I recently got the chance to prove those assumptions false. I volunteer in the National Parks, doing astronomy volunteer work. Chaco Culture National Historical Park had a broken Telrad, and our choice was to repair or throw away the Telrad.
At the park, I didn't have a multimeter or soldering iron, but I did have some wired alligator clips. I jumpered the wires from the battery holder directly to the LED, and it didn't light. OK, must be a bad LED. I took the Telrad home, planning to solder in a new LED.
Tools needed: multimeter, soldering iron, solder, small Phillips screwdriver.
Slide the top off the Telrad. Use a small Phillips screwdriver to remove the rear cover plate (left in above photo).
You remove the old LED by scraping away the black rubbery insulating material and desoldering the white wires at the front of this small cubical black housing (right in the photo), Peel off the reticle (mine fell off when I poked it with a screwdriver, but you may need to use tweezers) at the rear (left in photo) of the black cube, and use the eraser end of a pencil to push the LED from the rear to the front of the Telrad. Above photo shows a dental mirror reflecting the image of the LED upward for the photograph. You'd insert your pencil, eraser end first into the hole left from removal of the rear cover plate. It would come in from the left of the photo, and push the desoldered LED to the right (front of Telrad). I bought a 2.6 V red LED to replace the original, but when I tested the old one, it worked. Back to square one.
