Rotary weld positioner speed and/or high-frequency start issue (EE help?)

[keeena]

I have an MK Products rotary positioner that is supposed to be compatible with HF welding processes. I only recently welded aluminum on it for the first time and hit a problem: the positioner's RPM would drop significantly when welding. For example, I was set to 1 RPM, but during welding it dropped to about 0.3 RPM. The effect seems to correspond to the current. When backing off the pedal, the RPM would be less affected (though still very noticeable).

I dug into it and I ended up also finding two speed-control issues (#1 and #2 below) which occur as-is (just the machine itself, w/o any welding). My questions:

• Could the 2 speed control issues be the same root cause as the HF problem?
• If not: then regarding HF only - how are these devices generally isolated from HF? And are there things I can do to further isolate the issue or diagnose? e.g. change where the positioner is plugged in, inspect in the positioner's guts (possible an internal electrical ground lead fell off?), or do anything else?

1. The documented speed range does not match the RPM readout

	Per Manual	Per Machine Readout Display
Low Range RPM	0.5 - 3	0.3 - 2.3
High Range RPM	0.5 - 10*	1 - 7.4

*I think this may be a typo in the manual. There is an identical looking, newer revision machine and its manual says 1-10 for high. This seems more logical.

2. The machine's readout does not match reality when in High range only

Low Range

Readout RPM	Actual (Measured) RPM
0.3	0.225
1.0	1.0
2.0	2.0

High Range (deviates more as the RPM setting increases)

Readout RPM	Actual (Measured) RPM
1.0	1.5
3.0	2.875
5.0	4.6
7.0	6.25

Notes
The positioner is directly connected to the + lug on the welder (as it normally should be for TIG; DCEN) with a high-quality cable. The positioner is plugged into its own 120v circuit.

DC welding doesn't change RPM. My welder doesn't have selectable HF so I cannot rule out A/C vs. HF. Positioner can handle up to 300a weld current; I'm welding at far less than that.

My positioner's chuck has blackened jaws. Seems to conduct the weld current fine, but will be testing with a jumper to the turntable. I also want to try at different RPMs and both speed ranges.

The jog button spins the table at 9.8 RPM (indicated, I have not measured). Its not documented what it should jog at, but assuming that ~10RPM is the jog speed, then I suppose the speed control issue may just be the pot...though I'm not sure why only High would be affected?

The potentiometer's scale is 0-10. It doesn't do anything until position 2 in either range setting. At the #2 position is when you hear the transformer hum and get movement. Position 0 does show a readout of 0 RPM, so I suppose its OK, figured I'd point that out.

MK can/will take the machine for diagnosis and repair. I just wanted to have a bit of a go myself because I'm curious. And cheap. But mostly curious. I have a little bit of DIY EE and tools and am tempted to poke around.

Manual for my machine (has a high-level schematic): link

 

[markba633csi]

Sounds like maybe a grounding problem- definitely try plugging the unit into a different outlet, and possibly add an additional ground
wire to a known grounded cold water pipe nearby
You might also experiment with some ferrite snap-over noise suppressors on the positioner's power cord

 

[rabler]

Interesting that the speed pot is referenced to both internal ground and frame ground, with the interconnect having a jumper. That would support @markba633csi grounding problem theory.

 

[markba633csi]

Without test equipment it can take some experimenting to solve these kinds of things- MK probably has considerable experience with it and would have the best ideas on things to try- I would start a dialog with them

 

[keeena]

Interesting that the speed pot is referenced to both internal ground and frame ground, with the interconnect having a jumper. That would support @markba633csi grounding problem theory.

@rabler - Here's the manual for the newer revision machine: link. It looks identical on the outside, but there are some minor board design changes. When I looked both manuals the other day I also noticed 2 grounding changes. Notice the newer model has the jumper JP1 cut on the Panel Control board; my model has it intact. There is also resistor R50 on the Speed Control board's ground vs. jumpered on mine (also labeled JP1). I thought both of these were interesting changes. Easy enough for me to rework; not hard to imagine that they were changed in the new one because of an issue...

