smallfry Posted April 23, 2023 Share Posted April 23, 2023 My old Telwin MIG welder has decided to the play up. I bought it new in 1984 from my Mums friends, who ran a welding supply company. It was a lot of money relatively back then, and I distinctly remember him saying "it will last a lifetime". I am very annoyed to discover that he lied ! Main problem is that the 8 position rotary selector switch, apparently correctly called a "Rotary Cam Switch" has partially failed, which causes intermittent contact in some positions, and complete failure in others. Sometimes I get wire feed but no welding current, and sometimes vice versa. The whole machine and parts is no longer serviced by Telwin, and even the switch manufacturer has changed ownership and direction, and has no info on the switch. I can see many similar switches online, but they can be configured in a myriad of ways, depending on the application specified. Each segment of the switch (there are six segments on mine) has two cams and two pairs of contacts, and each cam has eight possible positions, so to get something off the shelf that I could adapt is unlikely. Statistically, I have more chance of winning the lottery. What makes it complicated, is that it has an automatic wire feed rate. There is no variable speed control as is normal. There is a bank of resistors and relays/contactors that the wire feed current is switched through, via the rotary switch. It has a min/max switch to alter the feed speed for overhead welding etc, but this never really worked quite right. I just got used to it. I wondered if it is possible to bypass all this and fit a "normal" rheostat instead ? All the components on the board are big old fashioned stuff like you would find in the back of an old CRT TV back in the days of Radio Rentals. Trouble is, even basic electronics like this mystify me. This I think would simplify the whole thing, as then it will just be a matter of sorting out the tappings on the secondary ? transformer. Because I need a machine to use, and I wanted to explore gasless MIG welding, I bought a little Draper inverter machine which is a lot smaller despite its higher output that does both, plus scratch start TIG. Also got a Euro torch and connector. I went for this because although I generally like to spend more on quality tools, financial constraints now dictate otherwise, also, now I am getting on a bit, I dont know if I would get the use out of it ! Anyway, it turned up on Friday afternoon, so I unpacked it all, but the 16 amp plug on, and set it up. Switched it on, and NOTHING. Checked the leads and power feed which was all OK. I then rang the supplier to ask about replacement and if I could take the covers off to chase the fault, not wanting to get into a bunfight over warranty, BUT, they had all gone home. So I took the top off anyway. There is power to both sides of the on/off switch, but that is as far as I can go with it. Overall impression is that it all looks rather flimsy and fragile compared to the old Telwin. There is a lot of tiny components on the circuit boards, which are all unprotected, and I do not feel it would last anything like as long. So yes, I will be able to get a replacement, but should I ? Should I use the opportunity to just get a refund, and fix the old one, which, once fixed, I am sure will see me out ? Quote Link to comment Share on other sites More sharing options...
Bowie69 Posted April 24, 2023 Share Posted April 24, 2023 Have you tried electrical contact cleaner on the rotary switch? Can often work wonders. Quote Link to comment Share on other sites More sharing options...
elbekko Posted April 24, 2023 Share Posted April 24, 2023 As Bowie says, some cleaning up, especially on a machine that's been in metal dust since the early '80s, can do wonders. I'd try mapping out the different circuits through the switch. When drawn out, it'll probably not be so complicated. Then you can either see about a fitting replacement switch, or if not possible, just have a bunch of toggle switches and a legend that you print out next to it for the different settings? Not idiot-proof, but that just means it needs one of these stickers. Quote Link to comment Share on other sites More sharing options...
FridgeFreezer Posted April 24, 2023 Share Posted April 24, 2023 If you're got a circuit diagram / manual for it it may well be possible to either work the switch out or do as you say and put "normal" controls in. I've just done a rebuild on my (probably) 50-year-old NBC MIG which has a similar setup of automatic wire feed rate with only fine adjustment, the circuitry in it is very basic but having eventually tracked down a manual there's some incredibly cunning twists to it that I would not have guessed from staring at it. Quote Link to comment Share on other sites More sharing options...
smallfry Posted April 24, 2023 Author Share Posted April 24, 2023 2 hours ago, Bowie69 said: Have you tried electrical contact cleaner on the rotary switch? Can often work wonders. That was my first thought, as the switch started to feel a bit graunchy, and progressively stiffer. I took the top and blew it all through with compressed air, then as the switch workings are fairly exposed, used contact cleaner while exercising the switch. This made the switch feel back to normal, but sadly seemed to make it electrically worse. I think some of the gubbins inside is now broken. Annoys me that I think if I had done this sooner, it would have been OK ! Quote Link to comment Share on other sites More sharing options...
