NHT SA3 Help - Strange sound on power off

[Mofi]

Hi all,

I have 4 NHT SA3 subwoofer amplifiers. The only thing is.... only one of them works as I think it should. Another doesn't work at all (yes the fuse is good). The remaining 2 have a unique and somewhat disturbing quirk. A local friend and hard core audio restoration (EE) guy recommends I recap them and see what happens. Looks like 72 caps and probably half of them are small ceramics that won't be replaced.

Here's what's going on. Since these are sub-amps there's a crossover network with variable controls. Main line from preamp IN, then output to "satellite" amp and power to subwoofer. On turn off, I get a weird "fart" from the satellite speakers about 10 seconds AFTER the sub-amp is turned off. It's fairly loud, same volume regardless of volume settings. At first I thought this was coming thru the subwoofer but it's feeding back thru the satellite amp/speakers (Acurus A150) whether it's turned off or not ! ! !

I substituted the power section of an NAD receiver for the satellite amp and got the same thing IF the receiver was turned on. If the receiver was off... no fart from the satellite speakers. Right off it's sounding like a capacitor(s) discharging. But who knows. Bad relay?

So, I'm asking if anyone on the boards has any experience with the NHT SA3 amps or a similar issue with signal feeding back into an amp on turn off. I do have a schematic for the SA3 and it can be found at this link:

http://cdn.growassets.net/user_files/nht/downloads/000/001/912/media0209.pdf?1405455385

You can see from the pictures that there's not a lot going on here. Only 2 small boards to deal with.


Thanks in advance for any help, opinions, guidelines, whatever you have!!

Randy

 

[petehall347]

looking like the relay isnt switching off at power off .. do you hear it click ?

 

[Mofi]

Good point! I'll try that. I don't know that I've listened for that relay on turn off.

 

[slimecity]

I don't know if planned obsolescence has anything to do with this but you also have two groups of resistors and/or small signal transistors getting VERY hot on the left side of the PCB.

 

[Mofi]

I saw those! One of the amps has those 4 big resistors elevated up off the board half an inch or so. I thought I'd replace and elevate those if and when I take that board out to do any work. I should probably replace those transistors at the same time. The bad thing about that schematic is that there aren't any values to use for measuring anything.

 

[Mofi]

Any thoughts about wiring in a cooling fan or drilling some extra vent holes in the top? The holidays have sort of gotten in the way of tearing into that amp. Plus I got a new workbench for Xmas. Have to get that setup!! A local audio guru suggested I recap the whole thing. I'd be looking at 35 or so electrolytics on both boards. Figure I can replace those transistors and bigger resistors at the same time when I have it torn down.