Miller 320A/BP TIG won't arc
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  1. #1
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    Miller 320A/BP TIG won't arc

    Hey guys, I was wondering if you could help me out with getting my welder to work. It worked fine when I picked it up from the guy I bought it from in March. Since then, it has sat at my parents place until recently. Now I have it wired and gassed up, and when I try it weld, it won't arc unless I touch the electrode to the work piece.

    Here are some of my observations:
    The fan turns on
    The high frequency sounds like it is working
    the gas timer works
    the "reset" switch is "on"
    I have the contractor control set to "remote"
    I checked the input voltages. hot to ground is 120v and hot to hot is 240V
    there is a big glass fuse at the back of the welder. I checked it for resistance and it is 0.
    I checked the ground and electrode cables for resistance and it is 0
    I checked the voltage between the ground cable and electrode with the pedal down and I get about 88V. This is on the medium amperage setting.

    There are some other things that I checked but those are the main things that I can remember.

    What really baffles me is that it worked fine in March, but now it doesn't work. I really hope that I hooked up something incorrectly and someone here will be able to tell me what happened. Or maybe there is something that commonly goes bad and is easy to fix?

    Thanks in advance,
    Austin

  2. #2
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    Re: Miller 320A/BP TIG won't arc

    First, do you have a manual for the machine? If not, download one from
    (http://www.millerwelds.com/om/o340j_mil.pdf), and check all of your setup carefully. Try starting with the switch on "Continuous," rather than "Start." If it works on continuous bun not start, the start relay CR2 or it's control circuit may be defective. All components refered to are as shown in Figure 7-1 of the manual.

    Second, be very careful about checking the open circuit output voltage at the torch on the TIG setting. If the HF is present (which it apparently is not) or becomes present, it will probably punch through the insulation of your meter and damage it or you.

    Check for 115 VAC at the INPUT of T4, the HF unit power transformer (NOT the HF coupling transformer in series with the electrode).

    To check voltages inside the machine, pull the power plug, clip your meter leads to the points of interest, connect power and turn on the machine, and observe the reading. Don't grope around inside the machine when it is powered up.

    Observe if arcing is visible at the air gaps. Check the setting of the potentiometer in series with the primary of the HF output coupling transformer. With the machine turned off and disconnected from the line, be sure you have continuity through the pot and check for continuity through the secondary of the HF power transformer and the primary of the output coupling transformer. These resistance readings may be well above zero because both are high voltage windings. We know the secondary is OK, since you say you can weld with touch-start.

    If these all check out, about the only things left are possible failed open capacitor C2 or shorted high voltage windings in the two transformers. They dont seem likely.

    Good luck. Let us know what you find.

    awright

  3. #3
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    Re: Miller 320A/BP TIG won't arc

    Quote Originally Posted by awright
    First, do you have a manual for the machine? If not, download one from
    (http://www.millerwelds.com/om/o340j_mil.pdf), and check all of your setup carefully. Try starting with the switch on "Continuous," rather than "Start." If it works on continuous bun not start, the start relay CR2 or it's control circuit may be defective. All components refered to are as shown in Figure 7-1 of the manual.awright
    I have a manual, and I double checked everything a few times. I think I have all of this ok.

    Quote Originally Posted by awright
    Second, be very careful about checking the open circuit output voltage at the torch on the TIG setting. If the HF is present (which it apparently is not) or becomes present, it will probably punch through the insulation of your meter and damage it or you. awright
    Yeah, I suppose that what I did was pretty stupid. Like you said, it must not be working or I would have seen smoke.

    Quote Originally Posted by awright
    Check for 115 VAC at the INPUT of T4, the HF unit power transformer (NOT the HF coupling transformer in series with the electrode).

    To check voltages inside the machine, pull the power plug, clip your meter leads to the points of interest, connect power and turn on the machine, and observe the reading. Don't grope around inside the machine when it is powered up. awright
    I’ll have to do some more digging how to check these. I found “figure J” in the manual, so I should be able to figure it out. I might have to take a cover off.

    Quote Originally Posted by awright
    Observe if arcing is visible at the air gaps. Check the setting of the potentiometer in series with the primary of the HF output coupling transformer. With the machine turned off and disconnected from the line, be sure you have continuity through the pot and check for continuity through the secondary of the HF power transformer and the primary of the output coupling transformer. These resistance readings may be well above zero because both are high voltage windings. We know the secondary is OK, since you say you can weld with touch-start. awright
    Arcing is visible at the gaps. The HF potentiometer has 2.5 Mohms at 0, 4 Kohms at 3, 2.5 ohms at 50, and 0 ohms at 100, so I assume that it is ok. I’ll have to check the rest of these things later since I have to figure out what they are.


    Awright, thanks for the help. I'll check into more of what you said when I get some more time.

