Ignoramus1782 wrote:
I just finished a mini project. I fixed a "broken" Miller XMT 300
CC/CV welding machine, which had the switch for the display
broken. That switch was switching the display between showing voltage
or current on a mini LED screen.

The broken switch had to be desoldered and a new one had to be
soldered in.

The issue that I ran into was desoldering. I have a "Pace SMD 2000
desoldering station" from my younger military surplus days. This
station has a tool that is like a soldering iron, but has a axial hole
in the tip and an adapter for a vacuum, and the built in vacuum. When
I push on a pedal, the vacuum starts sucking through the tip. So I
would melt the solder with the hot tip, push the pedal and...

My problem was that it barely sucked. Not enough to vacuum in the solder
from the circuit board.

You say "built in", that has the diaphragm pump in the power
supply box, or a Ped-a-Vac venturi vacuum generator that runs
off compressed air? Either way, there are likely disc-type
filters in the line that become clogged with flux residue.
The old ones had filters that you could pop open and replace the
elements in, the new ones are "non-repairable". But, I found
you can drip rubbing alcohol through the hole, let it sit a
while and repeat a couple times, allowing the alcohol to drip
out onto a paper towel. When the drips come out clean without
staining the towel, the filter is magically rejuvenated.
(Note the box may have ANOTHER disc filter hidden inside, it may
also be dirty.)

Second, there is a wad of felt in the glass tube inside the
handpiece that also becomes saturated with fine solder dust and
flux. I have not found those to be recoverable due to the
non-soluble dust. I got a bag of them with the machine, so I'm
probably set for life.

Third, the hole in the desolder tip fills with oxidized solder.
I have to rod the hole out every half hour or so when using one
a lot. I just use some handy solid wire and jam it in, twist it
around and pull it out, suddenly the "Suck" is audibly stronger.
As a stopgap measure, I used my 1/3 HP vacuum pump by connecting it to
the desoldering tool and turning on at the proper moment.

In the end, it all worked, the old switch was removed, a new one
installed, and the welder has a working selector of V vs. A display.

All this leads me to the conclusion that something is wrong with the
vacuum pump on this station. Would you say that it should provide very
strong suction?
I'll bet your filters are clogged. I have a motorized one at
work and a compressed air one at home. Whenever I hav a
problem, it is the filters or the tip.

Jon