NAD T753 Amp/receiver problem

[micden]

After being in storage for a while I hooked up my Nad T753 amp/receiver and encountered a problem. When I connected a speaker (wharfedale 8.3) as a test the mid range cone was sucked in violently (obviously magnetic) and released when I powered off. When I tried the system with headphones it blew the headphones ( fortunately my cheap set ). I am assuming the problem relates to both channels but I am not sure how to test without using a speaker. Just wondering if these symptoms are enough for someone to give me a heads up on the problem.....cheers

[Reschs]

DC on the output.
Check for a blown DC Rail fuse.

[Uncle Fester]

Strange that the protection circuit does not switch off the speakers with DC on the output.
Fuses these days usually on mains 240V side of the powersupply, not on the amplifier main power rails due to high surge currents for peak power demands.

The power supply itself could be faulty, possibly a rectifer diode open circuit, which you can locate with a multimeter.

Edit: Maybe the faulty protection circuit is a hint. It might be supplied with a voltage regulator that is faulty or has a faulty capacitor connected to it and that same voltage could be involved in the driver stages of the amp, pulling them to one side of the DC supply. Unfortunately I couldn't find a free schematic to download from my usual souces.

[micden]

Thanks guys..I just thought I would put it out there.It is a fairly complicated amp and I have very little electronics. Schematic is here and means zippo to me. I am following the posts with interest.

[TVguy]

Thanks guys..I just thought I would put it out there.It is a fairly complicated amp and I have very little electronics... and means zippo to me. I am following the posts with interest.

Quite extraordinary for a NAD receiver to impress DC on the outputs! The output stages are all relay protected and should not be energised when a catastrophic fault occurs.
If you have a multimeter, check the speaker output terminals for presence of DC voltage.
Do you hear the protection relays click? DC on speaker terminals is the stuff of cheap amps without protection. The circuit diagram clearly shows a belt and braces approach to voltage and current protection.
A failure such as this would point toward a system control problem; however unlikely that may be.

[Reschs]

Where are you located ?
If in Sydney or surrounds I can help you.

Otherwise, do you have access to a voltmeter.