  4. #4
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    Re: Miller 320A/BP TIG won't arc

    If I understand how you measured the resistances of the HF pot R5 (I think the HF intensity pot is R5 - it's hard to read the pot's ID on the schematic) it would appear the pot is defective, at least at the "0" position. Can you find the design value of R5 in your schematic, or from Miller tech support, or is a resistance value printed on the body of the pot? I can't find an R5 in the parts list. What does the pot look like? If it is a ceramic toroid with very visible wires showing along the contact track, it would never normally get to the high hresistance values you observed. Does it just look like a radio volume control? That type of pot could be anything inside. Some pots do have an "off" position at one extreme position of the dial, but I don't see a good reason for such a function in this machine.

    The 2.5 megohms at "0" seems way out of line with the function of the pot and the values at other settings. What settings of R5 were you using during your tests of welder functioning?

    The other thing that seems strange about the HF intensity pot is the extreme non-linearity your measurements indicate. If the scale is zero to 100 in a linear scale, your pot is only 2.5 ohms at 50% rotation (from "100"), but then 4Kohms at 70% rotation and 2.5 megohms at "0", or full rotation from "100." While non-linear pots are made, that's pretty extreme, making me think the HF intensity pot is defective. I'd check with Miller Tech Support for a description of the pot or a replacement.

    Good luck.

    awright

  5. #5
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    Re: Miller 320A/BP TIG won't arc

    Quote Originally Posted by awright
    If I understand how you measured the resistances of the HF pot R5 (I think the HF intensity pot is R5 - it's hard to read the pot's ID on the schematic) it would appear the pot is defective, at least at the "0" position. Can you find the design value of R5 in your schematic, or from Miller tech support, or is a resistance value printed on the body of the pot? I can't find an R5 in the parts list. What does the pot look like? If it is a ceramic toroid with very visible wires showing along the contact track, it would never normally get to the high hresistance values you observed. Does it just look like a radio volume control? That type of pot could be anything inside. Some pots do have an "off" position at one extreme position of the dial, but I don't see a good reason for such a function in this machine.

    The 2.5 megohms at "0" seems way out of line with the function of the pot and the values at other settings. What settings of R5 were you using during your tests of welder functioning?

    The other thing that seems strange about the HF intensity pot is the extreme non-linearity your measurements indicate. If the scale is zero to 100 in a linear scale, your pot is only 2.5 ohms at 50% rotation (from "100"), but then 4Kohms at 70% rotation and 2.5 megohms at "0", or full rotation from "100." While non-linear pots are made, that's pretty extreme, making me think the HF intensity pot is defective. I'd check with Miller Tech Support for a description of the pot or a replacement.

    Good luck.

    awright
    There is a wire in and out of the pot. I measured the resistance between the 2 wires. I'd have to look at it again to tell you what kind of pot it is.

    The "0" position wasn't exactly repeatable. I did it twice and got 2 different results. I'm not sure if my meter is repeatable at high ohms or not. Are you sure that the pot is supposed to be linear?

    Also, I tried welding at 100% (which had 0 ohms), and it wouldn't arc, so I assume that something else still isn't working. It worked fine 7 months ago...

    I should be able to look at it again on Tuesday.

  6. #6
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    Re: Miller 320A/BP TIG won't arc

    No, I have no firm basis for assuming that the pot should be linear. But the degree of nonlinearity you describe is extreme and more likely to be a result of a defect than intentional pot design. Loof at the numbers. If values around 4K or 2.5 megohms were useful for the HF intensity function, how could a change from 0 ohms to 2.5 ohms be of any value, and why would they use up 50% of the pot rotation for such a small change?

    If your measurements were valid, I would attribute the 2.5 megohms to an open circuit pot and the finite reading to leakage throught the circuitry external to the pot.

    It is more likely that the pot is unstable than that your meter is unstable. Pots are notoriously erratic, especially after long usage or storage under less than ideal conditions. Ohmmeters show their erraticism (?) more at low ohms than at high ohms ranges because any variable resistance in the range switch contacts would tend to be a low ohms phenomenon that would be invisible in comparison to the high resistance being measured.

    While I still strongly suspect the pot (or your measurements) as being defective, the fact that you got no HF at zero resistance - the highest HF intensity setting - strongly indicates that something other than the pot is the basic problem. That doesn't necessarily mean that the pot is ok - just that it is not the main cause of the loss of HF. While I am always suspicious when two unrelated things appear to go wrong at the same time, it is possible that the pot has been erratic for a long time, but nobody tried to use the HF at minimum intensity and, thus did not notice the pot defect. After all, if you or the prior owner were like most of us, they would crank the HF up to max and cruise at that setting (disregarding or ignorant of the resulting interference to neighbor's equipment and radios).

    Is the HF intensity pot R5 and did you find a value for it in the parts list? Can you post a more readable schematic that the PDF file on the Miller site?

