In this category this post was preceded by one on adapting miniature screw bases to LEDs
I was recently given one of these valuable units in pieces, as a reward from a kind person for restoring two others to a near-new and working condition. Happily, all the pieces were there except for a working bulb. The preceding post in this series explains how to replace the old incandescent bulb with an LED, using the base of the defunct bulb. Note that the central contact of the base must be connected to the negative of the LED, while in the scale lighting of most sextants, the central contact of the lamp is positive, a matter of no importance in an ordinary bulb, but an LED will not work if connected with the wrong polarity (though doing so will not damage it). An LED also consumes a great deal less current than an ordinary incandescent bulb, so the potentiometer (variable resistor, rheostat) of 10 ohms must be replaced by one of much higher value. This post explains how to proceed.
First, remove the cover by undoing the four screws at each corner (Figure 1)
This reveals the wiring to the potentiometer (pot) which must now be carefully unsoldered (Figure 2).
The knob may now be removed (2 grub screws) to reveal a large hexagonal nut that secures the pot. Undo the nut using a suitable box wrench if you have one. Otherwise it may yield to a pair of pliers, but try not to scratch the case. The new pot can now be put into place (Figure 3). Its spindle may be too long, even after you have inserted spacing washers between the pot and the wall of the case, so you may have to saw it off to length. There is plenty of room for the 16 mm diameter body of the pot.
The value of the pot’s resistance will depend on what use you plan to make of it. If you are only ever going to take sun sights, 1000 ohms (linear, not log) seems to be just right, but for star sights, even turned right down, the light seems still too bright and may wash out even bright stars. Three thousand ohms certainly gives a good range of brightness, but at the cost of reduced sensitivity. I compromised by using a 1000 ohm linear potentiometer and made an adapter to insert between the plug of the bubble unit and the socket of the sextant (Figure 4). The adapter contains a resistor of 1500 ohms to drop the voltage for star sights while retaining the sensitivity of the 1000 ohm pot.
Figure 5 gives a drawing to construct the adapter. Like all figures on this site, it may be enlarged by clicking on it, using the back arrow to return to the text. The 0.5 mm holes are first filled with solder and then the wires of the resistor are inserted and held in place while the solder is still molten. A mounted crocodile clip helps to avoid burned fingers. The plug has sufficient mass to remain hot for quite a few seconds. I forgot this and let go of the still-hot piece with such force that it flew across the workshop to be lost in some dark recess, and I had to make a new one.
The adapter may be left as it is or neatly enclosed in a piece of insulating sleeving (Figure 6), heat shrink for preference.