Hi first of all , I have to appreciate you for advices on my system choice some times ago .
Now my audio system is :
Marantz cd5001 cd player Yamaha kx493 cassette deck Yamaha ax596 amp Magnat Altea 7 ( 3 way ) speakers
Not as good as many systems , but this is what I could purchase then . Yesterday I decided to see how my old Dual CV40 integrated amp compare to yamaha . The Dual has a very nice real wood finish and is so simple . It has 2x24 watts music power ( 2x18 watts continious power ) and Yamaha is rated as 2x100 watts cont. but surprisingly I realised that the dual still has a very nice and clean sound and its bass is very rich ( all compared to my --very short-- sound experience - some may find my opinions awful ) .
BUT : when I connected the Dual to the speakers and powered it on , I heared a loud POPPP sound and saw the woofers large displacement . Now my questions are: 1-Is it harmful to my speakers ( I guess it is ) 2-If yes , which part is defective and how can I repair it all replies are very appreciated .
There's no way to diagnose a problem over a forum and your description leaves too much room for guessing. The most likely issue with any amplifier which has sat in storage for years would be the filter caps in the power supply. These are parts which tend to age even when the component is not in use. Of course, there could be other parts which are experiencing issues and, if the ampifier continued to play, those parts would be suspect.
Can this damage your speakers? Yes. Is it likely to damage your speakers? If you continue to use the amp with these speakers, don't expect to have working speakers for long. Even if the defective part is rather minor, the chances of other problems existing in the amp are high. You'll need to find a repair shop that is willing to take on a decades old product, which could be difficult. First, most shops simply don't care to be tied down to a vintage component which might have multiple problems. Second, parts and service manuals for older components can be difficult, if not impossible, to locate. Unfortunately, there is a difference between a vintage component worth the effort to repair and a simply old unit which has lost its market appeal. Your problems will not be made any easier given the fact Dual as a company long ago ceased to exist.
Use a search engine to locate a service/repair shop that specializes in vintage audio repair. Some problems in units such as your's have rather generic solutions. If the "POP" was from a filter cap, it's possible the amp can be restored. Of, course, you would need to ask yourself whether repairing this particular amp is worth the money given the fact you'll be paying for a tech to do trouble shooting to hopefully locate the problem and eliminate the prospect of other issues causing the part to fail. Expect to pay somewhere between $100 and $200 to have this amp repaired.
Storage is generally a death sentence to an electronic component. There is no guarantee there aren't other age related problems which might occur a week or a month or six months after you have the current problem repaired. Old electronics can turn into a money pit rather easily. Very few mass market components are actually worth the effort.