I have an old Heidenhain VRZ 731 2 axis DRO on my BP and the x axis
stopped working. So after determining that it was a sensor assembly
malfunction by switching the x and y sensors into the readout, I
proceed to open the assembly. The etched glass scale is intact so I
explore the actual sensor. It has some photo-cells a lens/collimator and
( I assume) some form of creating a light beam. I find Heidenhain's
website, and luckily a PDF of the manual for my unit. The only thing of
value in the manual is a pinout, a description of what voltages are to
be found on what pins. 2 of the pins are labeled lamp and carry a 5v DC
at 120 milliamps. So now I trace the circuitry and find a direct
connection to what looks like a grain of wheat incandescent lamp
positioned in a subframe assembly. I rule out a LED, because of the 5v
at 120 mA and no current limiting resistor in the circuit. So I start
making calls and finally get the Heidenhain tech in NY. I ask him if he
can repair my sensor. He says it's too old, not supported and I might be
well served with a newer model for $ 600.00 . I ask him if he knows what
the light is on my unit, he says it's a very special lamp, I ask him " a
5v grain of wheat microlamp?" He says it's a very special lamp and no,
they don't have a replacement, and I should buy the new sensor for 600$.
I thank him and hang up. So I find a 5v grain of wheat lamp in my local
electronics supply house, take apart the sub-assembly, remove the old
lamp taking care to measure where the original filaments where placed,
and superglue in my new lamp in the same position. A little bit of
soldering and pronto I power up the sensor, adjust the collimator so the
beam of light is perfectly centered on the photocells, replace the unit
into the scale and my x axis now works. A very special lamp indeed!

cheers
T.Alan