Speakers started buzzing.

Probably something else.
The buzzing is typical of an open circuit input "receiving" radiated 50/60 Hz from nearby mains wiring.
An alternative source of buzzing is the 50/60 Hz mains breaking through into the audio due to failed smoothing and filtering in the power supply.

Capacitors that have been unused for quite a while can lose condition and fail when put into use again and this might have been the cause of the thump/thud/boom you noted.

I doubt it is the cause but it is very unwise to operate an amplifier without a full complement of speakers unless the volume is kept at minimum...

The EC16B24304 number above is a discontinued Alps part. I found an equivalent encoder from Taiwan Alpha available on Mouser.com, part number RE160F-40E3-20A-24P. It has the same footprint, shaft length and keying. It fits perfectly. It has a lower rotational force than the Alps encoder and 24 detents per rotation instead of a smooth feel, but it is perfectly acceptable. I just made this repair on my Aiwa CX-NA222 stereo, and I've got my volume control back!

Register and download the manual for free at retrevo.com

http://www.retrevo.com/support/Aiwa-AV-DV75-Receivers-manual/id/1076dj797/t/2/

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We can't make assumptions that all is connected and selected properly.

The unit is going into protection mode. There is a fault detected in either the power supply or an overload on the output circuitry. Remove all connections to the amp (ie speakers, dvd players etc etc) Turn the unit on. If it still turns off the problem is internal and should be looked at by a qualified technician.

Try making sure tape monitor is in off position. Also I assume front speaker button is engaged. Another option is that analog setting needs to be changed to digital if digital output

There shouldnt be a problem with that at all. The subwoofer needs to be self powered. The Yamaha has a "Subwoofer Output" on the back. Just connect it there.

U can use the system, but u will need a separate sub woofer amplifier if your amp doesn't have one.

There may be a short circuit in the speaker terminals.

Disconnect all speakers and try it. If it works troubleshoot each speaker one at a time. Shorted wires are indicated.

This Protection activates if you hook up Unmatched Speakers i.e less or over impedance as required by Aiwa Amp. 2nd reason can be extra speakers attachment.

I think it's a grounding problem. Actually, it's a necessity for all audio-video equipments to get grounded properly. If you don't have a ground connection in your AC plug, your receiver will collect more and more static electricity. As a matter of fact, it decreases the sound quality that you can get from your equipment. I think that the effect of your reaction (slapping the receiver) isn't coincidental. When you touch any metal surface on your receiver (even while slapping) you condact the static electricity to the ground. I've had a same sort of problem with my receiver. Especially when I connected my laptop computer to the receiver, I heard an annoying distorted sound from the speakers. I found out that it was ground loop, which occurs when the negative (common) signal of the source coincides with the one of the receiver's. As a solution, I grounded my amp, changed all wires with new and shielded ones, and purchased an AC filter. Then, I got rid of the noise. I have to say that the AC filter (Belkin Pure AV Surge Protector, it also includes an AC signal filter)benefited me the most.