Hiya Liwei
I just had a look at your radio on your
website, and I must say it looks in fine condition!
I had a look at the service manual for the IC-740 and I see that the fan is in two halves, the propeller ("Fan Propeller(A)42081"), and the motor ("DC Motor M6B 12U22").
Unless the propeller has sustained damage, it is unlikely that it is the source of the noise, though sometimes fan blades can get covered with thick dust layer that can cause 'wobble'. The service manual appears to show a grub screw holding the fan propeller in place on the motor shaft, so just remove the propeller and wash it in warm soapy water.
The more likely cause of the noise is the motor itself. Pretty well all of these type of motor use phosphor bronze 'bearings' which are designed to self-lubricate. After many years of use the bearings get covered in a layer of hardened dust/dirt, and instead of the motor shaft running smoothly it tends to stick and race around the inside of the bearing, very often making a 'screaming' noise.
Also, these motors are very often pressed together, so they cannot always be disassembled to repair or clean them. If you cannot get the motor apart, fill up a small cup (or similar) with a suitable degreasing/lubricating agent (maybe WD40 or some similar penetrating oil), and immerse the motor in it, making sure that the motor fills with the solution, and leave it to soak overnight.
Next day, drain out the excess solution, and rinse the motor out with a degreaser such as
Isopropyl alcohol (auto stores sell a spray solution/solvent for degreasing auto-mobile brake drums and disks). You could leave the light oil in the motor, but if it does have phosphor bronze bearing rings then they work best dry, not oiled.
Only use alcohol based degreasers, since more potent solvents such as cellulose based ones might damage the insulation on the motor windings.
After a final drain out of any lubes/solvents, leave the motor somewhere warm so as to evaporate off any remaining 'moisture', and then reassemble and test.
If there is a good deal of side-play or wobble on the motor shaft then normally no amount of cleaning will help, and you will need to replace the motor (part number given above).
You could also try looking around for a cassette player motor of a similar size, or any one of a number of other scrap motors may work well.
If you don't mind spoiling the look of the radio a little, you could remove the old fan completely and then fit an
external fan on to the back of the radio case. Computer case fans or CPU fans are very good alternatives, and very quiet normally.
73, Mark.
