Having done this twice in the last 8 years and not found detail on how to do it, I thought it might be of interest. New anemometer c?$170 v reed switch a few pence/cents. Do check you have a reed switch fault (permanently closed or open) before embarking on this.
Order some reed switches before you start. They are of the normally open type. I use either REED SWITCH, 20W - KSK-1A35-1520 part SW02761 from CPC or part SW923 from Brimal in UK. They're both 220V 20W and only max 10.5mm. The length is critical as the gap in the 'jaws' of the PCB is very small. Reed switches are very fragile but, fortunately, also very cheap. So order 10 in case you break a few. Have a read of this to better understand the challenge you face:
https://standexelectronics.com/wp-content/uploads/Precautions_for_Reed_Switch1.pdfJust in case, I also replaced the tiny 47Ω chip resistors with metal film ones.
You'll need pliers, screwdrivers, soldering iron with small tip, solder, craft knife, an ohmmeter, a magnet, a compass and shrink wrap. A jeweller's eyeglass or a magnifying glass might help too.
I have some images of the process but will try and describe it such that you don't need them. In any case
http://www.lexingtonwx.com/anemometer/ has some good ones.
1/ Remove anemometer from whatever pole it is on.
2/ Carefully detach the cable from the pole attaching plastic semi-circle by wiggling it carefully free of the lugs.
3/ Using the appropriate Allen (hex) key, remove both the vane and the cups from the anemometer module.
4/ Remove the small screw which attaches the anemometer module to the upper part of the bent aluminium pole.
5/ Carefully pull the module away from the pole at the same time feeding the freed cable into the lower end to allow this to happen.
6/ Remove the nut and castellated washer from the vane end of the module. This will leave the vane potentiometer threaded shaft protruding through the centre of a small plastic disc recessed into the module. This disc needs to be removed and is held in place by a having had a few dabs of heat applied to its perimeter to melt it and thus 'glue' it to the module body.
7/ Using a small bladed craft knife gently cut around the perimeter of the plastic disc inside the module to release it from the module body.
8/ Gently tap or tease the plastic disc from the module body. Pull the Davis vane potentiometer from the module feeding cable into the module side as required.
9/ The reed switch is soldered onto a PCB. When you look into the cups end of the module you will see 4 legs of a plastic 'cross' and between two of the legs is the reed switch. The PCB is shaped like an upper case T. The reed switch sits across the top of the T. But it is inset in a recess in the top of the T which is about 11-12mm wide. On manufacture, the PCB was inserted tail first into the module until all that was left was the reed switch between two legs of the black plastic cross. Thus the tail of the T is out of sight in the module body. The plastic cross is NOT removable but the PCB is. It is only held in place by friction. So using a long thin screwdriver inserted in the vane end of the module aiming towards the gap between the appropriate two legs (the two where the reed switch sits) you can press on the end of the tail of the PCB and, surprise, surprise, the PCB with reed switch attached will simply slide out of the cups end of the module. If there isn't enough slack wire cut it. You can always join some later.
10/ Now for the real fun. Firstly, I unsoldered and removed the reed switch. Then I unsoldered the wires which connected the PCB to the vane potentiometer noting which was red and which was black.Then I dabbed solder on to the chip resistors to short them out (you can miss this if you are not going to replace them). I then soldered a 47Ω metal film resistor to each of the PCB tail end contacts and re-soldered the red and black wires to the other end of the appropriate resistor. I used shrink wrap to insulate the joins. Then, following the advice in the pdf above, I very gently soldered a new reed switch into the recess in the top of the PCB T. The orientation may be important and you can check this both visually and with a magnet and an ohmmeter.
11/ When you think you've succeeded without damaging the new reed switch (the glass envelope is the issue) , put an ohmmeter across the red and black wires and, moving a magnet to and from the end of the PCB T check the reed switch is working. It should be normally open (∞Ω [max] with no magnet and closed (0Ω) when the magnet approaches.
12/ Ever so gently insert the resistors and PCB back into position and reverse the disassembly. When you put the plastic disc back into the vane end recess, gently dab your soldering iron to re-melt it to the sides in a couple of spots. You don't have to be over zealous because: a) you might need to do this again and b) it is sits positioned down when working and c) the vane potentiometer is quite a nice friction fit and unlikely to fall out vertically upwards.
13/ When you re-attach the unit to the pole you will have to re-orientate it with a compass.
Voil?, anemometer fixed!