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WMR928N Repairs

Discussion specific to Oregon Scientific weather stations
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Posts: 51
Joined: Tue 03 Jan 2017 8:12 pm
Weather Station: OS WMR928N
Operating System: OPi Armbian 5.75 Ubuntu Bionic

WMR928N Repairs

Post by SunDial »


As most WMR928[N,X] weather stations are somewhere in the region of 20+years-old now I thought I would start a thread that suggests ways to repair failed or failing parts of this weather station. If you would like to comment or have suggestions of your own please feel free to reply to this thread.

Here are a few things that I have repaired on my WMR928N:
  • The indoor thermo/baro/hydro (BTHR918N) meter's original humidity sensor deteriorates within a few years causing low humidity readings
After a very long time trying to source a compatible sensor to replace the original one (HR202L, HDS10, HCZ-H8, HCZ-J3A, etc did not produce the correct humidity reading even after changing the dependent 1% resistor for the sensor), I found one in a cheap temp/hydro meter available on Ebay and AliExpress that is the same spec as the original and more reliable - it has not degraded at all in the two-plus years I have had it installed. It can currently be found on Ebay by searching for "Mini Digital LCD Thermometer Hygrometer Meter" - just look for the porthole/round type in your search results. Unfortunately this cheap temp/hydro meter is unbranded so I have been unable to discover the part number of the actual sensor. It is, therefore, a bit of a gamble to assume all of this type of thermo/hydro meter includes the same humidity sensor type, but the meter is cheap so you may feel it is worth a punt to buy one. Once you have the new thermo/hydro meter, (with the batteries removed from both sensors) de-solder its humidity sensor (square white wafer with interleaved lines on one side), de-solder the faulty humidity sensor in your BTHR918 then solder the new one to the pads where the faulty sensor was. You may need to adjust the trimmer in your BTHR918N to fine tune the humidity reading to a known accurate humidity sensor source.
  • The STR928 and/or STR938 solar panel housing comes apart due to the plastic screw pillars failing, allowing moisture ingress which often leads to the transmitter failing.
This requires quite a bit of work but is a permanent fix if you are willing to put in the effort. The plastic screw pillars are very prone to splitting and cracking away from the main housing - this is what causes the transmitter housing pieces to come apart allowing moisture in. The best thing I have found is to knock off all of the plastic pillars and replace them with M2 x 9mm brass hex pillars. The pillar craters left in the housing plastic helpfully mark where you need to drill your 2mm holes for 6mm long stainless steel machine screws to go through into the brass pillars. You can order M2 x 9mm long brass pillars and M2 x 6mm stainless steel screws quite easily from sellers on Ebay. You need to then carefully prize the solar panel away from the centre housing enough to access the back of it (CH piece) to insert the pillar screws, being very careful not to break the solar panel wires. DO NOT pull the solar panel wires completely through the hole or you will not be able to pull the wires back when you finish inserting all the pillar screws (x8). You may need to carefully break away the glue surrounding the solar panel wires to free it a little but be extremely careful doing this as the wire is very thin and weak. I then used a little bathroom sealant around the solar panel wire to reseal it and also around the screw holes before screwing the pillars to the plastic case to ensure all were watertight. Once the pillars are in place and you have repaired/replaced any other components, wiring, etc in the BTHR918N, paste a little silicon gel around where the rubber ring lies in the groove and you can then screw the battery part of the case to the solar panel side. The BTHR918N should now be better than new in terms of its structural strength and stay securely closed to prevent moisture getting into the electronics in the future,
  • The STR928 and/or STR938 no longer transmits due to moisture getting into the electronics and corroding wiring and/or components.
As the cause(s) of this fault can vary I will provide some general pointers to help you track down the fault. First check all wiring and battery contacts for corrosion. Replace and/or clean where necessary to restore electrical continuity between the battery, solar panel, LED and the printed circuit board. Once that is done, if you find that the transmitter is still not working, with a magnifying glass or magnifying eyepiece examine the printed circuit board and its components for any signs of corrosion. Pay special attention to the small inductor coils that are inline with the antenna. Perform continuity tests with your DMM on each of them and check if any of the circuit board's tracks has corroded away. If so, repair the tracks with small pieces of wire.

If you are not experienced or confident with electronics repair the next suggestion may be something better left to an expert, if you can find one to help you. If an inductor has failed these can be replaced, but you will need to work out the values of any that have gone open circuit. Under your magnifier, count the number of turns of the inductor and complete the details in the form here: https://www.translatorscafe.com/unit-co ... nductance/, or here for more a more detailed inductance calculation: https://hamwaves.com/inductance/en/index.html#input, estimating the other form details as best you can based on measurements you can make manually or just estimate. Of course the result may not exactly match the original inductor value, but it should be close if you have the parameters set well in the form. RS Components stocked the inductor I needed to replace in my STR928 and it works well. Some experimentation may be needed to complete the repair should you find the value you use for your replacement does not restore the transmitter to working order.
  • The cover on which the adjustable arm is attached to the STR928/STR938 allows moisture into the solar transmitter electronics/battery compartment.
The plastic rubberised seal around the cover becomes less flexible over time and eventually fails to create a watertight seal when screwed to the back of the sensor transmitter. I use a line of bathroom sealant around the the seal before screwing the back onto the sensor. This provides a watertight seal but can be easily broken for when batteries need to be replaced or to carry out a repair.

Only attempt repairs on the sensors if you are confident you can do so and have some experience of carrying out DIY repairs on similar projects. These repair suggestions are provided in good faith to anyone in the knowledge that if they fail to repair whatever it is that is faulty they were happy to have taken on the challenge, come what may. In other words, don't blame me if your repair attempt makes things worse! :)

That's all I have for now.

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