dear japangirl hing thanks for your info & suggestion!
i) the 3x forum did have some good info
ii) as u suggested, I just came back from trying to open the top cover. unfortunately, as the amp is so old and was not stored properly (it's been stored in open-space with very high humidity..... ), the screws on the top cover are already vey rusty. Therefore, i can't unscrew them without further damaging the screws. I understand it might need some force to do that but I afraid it may damage the screw surface totally. As I'm not very experienced in this, the only option left to me now is to take it to the expert~ can u help??
Is it safe to do so?
Coz i read some articles saying wd40 may lead to risk of current leakage if use in hi-fi coz of its non-vaporize & conductive nature... Is that correct?