I know what you mean about the dirt sucked into it. Wow!
What a cleanup job that was.
Well, it happens I was taking a 2 year "Machine Technology" course
at the local college that also has a welding tech course.
Guess what ALL stick/tig the machines are?
With permission, I opened up an idle machine, since these are
relatively new to see what the new guts look like and what the fan
control is as I had heard that you could have bought a kit to add
the fan control. The manuals they had also had the diagrams, so
I came away with the info and did some shopping. It's fairly easy
to do. Add a relay (120V fan on/off) and 2 NC temp sensors.
One goes on the big tryristor heatsink and the other gets buryed
in the transformer coils (lots of gaps to jamb it into).
Tracing out the wiring is the hard part.
I love that quiet now.
RichD

On Sep 25, 4:14 pm, "Pete C." wrote:
RichD wrote:

I repaired my Miller Synrowave 250 the second time it broke.
I have the early production one (old style board). The output
just goes ON at some fixed setting. Panel control or foot peddle
does nothing.
The schem. did come with it. Turns out they used a now obsolete
op-amp that was prone to fail. No pin for pin modern replacement.
I cross wired a new op-amp type in to check it out, and all was well.
Ebay turned up plenty of the old type. Swapped it out and.. Viola!
I put the thermal fan control mod in it while it was open.
RichD, Atlanta

Not sure whether my 250 is new style or old, it's about 1995 vintage. I
should add the thermal fan control on mine since in home shop use it
would hardly ever come on and would reduce dust ingestion substantially.