On Thu, 04 Aug 2011 13:12:09 -0500, Karl Townsend
wrote:

On Thu, 04 Aug 2011 13:24:06 -0400, Spehro Pefhany
wrote:

On Thu, 04 Aug 2011 11:50:52 -0500, Karl Townsend
wrote:

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
http://metalworking.com/dropbox/miller250spoolgun.pdf

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
http://metalworking.com/dropbox/miller250prints.pdf

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

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.

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

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.