72 Silicon chip Australia’s electronics magazine siliconchip.com.au
very thick insulation due to the high
voltage (tens of kilovolts!).
To avoid strain on the new splices,
we taped the wires to the outside of the
tube (as the original wires had been).
This completed the installation, but
we still needed to check the alignment
of the laser with respect to the mirrors
and other optics.Calibration
Since the new tube has a smaller di-
ameter than the old one, we expected
that the line of the laser beam would
be shifted somewhat. Removing and
re-fitting the rubber pads may have
also caused some variation. So we
took no chances and checked the en-
tire beam path.
This is done by placing a piece of
paper (such as a self-adhesive label)
over the mirror in the optical path,
briefly firing the laser using the PULSE
button, then checking that the laser
strikes near the centre of each mirror
along the way.
The first mirror is accessible from
the rear of the machine, the second
through a panel on its left side, and
the third is on the moving carriage,
directly above the lens which focuses
the beam onto the workpiece.
We found a good guide at siliconchip.
com.au/link/aao9 The button press
triggering the laser burst needs to
be very brief, or the sticky label may
smoulder or catch fire. After trying
with a second sticky note, we found
that the laser was aimed close to theconnecting to the new tube. The new
tube already had wires fitted and insu-
lated to the anode and cathode, much
more nicely than the old tube, so we
wanted to keep as much of the wiring
as possible.
After this, having already detached
the cooling water tubes, we carefully
removed the tube by lifting it out.
The new tube has a warning to refer
to the user manual, but there was no
manual included. So we were going
to have to figure out the installation
process by ourselves.
We fixed the new tube using the
existing clamps. Its slightly smaller
diameter meant that the hex machine
screws had to be screwed in further
than previously, but we were able to
clamp it securely.
We then attached the water pipes.
These merely push onto the barbs on
the tube. All seemed in order, so we
turned on the pump and refilled the
small outflow tank with fresh water.
The cooling circuit quickly filled,
and the bubbles made their way to
the outflow pipe. It’s important to get
rid of air bubbles so that there aren’t
any hot spots inside the laser tube.
Our cooling system monitor reported
no faults, so the flow appeared to be
adequate.
We then soldered the two new wires
to the existing power supply connec-
tions, added heatshrink tubing and fit-
ting the protective anode wire housing
over the join.
These wires are very fine, but haveWe suspected breakdown of the high
voltage insulation around the anode
connection, and attempted repair by
adding some silicone sealant.
Unfortunately, while that stopped
the discharge, the laser still wasn’t
working, so we suspect that the criti-
cal CO 2 gas may have escaped through
a small hole. Since the tube is blown
glass, it’s almost impossible to service.
So we bit the bullet and ordered an-
other tube.
After a few anxious weeks, the new
tube arrived in one piece. This one was
slightly different to that originally fit-
ted to the K40.
For example, it has a slightly small-
er diameter and is also a bit shorter. It
generally looks a bit better made, and
the high voltage anode lead features a
locking insulating sleeve that protects
and insulates the wire join we would
have to make.
Thus began the delicate process of
removing the old tube and replacing
it with the new tube. We started by
draining the cooling water circuit, us-
ing the cooling pump to empty it into a
bucket. We then opened up the hoses
near the laser tube and forced air in,
to get the rest of the water out.
The tube is held in place by clamps
at either end, with the glass tube pro-
tected by rubber pads which fully en-
circle it.
We cut the two supply wires (a red
anode wire and a black cathode wire)
near where they are terminated to the
old tube, giving some extra length for
Left: the old laser tube with the water cooling tubes and supply wires detached.
Middle: the glass tube is mounted in a saddle lined with rubber pads, which had to be carefully removed.
Right: the anode wire join for the new laser tube.