I had asked a few servicemen and all I got was a short "...there's an internal thermal fuse...". I trawled the web for months but just could not find any info on where the fuse was and how to replace it. Finally I decided to sacrifice one of the fans and dissected all the wires going into the windings.
From this came the schematics of the fan, but only up to the motor windings.
This instructable comes in 2 parts: (I)Troubleshooting and (II)Fuse replacement.
Step 1: Materials Required
Flat tip screw driver
Scissors or Pen knife
Soldering Iron and some Solder
Some string about 0.5mm in diameter
Spare wire, about 60cm long
Capacitor 1.4uF 450VAC (if required)
Thermal Fuse 130degree C, 1A, 250Vac
(note: I could only find a 2A 130deg replacement @$1.50SGD a piece from the local electronic component shops. Sorry I don't have equivalent part number from Mouser, RS n Digikey)
Step 2: Troubleshooting
If you are NOT trained in electrical repair, please STOP here.
It would be wise to spend the money for a replacement fan and give the faulty one to someone who knows. We are dealing with 250VAC, so be extra careful. You proceed at your own risks.
The Schematics :
At this juncture, I assume you know and have verified that the timer and speed selector are working fine (i.e. voltages are present at output of the speed selector (grey, orange and red wires)), see schematics and picture.
The colors of the wiring used in my fan may be different from yours, so make a note.
Troubleshooting the Thermal Fuse :
1. Disconnect power to the fan.
2. Open up the control unit cover.
3. Take your time to trace the wiring and note down its color and location. It should be similar to the schematics above.
4. Using the spare wire, connect from point “A” to “B” to shunt the fuse. For my case, “A” was the blue “neutral” wire on the terminal block and “B”, the grey wire on the big squarish capacitor.
Once done, make sure your fan (without the blades of course) is resting properly and securely as we are going to power it up.
5. Connect and power up the fan motor. If the fuse is faulty, the motor will rotate, mine did. If it did not, it might also be that point “C” was used instead of “B”, since you could not know which terminal it was. No worry.
6. Disconnect power. Move point “C” to “B” and power up the fan again. If it still would not rotate, it’s likely the capacitor.
7. Disconnect power and remove the spare wire.
Replacing the Capacitor:
1. Disconnect power.
2. De-solder (or cut) the wires at capacitor terminals and connect them to a new capacitor (same type).
3. Power up the fan. If the motor still refuses to rotate then I believe the problem is with the motor windings and it's time to say bye-bye to the fan.
Step 3: Thermal Fuse Replacement
Removing the Thermal Fuse:
Disconnect power to the fan before proceeding.
1. To make fuse replacement easier, we need to free the motor from the control unit keeping its wiring harness as long as possible. The harness has 5 wires going into the motor housing. By tracing this harness, starting from the motor housing, I found the Ground (green/yellow) and Neutral (blue) wires ending at a terminal block and another three (grey, orange, red) ending at the speed selector.
2. Remove the Ground and Neutral wire (no cutting required) from the terminal block. De-solder the grey, orange and red wires at the speed selector, note their location first. (You may have noticed that I cut the whole harness about 10cm from the motor…big mistake...but that was way before I decided to do this!!)
3. Using a flat tip screw driver as a lever, pry the motor housing mount rod away from the control unit.
4. Remove the oscillating unit and detach the capacitor.
5. Remove the metal housing screws. Carefully remove the rotor and the windings slowly by giving it enough slack from the harness.
6. The windings are tied by strings and coated with some kind of lacquer. We need to cut some of them (see the “X” points in the picture) using pen-knife or sharp tip scissors. Look for a bulge (where the fuse is) and notice where it ends.
7. Carefully slice open the black sheath cover to expose the brown woven sheaths, also tied with strings.
8. Trace the blue Neutral wire from the harness towards the windings. You will notice it ended inside the biggest sheath which is on top. This is our target. Cut the strings tying this sheath.
9. This big sheath, which is flattened, holds the fuse and another sheathed wire. Use fingers to lift it and press on its sides to form back its tubular shape.
10. With wriggle, press and pull action, slowly remove this sheath and the thermal fuse will be exposed. Put the removed sheath aside to be reused later.
11. Notice that one of the fuse’s leads is also sheathed to prevent electrical contact. Slice the SIDE of this sheath lengthwise about 10mm. Expose the lead through this cut.
12. With both leads exposed, cut them off as close to the fuse as possible. This will give us enough room to solder the new fuse.
13. Trim the leads of the new fuse to about 10mm and tin them with solder.
14. Solder the new fuse, polarity is not important but you may follow the old fuse direction.
15. Move the cut sleeve to cover back the lead exposed in step #11above.
16. Cover the fuse and its accompanying wire with the big sheath removed in step #10 above.
17. String the new fuse assembly to the windings as neat as you can. Fold back the black cover and string again. Ensure no string or black cover sheath is protruding into the rotor area.
18. Carefully place back the windings and rotor into the motor housing.
19. Secure the housing and mount back the oscillator and the capacitor.
20. Insert the motor housing mount rod to the control unit, a little pressure is needed here.
21. Dress back the wiring harness and reconnect the Ground and Neutral wires to the terminal block. (For me, I had to solder back all the wires and insulate them with heat shrinking tubes. I also had to de-activate the oscillator to prevent them from rubbing against each other)
23. Re-solder the grey, orange and red wires back to the speed selector.
24. Power on the fan to test. Cover back the control unit and you are done.
Step 4: Finally...
To prevent overheating, I modified the timer to disable the permanent "ON" selection. By using the timing mechanism there will be resting time in between, when no one is in the room or no intention to use the fan for a while.