The VP2 SuperCap Fault
The supercap fault has been widely discussed in various online WX forums. The stations at most risk of the fault were those made in the first few months of the VP2 production (beginning Dec 2004). The problem is not that the supercaps are short-lived (should have a useful life of 10+ years) but is that they seem to be very sensitise to undue leverage on the leads, which can cause internal damage. This may have happened when the components were put on the circuit boards during manufacture and not taking care in the handling of the supercaps and some damage might have been caused.

The faulty supercap only starts to have noticeable effects after the board is in use 1+ years. There is no way that testing at the time of manufacture would have picked anything up. It was only in 2007/2008 that the issue of the faulty supercap come to light.

Once Davis knew about the fault they took steps to modify the board assembly, including gluing the supercap to the board to stop vibration damage. The VP2 stations made from July/August 2007 onwards should be less likely to develop the fault. It is likely that unmodified stations of 2005/2006 issue will continue to present with the supercap fault.

Diagnosing The Fault
If you suspect fault, then you will need to carefully inspect the transmitter's SIM circut board. To do this, you will need to remove the inner cover from the circuit board (cover held in place by 4 screws). Removing the 4 screws will allow the board to become loose, but it should still be in place and retained in by the wireless antenna. Look closely at the long cylindrical component (black in this image but exact colour may vary) to the right of this area is the supercap. Check the supercap leads (these should look clean and bright). But if you can see some exudate apparently leaking from the supercap body out on to one of the leads (usually the anode in fact) then this is a clear sign that the supercap is in trouble.

DIY SuperCap Repairs
The supercap fault in one of the issue on the VP2 system that can to fixed by replacing the supercap. Normally this would be done by Davis Service Centre. But the repair can be done by yourself. They are some key points to be aware of:

The 10F 2.7v supercap is not widely available and you may have issues trying to get one from you local electronic supplier. The official Davis spare part (7370.093) might also be availdable from certain suppliers. Being in New Zealand I found a supplier with a 10F supercap, but at 25v and not the 2.7v that is required. I ended up using a 10000uf (10 Farad) 25v one

You need to becareful handling the supercap. Never bend the leads from the point they join the supercap base. Instead make the bends away from the supercap body by using two sets of pliers. (One holding the lead between the body of the supercap and the other to make the bends in the leads). Failure to observe this precaution will cause the same cycle of latent damage to the supercap failure.

The supercap fault question was put to Davis and got this informative response.

We don't recommend customers install supercaps themselve. We prefer to repair it. In theory it should be OK (but we can not guarantee it). There are considerations.

Special care needs to be taken when working with these parts. Improper handling can lead to capacitor damage. When forming the 90 degree bend in the leads, special care needs to be taken to assure no stress travels through the rubber seal on the bottom of the part into the part, as this can damage the seal. This is typically done by holding the part with narrow-tipped pliers near the rubber seal, and bending the leads against the pliers. When installing, special care needs to be taken to (again) assure no mechanical stress travels into the part. Similarly, solder time needs to be minimized to keep thermal stresses from the part.

The part also needs to be affixed to keep mechanical shock from producing stresses between the part and the leads that are soldered to the board. This can be done with double-sided foam tape, silicone gel, or similar affixing.

Aside from this, one or two super-capacitors can be installed. Also, several values can be installed. The night run-time is proportional to capacitance. The reason for our selection was need/cost. The boards was designed to take 10F, 25F or 50F parts, up to two each. They can be mixed/matched in any combination.