Hey guys,

My most loved VA 1012XII (Velodyne active subwoofer) which has performed flawlessly for over 15 years

has developed a uncontrollably thumping when switched on and only

stops when unit is switched off .

-All inputs and outputs have been disconnected-

Outlet PWR good and clean.

Any suggestions or information would be highly valued.

Cheers.

 

I would look at the LM324 Quad Op Amp used in this system. These OP Amps are prone to oscillating, and a slight change in component values in the low pass filter circuit could put this OP Amp into oscillation mode.
...
Here is a design checklist... Pay attention to item 9 and 10.
..
http://www.circuits.dk/resource_electronic_design_checklist.htm
....
The issue of LM324 oscillations is also covered in this thread...
...
http://dics.voicecontrol.ro/process...ed_current-sink_output_feedback_resistor.html
....
My first guess would be a bad electrolytic filter cap.... Power supply bypass and feedback loops can have bad electrolytics (trust me the world of electronic repairs is full of Bad Cap stories - I have probably over 100 stories of my own).
..
You can test electrolytics by simply jumpering a known good onbe accross the cap... Or look and see if the cap is distorted or swolen.
...
My guess is in the bad cap area (almost all amps that use LM324s are highly prone to this issue). The second thought would be bad power supply ripple/isolation - leading us back to bad caps.
..
Hope this helps... and... Good luck.
..
Dave
Phoenix, AZ