AL spoolgun stuck on high

I picked up a millermatic 250 welder with an AL spoolgun at an auction a while back. I tried the spoolgun a couple days ago. It works but the feeder is stuck on "HIGH" That is it feeds the AL wire at full speed when welding and the speed knob has no effect. It welds great if you can use wire that fast.

I first tested the spoolgun itself

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The reostat is 10K ohm and the wiper goes from 0 to 10 K just fine. I seen no obvious mechanical issues. (I was really hoping for a broken pot, etc.) This was tested right at the plug to the welder.

I located the electrical prints for the welder

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The spoolgun connection is shown right at the bottom of the print. Before I give up and send it to the welding supply for repairs, are there any other simple possible troubles to look for?

Karl

Reply to
Karl Townsend
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Didja try the gambits offered in the Owner's Manual on page 16?

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Low, high, or erratic wire speed. Readjust front panel settings (see Section 3-1). Place Low Range/Full Range switch in correct position (see Section 3-1). Change to correct size drive rolls (see Section 4-5). Readjust drive roll pressure (see Section 2-12). Replace inlet guide, contact tip, and/or liner if necessary (see Sections

4-4, and 4-8). Check position of input jumper links (see Section 2-9). Have Factory Authorized Service Agent check main control board.

--Winston

Reply to
Winston

Maybe blown SCRs. See what they measure anode to cathode (and with leads reversed) on a multimeter using the diode (ohms) range. With power disconnected, obviously, or the meter will be smoked.

Reply to
Spehro Pefhany

4-4, and 4-8).

Probably good to point out the basics. I hadn't double checked the input jumper. Its OK

Reply to
Karl Townsend

You'll have to dumb this way down for me.

Reply to
Karl Townsend

I also should point out the standard gun works fine. If I follow the prints right it uses the same SCR??? to control wire feed speed. Please know, I can hardly read a print.

Karl

Reply to
Karl Townsend

Is there a part marked Q1 on the PCB that is connected to the motor terminal (59) ? That should be a 10A SCR if I got the right schematic.

Reply to
Spehro Pefhany

(...)

Suspect a broken wire or open connection between RC4-4 and the pot on the gun.

Advise photograph existing wiring, unplug and re-plug connections. Please consider good quality contact cleaner treatment of connectors, too.

--Winston

Reply to
Winston

No sign of smoke by this device. The are four pins on Q1 so I did ohms both ways (a diode gives different reading depending on which way) accross each pin:

top pins gives 144 K ohm bottom pins 137 K ohm right side 0 ohm left side 6.1 K ohm

You must have found a more complete schematic than I have. Where did you find it?

Karl

Reply to
Karl Townsend

Good suggestion. I'll get some contact cleaner and try this.

Reply to
Karl Townsend

I'm guessing it's similar to the Snap-on 250A welder:

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Reply to
Spehro Pefhany

LPS or better. Not Radio Shack brand!

:)

--Winston

Reply to
Winston

As wired (based on the schematic you posted the URL for) this is not a rheostat, but rather a potentiometer. (I expected that because rheostats are seldom that high in resistance, but I had to check the schematic to be sure.)

The difference between a pot and a rheostat is that a pot uses all three terminals to divide a voltage to an variable output. A rheostat is in effect only a two terminal device -- a variable resistor used between two points (though sometimes the CCW end will be connected to the wiper to minimize how high the resistance shoots when the wiper makes poor contact.) And because a rheostat is usually (though not always) running with more current, it will frequently be constructed as a wirewound on ceramic instead of the carbon resistance track common with pots.

So -- did you check all three pins? It should be 10K between pins 50 and 51, and pin 30 should measure resistance to the two other pins which add up to a total of 10K. I suspect that pin 51 is not making connection, so you will get full speed all the time. They don't say, but I expect that you should measure 10V DC (though it may be 5VDC or some other value) between pins 50 and 51 at the pot. With the voltmeter connected between pin 51 and the wiper of the pot (pin 30) you should get a voltage which varies as you turn the pot from fully CCW to fully CW.

The one marked IPM (which I presume is "Inches Per Minute")? I wasn't sure at first whether you were talking about the drawing as it popped up in the PDF reader or after rotating so the lettering on the drawing was right side up.

Check the full connection of all three points of the pot to the control.

Good Luck, DoN.

Reply to
DoN. Nichols

(...)

Apparently that circuit works fine. Karl mentioned that the local controls work properly when the spoolgun interface is not connected. That is, just using the built-in wire feeder and IPM pot you cited.

We still both hypothesized the same thing. If there were a break in Circuit 33, RC4-4, then the welder would be commanded to go to full speed all the time when the spoolgun was connected.

If there was a way to check those connections at the welder control board, that would be very interesting information indeed. Either way, it could not hurt to disconnect the control board and wash down the connectors with some good quality connector cleaner. LPS!

--Winston

Reply to
Winston

Thanks DoN and Winston. I'll check all this out.

Karl

Reply to
Karl Townsend

Please keep us posted, Karl.

--Winston

Reply to
Winston

VICTORY!!!

Well, sort of. The female amphenol plug is the loose connection. I ran a drill bit in/out to remove swarf and dirt. Then sprayed BrakeKleen. Put plug on real tight. It works. My solution for now will be to put a bracket on the side of the welder to hold all the spoolgun cable. Then never take the connection apart again.

Failing that, I'll hire somebody to re-solder on a new amphenol plug.

Karl

Reply to
Karl Townsend

YeeHaw! :)

--Winston

Reply to
Winston

Great!

Out of curiosity -- is the plug fitted with soldertail pins or crimp pins?

And is the female connector on the panel or the spool gun?

It is a good idea to tie a plastic baggie around the connector when it is not plugged into the panel.

Good Luck, DoN.

Reply to
DoN. Nichols

Soldertail. I don't do this.

On the panel.

Reply to
Karl Townsend

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