smallfry Posted April 24, 2023 Author Share Posted April 24, 2023 1 hour ago, elbekko said: As Bowie says, some cleaning up, especially on a machine that's been in metal dust since the early '80s, can do wonders. I'd try mapping out the different circuits through the switch. When drawn out, it'll probably not be so complicated. Then you can either see about a fitting replacement switch, or if not possible, just have a bunch of toggle switches and a legend that you print out next to it for the different settings? Not idiot-proof, but that just means it needs one of these stickers. That is exactly the sort of solution I had in mind ! Most of the things I have here should have one or other of those stickers ! Quote Link to comment Share on other sites More sharing options...
smallfry Posted April 24, 2023 Author Share Posted April 24, 2023 7 minutes ago, FridgeFreezer said: If you're got a circuit diagram / manual for it it may well be possible to either work the switch out or do as you say and put "normal" controls in. I've just done a rebuild on my (probably) 50-year-old NBC MIG which has a similar setup of automatic wire feed rate with only fine adjustment, the circuitry in it is very basic but having eventually tracked down a manual there's some incredibly cunning twists to it that I would not have guessed from staring at it. I have got the manual somewhere. I dont know if it has a schematic diagram in it, but it is in a safe place. So safe that I can't find it ! What chance would I have of working out any cunning twists, if an electronics genius such as yourself did not spot it ! Quote Link to comment Share on other sites More sharing options...
Bowie69 Posted April 24, 2023 Share Posted April 24, 2023 Which model is the Telwin? Quote Link to comment Share on other sites More sharing options...
FridgeFreezer Posted April 24, 2023 Share Posted April 24, 2023 2 hours ago, smallfry said: What chance would I have of working out any cunning twists, if an electronics genius such as yourself did not spot it ! Depends what you end up trying to do - it may not matter or it may be labelled / described in the manual as the NBC was. In mine it was the way they'd wired the relays that trigger the wire feed motor in sequence with the gas & main contactor so it actually brakes the motor when you release the switch to get a controlled burn-back of the wire to the tip, very elegant for something so low-tech. Studying the schematic you can see it must be the case, but it's easy to skim over and miss the intention / significance. Quote Link to comment Share on other sites More sharing options...
smallfry Posted April 24, 2023 Author Share Posted April 24, 2023 10 hours ago, Bowie69 said: Which model is the Telwin? Its a Mastermig 120 from 1984. Quote Link to comment Share on other sites More sharing options...
smallfry Posted April 24, 2023 Author Share Posted April 24, 2023 8 hours ago, FridgeFreezer said: Depends what you end up trying to do - it may not matter or it may be labelled / described in the manual as the NBC was. Still cannot find the manual. I dont think mine has anything as sophisticated as that, but I have no idea. I want to bypass the "fixed" wire feed, and fit a rotary knob rheostat, or is it a potentiometer ? I have found the highest output tapping and bypassed the board, so now the wire feed is on full tilt on all power settings. I need to choose a control for it. At the moment I am trying to work out how the welding current works. There are some bridging links on the rotary switch but the faulty switch makes it difficult to make sense of it. Quote Link to comment Share on other sites More sharing options...
FridgeFreezer Posted April 25, 2023 Share Posted April 25, 2023 14 hours ago, smallfry said: rheostat, or is it a potentiometer Could be either, the issue will be if it's dissipating any power to drop the speed - the fine adjust on my NBC is a chunky ceramic one rated to about 10W If the resistors you bypassed are chunky ones that is a clue that there's power being turned into heat in there Quote Link to comment Share on other sites More sharing options...
Bowie69 Posted April 25, 2023 Share Posted April 25, 2023 There's manuals out there for slightly later models, which still contain a rotary switch connecting to different take-offs on the transformer, I suspect, given how there is nothing new under the sun, that a later manual will have a virtually identical electrical wiring layout compared to yours. 1 Quote Link to comment Share on other sites More sharing options...
Gazzar Posted April 25, 2023 Share Posted April 25, 2023 Mig welding forum. 1 Quote Link to comment Share on other sites More sharing options...
FridgeFreezer Posted April 25, 2023 Share Posted April 25, 2023 3 hours ago, Gazzar said: Mig welding forum. https://www.mig-welding.co.uk/forum/forums/welders-and-welding-machines.15/ 1 Quote Link to comment Share on other sites More sharing options...
fmmv Posted April 27, 2023 Share Posted April 27, 2023 You could probably fit a PWM controller for the wire feed, the sort that might be e found on ebay. Supply voltage may be 24v or even 42v. However re your switch try calling TechnicalArc at York, they sell those kind of switches. Quote Link to comment Share on other sites More sharing options...
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