I've just registered on the forum as I read quite a few posts on the W-8681 / WH1080 about various faults, so decided to make a short write up on the fix I did to my unit.
The problem, as has been found in the past by other users is that the external humidity sticks at 99%. I removed the sensor from my mast mounted transmitter to check it out. As was noted in another post, its a bog standard capacitance to frequency converter using a 555 timer. The other IC on the sensor board is a serial eeprom, which I am presuming contains the calibration data which is mentioned in the humidity sensor data sheet. I tested the actual sensor element with a capacitance meter, and found that it was reading 1 or 2pF, interestingly it was not a dead short, but some sort of failure, the nominal reading should be about 180pF.
The solution in my case was to open the display unit, remove the humidity sensor PCB and simply resolder this into the mast-mounted transmitter - this resulted in the external humidity reading now being correct. Initially I did not re-fit the faulty sensor inside the display unit -this turned out to be an error. Although the external temp/humidity readings were fine, neither wind speed or rain rate was measured, although wind direction was OK. I can only assume that the eeprom on board the humidity sensor PCB also contains some calibration data for wind / rain sensors - refitting the board fixed these two readings. A 200pF capacitor combination was fitted in place of the faulty humidity sensor and the internal reading how shows 99%.
I noticed that the external mast-mounted transmitter PCB has very poor soldering in general. There also seems to be a incomplete covering of conformal coating on the PCB - both of these issues were fixed. The 1n4007 SMT diode was replaced as it appeared that dampness had caused damage to one terminal. I tried initially bypassing the diode, with a new set of AA batteries, the TX light comes on and there is no modulation at all of the data. Adding the diode fixed the issue. I also modified the plastic box by drilling a few more holes for air flow, and the unit does indeed seem more responsive.
When I get some more time, I'll take out the eeprom, dump out the data and see if the calibration curve can be determined - I will inject a low-level square wave into the indoor LCD display unit to work out the various frequencies needed for levels of humidity display, once this is known, it should be possible to create a more reliable sensor setup.
Thats all for now, I'll report back once I've tested the system for a week or two.