AR ARS108PSB Powered Subwoofer Repair

[Smoky]

What's next to fix is this AR subwoofer. It was built around 2001 and it seems to play pretty well but when it is turned off, a "thump" is heard a few seconds later. There was no signal present at the input of the subwoofer prior to.

I'm going to open it up and check if something looks loony





The first thing I did was measure the load of the 8" speaker. It measures around 4 Ohms. The speaker appears to be in very good condition too:



The plate amplifier itself was blanketed with fiberglass insulation. I brushed and vacuumed it out. Maybe, this isn't the type of sound deadening to be using inside of a "vibrating" speaker



I turned on the subwoofer while it had no signal being sent to the inputs and I measured ~70mVDC at the speaker terminals using my DMM:



I then sent a 20Hz sine wave into the plate amp and my oscilloscope measured a near perfect output from the subwoofer. If I raised the amplitude, I could literally bounce my Hakko soldering iron off of the bench!



I'm going to search the web for a schematic and see how the components are arranged. I do see some quality caps like Jun-Fu and G-Luxon inside too

I also made a video of the way the subwoofer acts when it is shut off:

[jdragoset]

Is this a change from when new?
The power-off thump does not seem to be very high amplitude to cause woofer damage.
Many years ago I had put together some Southwest Technical Products (FET preamps, Universal Tiger amps, Altec Lansing speakers), Had a mega cone excursion when all powered on together but installing an Amperite delay relay with a switching relay to power the amp 10 seconds after the preamp solved the problem.

[Smoky]

I'm not sure Jdragoset, I bought it used. It has probably been stored for some time.

What I have noticed too is that the DC Volts at the speaker terminals appears to be dropping the longer it is on. Maybe the capacitors are re-forming themselves? I do recall it being well over 160mV's at the speaker terminals when I first powered it on yesterday.

The next time I work on a powered subwoofer, I'll disconnect the speaker from the plate amp and use a dummy load instead

[mzacharias]

The thump at turn-off is probably normal if there is no protection relay. Perhaps even if there is a relay.

Most amp protection circuits employ a fast turn-off to avoid thumps of this sort, but it's not required.

DC sensing during operation pretty much is.

So long as there is no appreciable DC at the woofer terminals during operation, probably nothing to worry about.