Miller 350LX HF wiring

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Hello everyone,  I recently acquired a miller 350lx, however there seems to be a problem with the hf start. Looks like the wiring has been messed with and I would like to return it to the original setup. The hf will start, but will eventually start arcing on a bolt beside the points. There is a lead connected from the mica capacitor hooked up to the back bolt the hf is arcing on. I believe this is the lead that hooks up to the rheostat? I've repaired the rheostat so it should work now if I hook it up. Ive attached some pics, hopefully they help. I can add more if you like. If anyone has any pictures of there machines hf parts that would be amazing!! Any help would also be awesome!! Thanks a lot,Cale
Reply:Oh yeah I see it. Take that gold washer out from under the gold screw. It looks like the point block is arcing to ground. The point blocks should be isolated from ground.Weld like a "WELDOR", not a wel-"DERR"

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Reply:Talk to Miller before you remove the washer or alter the circuit.  It appears to be original wiring connected to the mica capacitor that is part of the coil-capacitor resonant circuit that generates the HF.  It does not appear to be connected to ground.  My guess is that it functions as a “safety valve” for excessive high voltage HF by providing an alternative path for the HF spark.  HF gets induced throughout the entire electrical system of the machine and you don’t want it sparking somewhere it can cause a problem, like your main transformer.  I have repaired other HF welders that had the primary windings burned out with evidence of HF arcing inside the transformer.  High voltage will always find the easiest path to ground.The voltage is related to the spark gap distance, so make sure that your gaps are adjusted to the minimum standard width.  If you have a HF intensity adjustment knob, make sure that it is set to the minimum level to support your welding conditions.
Reply:

Originally Posted by Bradley256

Talk to Miller before you remove the washer or alter the circuit.  It appears to be original wiring connected to the mica capacitor that is part of the coil-capacitor resonant circuit that generates the HF.  It does not appear to be connected to ground.  My guess is that it functions as a “safety valve” for excessive high voltage HF by providing an alternative path for the HF spark.  HF gets induced throughout the entire electrical system of the machine and you don’t want it sparking somewhere it can cause a problem, like your main transformer.  I have repaired other HF welders that had the primary windings burned out with evidence of HF arcing inside the transformer.  High voltage will always find the easiest path to ground.The voltage is related to the spark gap distance, so make sure that your gaps are adjusted to the minimum standard width.  If you have a HF intensity adjustment knob, make sure that it is set to the minimum level to support your welding conditions.
Reply:I agree that the screw is grounded to the bracket and frame,  but Miller specifically put a wire in there to connect to the capacitor probably to be sure that this connection did not fail.  There must be a reason for this and I think it is to limit the HF voltage so it doesn’t arc inside any place it could cause damage.  Arcing to the washer is the symptom and not the problem.  If you break the arc by removing the torch without letting off the foot pedal, the voltage will go way up with the arc length.  The HF is still on, and the high voltage HF will find the easiest way to ground.  In my opinion this circuit is there to limit the arc to this gap under circumstances where the HF voltage is too high to prevent it arcing somewhere that it will cause damage.  I got a free Linde HeliArc 250 that had a main transformer burned through the insulation that may have been caused by stray HF arcing inside.  $60 of magnet wire and insulation later, by Linde was working and I had it for 12 years.  The points gap was set way too high.
Reply:Circuit has been modified from factory. The allen screw with the washer should be a nylon screw. Look at the diagram on the cover panel or in the owner's manual.put everything back the way it should be. The rheostat was removed in newer units.To many of them failed. Gap should be set at 0.008" The secondary of the HF high voltage transformer is grounded at the center inside the transformer. just looked at a diagram.Depending on the serial number the ground is either inside the transformer or a wire is added to the spark gap on one side. wire number 4 is the ground strap.it goes to the spark gap side that also connects to the HF capacitor. the other HF cap terminal connects to the HF coupling transformer. The other spark gap has the HF high voltage lead and the coupling coil lead connected to it. then the two spark gaps are jumpered together. Ground strap was added at serial number LB214644all though looking at the picture the grounded wire might just need to be moved to the top spark gap. Looks like the HF cap wire. Nylon or plastic screws /fasteners are used to mount the assembly to the metal plate.Last edited by ccawgc; 53 Minutes Ago at 02:57 AM.