specs on the varistor, but I would think if you just add the five capacitors, it will be just fine. It obviously worked that way for many years. C5-C8 would be four .01mfd ceramic caps (C5-C8), probably 200 volt rating or more across each of the diodes. Ceramics are perfect for that application. C9 should be a 0.5 mfd @200 volts. C9 was a paper/oil capacitor, so I would probably use a modern polypropylene or polyethylene--shouldn't make any difference which. You will just have to make up a connection scheme to get them mounted.

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              • #82
                My Helga is a mid-70s model, so if I can help with some pictures I will. I’ll just need some guidance on what exactly you need since I’m an electronically neutered booger eater on that stuff, especially when compared to the big brains in here.

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                • #83
                  Mine works flawless,,,,,,, as a stand for other machines,,, ha. Also looks really impressive. Click image for larger version

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                  • #84
                    Originally posted by ryanjones2150 View Post
                    My Helga is a mid-70s model, so if I can help with some pictures I will. I’ll just need some guidance on what exactly you need since I’m an electronically neutered booger eater on that stuff, especially when compared to the big brains in here.
                    The main power rectifiers should be visible if a side cover is removed. Going in order from the back, you have the fan, the balancing resistors and then the power rectifiers. I have no idea what form they take in your machine, but it should involve some pretty impressive heat sinks. There are 4 rectifiers and each one should be paralleled by a small capacitor. A picture of your setup and the value on those capacitors would really help. Aeronic41 has posted some values and this info would really nail it down. Cheers, Alfred

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                    • #85
                      Originally posted by Sberry View Post
                      Mine works flawless,,,,,,, as a stand for other machines,,, ha. Also looks really impressive. Click image for larger version

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                      I must be going blind, I don't see it.

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                      • #86
                        Ryan, here's some guidance on what to photograph.

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                        • #87
                          Thanks for those diagrams, I really had no idea how the new setup was put together. Looking at the heat sinking arrangement makes me think that mine is completely overdone with the new rectifiers. I guess your initial values for those caps were right on. Any idea what the breakdown voltage is for those? I see the smaller ones are 200v so the .5 is probably similar.

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                          • #88
                            Yes, I think you'd be quite safe with 200 volts--could go 400 if you want, probably cost about the same.

                            Too much heat sink is WAY better than not enough. It will be good assuming you can get them mounted somewhere without too much difficulty.

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                            • #89
                              Actually mine is a synch but I have used several 330s. I mean in this thread cause it's so slow here and this is active. I do appreciate the know how here. I am also still a dirt eater in this respect, takes too many brain cells and I cant spare them.
                              I do fix a lot at componant level but I got help. I bought mine 100$ long time ago with problem but it came with manuals and my guy fix a board in it. Cost me another 100.
                              it's been worth it as I had some real work I was doing on a buds 330 and it saved me a quite a bit of time and some cost. Size, weight, power not a problem. Space not really a deal especially since i pile on top.

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                              • #90
                                If I was going to hang out the shingle might want something adjustable frequency . It's a great thing for me, it's there if I need it. It's working. Paid for, too low in value to sell it compared to it's worth as a tool.

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