They also moved the RPM meter to the Speed Control board and they added a DB9 connector for other functionality. Other than these few items: the boards look identical.

I did start to open it up; visually checks out. Speed pot is OK.

Without test equipment it can take some experimenting to solve these kinds of things- MK probably has considerable experience with it and would have the best ideas on things to try- I would start a dialog with them

I did chat with them the other day. Their only offer was to take it for a free evaluation and quote for whatever needed repair. I suspect they would just want to throw new board/s at it, but looking at the schematic differences maybe they would do the ground rework that I mentioned above? Both the Speed Control and Panel Control would be swappable as a pair from the newer model.

My unit was made in 2002; the minor rev changed happened around 2010. When did HF hit the scene? I had asked them a couple years ago if my model was OK with HF start and they said yes, but its old enough that perhaps they are confusing it with the 2011+ model.

 

[markba633csi]

I would try the changes and see- your unit is old enough that the factory may have found some issues with HF susceptibility and yours
doesn't have the latest and greatest
Of course they want the opportunity to get some more money out of you if they can...

 

[rabler]

@rabler - Here's the manual for the newer revision machine: link. It looks identical on the outside, but there are some minor board design changes.

Since you've implied you're comfortable with DIY electronics, I'd try removing the jumper and replacing it with a 10K resistor as in the rev 2 board. If that doesn't work you still have the option of sending it in, just replace the jumper first. I could easily see HF creating some weird grounding behavior in a welding current scenario. Out of an abundance of caution and ridiculous overkill, I might use a 1 watt rather than typical 1/4 watt resistor.

but looking at the schematic differences maybe they would do the ground rework that I mentioned above?

It is a little unclear, they show the jumper JP1 on both the speed control block (printed circuit) and the panel control block (printed circuit). The resistor is probably one of those PCBs. So I would guess they probably just would swap one or both components, and charge you accordingly. Either way, the cost of shipping both ways, plus labor opening the box at their end, is going to far exceed the cost of a resistor. Of course you *might* get better RPM range in high with that.

 

[keeena]

Hard stop!

I opened the unit up and noticed those 2 jumpers were not shorted. ?? A look at the board #s shows that mine has the newer boards in it (the manual I linked in post #5). They must have started making board changes after the manual for mine and prior to the 2011 manual. It makes sense too, because my unit has the external panel fuse (the first manual I linked does not show a fuse).

Anyway: good news because the newer model's manual has a complete schematic for the speed control board (page 12). But still: I only know a little about what I'm looking at.

The only thing visually that jumped out at me was capacitor C4 (orange-red, middle of the pic) on the speed control board. Its a tantalum cap, 33uf 35v...she looks a bit bulbous. This look normal or is it on its way to letting out the magic smoke?

 

[WobblyHand]

Hard stop!

I opened the unit up and noticed those 2 jumpers were not shorted. ?? A look at the board #s shows that mine has the newer boards in it (the manual I linked in post #5). They must have started making board changes after the manual for mine and prior to the 2011 manual. It makes sense too, because my unit has the external panel fuse (the first manual I linked does not show a fuse).

Anyway: good news because the newer model's manual has a complete schematic for the speed control board (page 12). But still: I only know a little about what I'm looking at.

The only thing visually that jumped out at me was capacitor C4 (orange-red, middle of the pic) on the speed control board. Its a tantalum cap, 33uf 35v...she looks a bit bulbous. This look normal or is it on its way to letting out the magic smoke?

View attachment 463213

Hard to tell. Usually tantalums look sort of mustard color like on your board. But it might be ok. Only way to know is to remove it and test its capacitance. The + pin seems to be in the square pad, so if they are complying with convention, it seems like the polarity is correct.

I'd be checking the jumpers carefully first though. From a quick look, that board seems to have some rework on it, so you might mark up / update your schematic, or at least check that it's accurate. If nothing else, I'd inspect the board to see if there's any revision ID on it. Might help you with your conversations with the vendor.

 

[markba633csi]

Dipped tantalum caps often have that shape- it's fine