Forum Ländlestrahler

On the weekend I was clearing out some old storage in my father-in-law’s garage and came across a Denon PMA-920 amplifier. It looks seriously impressive — a heavy, old-school Japanese unit. I tried powering it on: the lights come on, the relay clicks, but the sound from the speakers is very muffled, kind of like it’s coming through a barrel. After about five minutes of operation, there was also a faint smell of burnt dust. I really want to restore it and use it in my office, but I’m afraid of blowing the output transistors if I keep running it in this condition. Does anyone happen to have a detailed service manual with factory test points and a proper disassembly guide, so I can clean it properly and check the main power rails without probing blindly?

And one more thing, could this muffled sound be caused by dried-out coupling capacitors in the preamp stage, or should I first be looking at the output stage and bias settings? I’m still only at a basic level when it comes to this kind of circuit design.

The burnt dust smell is nothing to worry about, really. It's quite likely just some dust build up on the heatsinks. But that muffling sound might be a bit more serious issue. The inputs selectors might have oxidized or those old capacitors you've mentioned. Have you connected it to some good speaker set already? So, about that service manual for Denon PMA 920 amplifier... there's a website called ManualMachine. Besides the voltage tables, it also contains a detailed procedure to set the bias current of the unit so it doesn't get hot. First thing I'd do - is clean all the switches and buttons thoroughly with a proper contact cleaner. In most cases, around 90%, this alone solves the problem and the sound is restored to normal. And regarding the capacitors of this model, they are indeed quite good ones, so don't get carried away and change all of them just for the sake of it. If you have possibility, use an ESR meter to test them before you decide to discard any.