Repair of the oscillator assembly in an Air-O-Swiss room humidifier model 7142

Our Air O Swiss room humidifier model 7142 was turning on, the front panel worked, air flowed, but did not give mist. I read Dennis Aanderud's post on fixya and that gave me a hint for further repair. I decided to open the unit and check the oscillator assembly to find out if it could be repaired. On inspecting the circuit in the oscillator, I found that one of the components in it, a transistor, was damaged by water leakage. I was able to successfully replace that component and the unit now works normally.

For all who wish to repair their units here are the steps I followed: 

Step 1. Remove the bottom : Remove the five screws (I think the type of screw the unit has are 'Double-square' with a post in the center of the head. You can use a small flat head screwdriver that will fit in and rotate the screw. This may break the center post, but you can still screw and unscrew without any problem).
After unscrewing, apply a slight force to remove the bottom plastic and remove it only slightly from the main body. There is a fan seated in the bottom part but wires are connected to the above portion. Now, try to remove the bottom slowly and grab the fan and place it on the side.

Step 2. Remove the oscillator assembly : The oscillator assembly is the part to which the transducer is attached. Gently  remove the wire connector and then the four screws and the oscillator assembly should be loose. In my case the plastic surrounding the transducer was cracked.

Step 3. Open the oscillator assembly : Remove the screw at the side and you should see the PCB. When I first saw the PCB in my unit there was water all over it (due to the filled water tank) and it had white powdery like coating on it in some places. Remove the transducer wire connector, then the two screws on the PCB, and the two screws which fix the aluminum heatsink to the case.  
To remove all the white dust on the PCB I put some rubbing alcohol on it and cleaned using an old toothbrush. Water entered the assembly due to the cracks in the plastic surrounding the transducer and created this dust. I covered the crack in plastic using silicon glue.

 Two major elements in this circuit which can get damaged are: [1] the ultrasonic transducer (attached to PCB by a connector) and [2] the transistor (The big black part having three leads and connected to the aluminum heat sink).

Step 3a. Check if the transistor is OK : The transistor part number is given on it's face. Mine had the letters and numbers SK C3835. I found out that the transistor is of NPN type by finding the datasheet on the Internet.

(A rectifier diode is an electronic component which has a P type semiconductor connected to a N type semiconductor. and it gives low resistance to current only when (1) P type is at higher voltage than N type by +0.7 volts and (2) current flows from P to N. The other way around it gives a very-very high resistance. An NPN transistor is an electronic component which has P type semiconductor sandwiched between two N type semiconductors. The center P type is called Base and other two terminals are called Emitter and Collector. These three terminals are marked on the PCB as 'B', 'E' and 'C' respectively. You may think of the connection between the 'B' and 'E' terminal as one diode and that between 'B' and 'C' as an another diode.)

I tried to find out if the transistor is damaged by checking the resistance between each pin both ways; The +ve connected to one of the pins and -ve connected to another one and then reversing this.
Keep the multimeter in the lowest ohms range and check the resistance between each of the three terminals of the transistor. You do not have to measure the resistance but just check if none of the readings give a small resistance value (like 0, 5, 20, 100 ohms, etc).
 
(With +ve lead connected to 'B' and -ve to 'E' you will not get a low resistance reading as about 0.7v must be applied before the P-N connection conducts. If the transistor is good, then I think the only way you might get a resistance value reading, with the +ve lead on the 'B' pin and -ve lead on either 'E' or 'C' pin, is if you put the ohmmeter on the Mohm scale; For all scales below Mohm it will be a 'beyond scale' reading. You may be able to use the diode checker if your meter has that functionality but I am not sure how it works when checking a transistor.)

I found that my transistor had gone bad as I got the resistance between the 'B'(Base) and 'E'(Emitter) pins around zero ohms.
I found that ebay and some other websites were selling a Sanken 2SC3835 transistor for humidifiers. So, I searched for this part on electronic component supplier Digikey.com and luckily it was available for $2.73 plus USPS first class shipping. This component had similar lettering and numbering as the original part. The datasheet of this transistor also mentions that it can be used for humidifiers. The link to Digikey page for this part is http://www.digikey.com/product-detail/en/2SC3835/2SC3835-ND/3929392
   
Step 3b. Check if the transducer is OK : I had two of these units. One was working. So, I tried to find out if transducer had gone bad by swapping it in the oscillator unit of the working one; and it was working. I also tried the transducer of the working unit in the damaged oscillator unit and this did not work. If your transducer is damaged you can buy one from some Korean websites. From the information (pictures) given on these websites it looks like the unit they sell would compatible. Please verify before you purchase. Here is the info:

1. http://www.lattron.com/eng/index.php
I think this company makes compatible transducers, but I am not sure.

2. http://itempage3.auction.co.kr/detailview.aspx?itemno=a591832155
This one sells similar looking transducer. The oscillator in one of the picture looks the same as in Air-O-Swiss model 7142. The site is in Korean. You may have to use a 'Buy from Korea service' like http://sasaeyokorea.com/ to purchase from here.

3. http://item2.gmarket.co.kr/English/detailview/item.aspx?goodscode=249277352
This one also seems to sell compatible transducer and looks like they ship to USA. Please verify before you purchase.
 
Step 3c. Check if other components are good : You can check if other elements have gone bad by using a multimeter to check for the resistance(and capacitance) values, if fuse is OK and if there is no break in connection between elements. See the connection diagram I made for the oscillator unit for the resistance values. You can find the resistance values using a resistance color code chart.

Step 4. Replace transistor : I used a de-soldering iron to remove the damaged transistor from the PCB and removed the transistor from the aluminum heatsink. In low humidity there is danger of semiconductor components getting damaged due to static buildup, so to 'ground' myself I touched the screw on the electrical wall plate before removing the transistor from its anti-static package. I did this a few more times till the new transistor was soldered to the PCB. I applied little thermal paste to its metal back and spread it in a thin layer such that it covers the entire surface (Every metal surface has tiny hills and valleys on it's surface and thermal paste is used to cover these for better heat transfer by lowering the thermal resistance between the two surfaces by filling the tiny air gaps. I used a thermal paste that is used for attaching microprocessors to their heat sink's in computers. You can buy similar or look on ebay.) Screw the transistor to the heat sink and take care that the edges are parallel to the edges of the heat sink just like the original part was attached. Using a small plier bend carefully each of the three pins from a point on the lead and to an angle just like that on the old one. Insert them in the PCB and keep leads that come out on the solder side just the same length as in the old part. Solder the leads.

Step 5. Cover PCB in plastic : I covered the PCB in plastic 'Cling' food wrap to prevent water damage and covered the transistor leads in a small length insulation tape. Make sure water does not have a path to enter and remain there. If this is unavoidable then make a hole in the plastic wrapper at a suitable spot for drainage.

Step 6. Put everything else back together : The fan has a seat in the base and also in the upper part such that both hold the fan when the unit is put back together. You may bring the unit close to the base and put the fan in it's seat in the base and join OR you can put the fan it it's seat in upper unit and then attach the base being careful that the fan does not fall out of the seat.

After all above machine was working as before.