    Good luck.

    awright

  7. #7
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    Re: Miller 320A/BP TIG won't arc

    I checked the Pot again. This time I unhooked a wire so I am measuring the resistance through the pot itself.
    100 = 0.0-0.3 ohms (checked it a few times)
    70 = 1.9
    50 = 2.6
    40 = 3.2
    32 = 9.5
    30 = 336
    20 = 2300

    It appears to go haywire in the low thirty's and below. Above that it seems linear. I'll just keep it at 100 I assume.

    But I have some good news I think... I got out the manual and started checking resistance and capacitance on what was accessible. On the item number 208, which is described in the book as “CAPACITOR, mica .002 micro f”, I noticed that they must have got hot at some point in their life since it looked like a fluid melted from it. I unhooked one of the straps connected to the capacitors checked the capacitance, and it doesn’t match up with what the book says it should. There are 2 of these capacitors, the left 1 gave me 2.39 nf and the right one gave me between 3.5 and 4.11 nf (it wouldn’t stay stable). The book says it should be .002 micro f.

    In summary:
    .002 micro f = 2.0 nf which is what the book says it should be
    2.39 nf is the capacitance of the left capacitor (19.5% off)
    3.5 to 4.11 nf is the capacitance of the left capacitor (~85% off)


    Also, I’m pretty comfortable the my meter is reading correctly since the capacitance of the other C3 Capacitor is somewhat close, 10.85 micro f and the book says it should be 10.0 (8.5% off)

    Here is the schematic from under the front cover:
    http://www.cardomain.com/member_page...62_14_full.jpg

    Here is figure J from the manual which is the HF circuit
    http://www.cardomain.com/member_page...62_15_full.jpg

    Here is a picture of the capacitors that I think are bad. Notice the seepage.
    http://www.cardomain.com/member_page...62_16_full.jpg

    Am I right in thinking that one or both of these capacitors are bad? If so where can I buy replacements at? Will Miller still sell them and do you have any idea on price?
    Thanks for your help, Austin

  8. #8
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    Re: Miller 320A/BP TIG won't arc

    Boy, that's annoying! I got halfway through a reply and lost it when I tried to bring up the thread to look at the schematic. Sorry if I end up with redundant posts.

    The capacitance tolerances you describe are not unusual and should not, in themselves, cause problems or concern. The exact capacitance of series coupling capacitors is not very critical due to the nature of their function. However, the fluid leakage is a cause of concern, especially since these are 5 KV capacitors handling high peak voltages from the HF power transformer and fas, high voltage pulses at the instant of breakdown of the spark gaps.

    It's not clear to me what might be happening in the circuit. You report that you are seeing sparks in the spark gaps and that you measure capacitances in the ball park of design values. That would indicate that the caps are not leaking excessively (electrically) because if they were, I would expect them to prevent the voltage across the spark gaps from ever reaching breakdown potential. Your capacitance tester, however, probably only applies a low voltage to the caps under test and would not uncover high voltage breakdown of the caps. It would help if you could rig up a high voltage test of leakage of the capacitors, but that is not a trivial task and can be fatal to a careless troubleshooter.

    I would suggest just replacing the C2 caps and getting the replacements from Miller. It is not easy to find HV caps of a particular value through surplus channels, and I suspect you'd have difficulty finding them in ordinary electronic supplier's catalogs.

    I assume that Miller would be able to supply replacements. I have no idea about cost. If Miller says they don't have exact replacements you might talk to a technician in the Miller service department and see if he will be cooperative enough search parts lists of other, more recent machines to look for an available equivalent.

    One good reason to replace C2 soon is that the power input transformer to the HV/HF circuit could be burned out if it feeds excessive current to a leaky capacitor over a long period.

    awright

    Almost forgot to ask if you can find the description of the HF intensity pot in the parts lists.
    Last edited by awright; 10-19-2006 at 05:58 AM.

  9. #9
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    good news!

    I spent at least 12 hours trying and checking and checking and talking to different people and service departments etc. I even got my EE buddy to to bring over a scope and diagnose. Finally I called Miller Tech support. The guy was very knowlegeable and helpful. We talked for about 16 minutes. The jist of the conversation was "it sounds like it should be working blah blah, but check these things..." Ok will do.

    But before we ended the call the tech support says "One more question. What kind of shielding gas are you using?"

    "I don't know, it is a mix." I looked at the tank, it was 75% Argon and 25% CO2.

    "That's a big no no. You need straight argon"

    "Really?"

    "yes"

    "is this my problem?"

    "50/50 chance"

    So I gapped the points to 0.014" like he said. Didn't work. Then I went and bought straight argon. Bingo!!

    I would feel like an idiot, but I'm just too happy!

    Thanks a ton for your help!

  10. #10
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    Re: Miller 320A/BP TIG won't arc

    Congratulations! I can't think of a better outcome.

    Have fun.

    awright

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