Tig 300/300 PC control Bd Fuse blew

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I have a tig 300/300 that blows the 20 amp fuse on the control board when attempting to tig weld. As a result it doesn’t flow any current for welding. There is no control of amperage at the dial on the welder or at the foot pedal. I put a resettable fuse in for trouble shooting, when the welder is idle its fine, soon as i energize the foot pedal and apply the current it pops. If the current adjustment is by the dial on the welder then the breaker pops as soon as I engage the contactor. Even though I get no amperage, i do get 74 Volts at the terminals where the welding leads attach when the contactor is engaged. Before I go looking into replacing the PC boards I was wondering if there were any checks that can be done to determine they are at fault. Any help would greatly be appreciated.
Reply:Please post the code number of the lincoln welder you are asking questions about.Lincoln made the tig 300/300 for many years. This first model is very different from the last.The first models have a rounded top. First ones do not have a pc board the next ones have just a small pc board. the later one had a square top and several pc boards.Which one is yours and we might be able to help you.I do not have a lot of time to check the boards during the week so send me a pm and I will give you some tests. If you have a friend that knows electronics get him to assist you.Smoked meters and a fried body are no fun.
Reply:The Model is Tig 300/300, Code # SPL-B443-84, Serial # AC-576738. I took the two PC boards out to have a look. The timer board looks fine; the control board which has the slow blow 20 amp fuse on it had finally burned one of the solder trails on the back side from shorting out. I traced that section back to the large block with the three diodes. I tested them with a diode-meter-checker and two of them are a direct short. 0.0 in both directions. The one that is still on the board checks out. Now the million dollar question is what’s the voltage and amperage rating of these little diodes? I checked with Lincoln and they want $32.00 a piece. These are going for a few bucks on EBay; just don’t know which ones to get. Attached Images
Reply:I don't know anything about that machine, but those diodes should have some sort of part number on them.  Given the part number we can look up the current and voltage ratings and maybe find a suitable replacement.Dynasty200DX w/coolmate1MM210MM VintageESAB miniarc161ltsLincoln AC225Victor O/A, Smith AW1ACutmaster 81IR 2475N7.5FPRage3Jancy USA1019" SBAEAD-200LE
Reply:Part # on the diodes are T12705-24, I searched a bunch and can’t find anything. they seem to be Lincoln specific. I searched the Lincoln website and they say to use suitable sub 9ST12705-67 which is $32.00 a piece. Unfortunately they show any voltage or current ratings.
Reply:The fuse on the pc board is 20 amps, the control voltage is 0 to 30.Use any diode with the same size stud. That is rated higher than 20 amps and the highest voltage rating you can get in that size. 100 volts or more. Look on the side of the diode for a diode symbol. The replacement diode must have the same symbol going the same direction.Stud diodes come two ways anode or cathode to the stud. Standard or reverse polarity.2 of the diodes and the scr's on the other plate make up a bridge rectifier that is controllable.the 3rd diode could be across the center of the bridge and backwards. Its job is to protect the other parts from voltage spikes.  the control winding of the mag amp should measure 1.1 ohms. You may want to put some dc voltage on it from a fused bench power supply and bring it up slowly to make sue it is not shorted out coil to coil or to ground. Before buying the diodes.
Reply:I searched for your part number and came up with squat.  Well... I found a bunch of stuff on the lincoln website, but nothing that converted that number into an independent manufacturer's part number.I did a little digging on digikey and found a 200V 40A stud mount diode for $6.56.  http://search.digikey.com/scripts/Dk...ame=1N1186A-NDAs ccawgc said we'll need to figure out which way the current is supposed to be flowing in the diode.  You probably can't tell by testing the ones that have already been burned up.  Can you tell from the wiring which way they need to go.  Like the diodes are wired to an electrolytic capacitor with + on one lead, or some other tip-off from another component that has a non-lincoln part number.Last edited by AndyA; 11-09-2010 at 10:00 PM.Reason: oops on the linkDynasty200DX w/coolmate1MM210MM VintageESAB miniarc161ltsLincoln AC225Victor O/A, Smith AW1ACutmaster 81IR 2475N7.5FPRage3Jancy USA1019" SBAEAD-200LE
Reply:Ccawgc thanks for the info, I checked the control winding and it measured out at 1.5 ohms. Its not grounded to the case or the secondary windings. All three of the stud mount diodes are forward biased with the same part #'s.AndyA, thanks for the link on the diode. I actually found some on Ebay, and purchsed 3 before I saw your post. I figure I might as well replace all of them. With my luck if i just put the two in, next week the third one would fail.I should have them ny the end of the weeek, I'll keep ya posted. I think this going to be a good fix. I'm no electronics expert, but everything else looks good and seems to check out with the meter.Steve
Reply:Well I put the new diodes in and two of them went bad again after attempting to strike an arc. The ones I put in were rated at 75 amps, 100 volts. I found a schematic for the control board and it shows the diode rating at 25 amps, 600 volts. Could the diodes been spiked with too much voltage? Looks like I might have to send the board in for repair. Anyone have a recommendation for reliable board repair service?
Reply:try  www.industrialelectronics.comthey say around 60+ on 1 board  and 40+ on other boardLast edited by prop-doctor; 11-16-2010 at 12:35 AM.idealarc 250/250 ac-dc tigidealarc 250/250 ac-dc tig #2 used for sticklincoln sp100hh125dual arbor grinder polisher30 yrs of hand tools52 pitch blocks 6p-26prake gauge -pitch gaugeG&D prop repair 918-207-6938Hulbert,okla 74441
Reply:If yours were only rated at 100V reverse voltage and more than that got applied, they probably went poof!Dynasty200DX w/coolmate1MM210MM VintageESAB miniarc161ltsLincoln AC225Victor O/A, Smith AW1ACutmaster 81IR 2475N7.5FPRage3Jancy USA1019" SBAEAD-200LE
Reply:Well, I took both boards out, packed them up and shipped them out to Industrial Electronics today. I figured that if I keep trying to fix it myself I’ll be into it more than the cost to fix it professionally.  We'll see what happens. They told me 3 to 4 weeks for service, hopefully they can get done sooner.
Reply:not voltage. 30 X 1.414 = 42.42 volts. that would be peak unfiltered volts.when repairing these types of machines, I like to use a variable DC bench power supply and operate the welder and make sure it works. before repairing or replacing the pc boards.A 600 volt diode is tougher than a 100 volt diode when hit with voltage spikes.To high of current and to high of voltage will kill a diode. If the Mag amp has a coil to coil short it will blow the fuse and possibly the diodes It can only be found by using the bench power supply and watching the current.
Reply:The closest thing I have to being able to apply DC power to the coil is a battery charger. I hooked up 12 volts and it only drew seven amps. Since I didn’t have the boards installed there isn’t high frequency start or gas. I tried it out in stick welding mode and it works as advertised. It should work after the boards come back.
Reply:Well I finally received the boards back from repair, of course with my luck it worked for about 30 seconds and does the same thing it did before. Soon as the fuse blew I check the diodes and there shorted. I checked the incoming voltage to the board and its 124V AC; I can put DC voltage to the output side of the board that energizes the coil for current and weld with no problems. So I’m guessing that somewhere the board is shorting to ground.Anyone have an idea what else to check for on the welder? There is isn’t much on the back side, a few switches, potentiometers, and bunch of wires.