JBL PB12 Subwoofer has constant machine gun "thumping" connected to system or not. Serial number begins with "AM0035" and was told these amps cannot be repaired.Is this true? What now?

First try to find the source of your problem. The easiest way would be to try your sub with a different audio source to see if the problem still occurs but is sounds more like there's a problem with the receiver.

There is a quick way to find out if the parts are hard to get. Look inside a make a note of the IC and Transistor numbers. Don't bother with the resistors and caps. Put the numbers into Google and see what comes up.

Check inside to make sure there are no capacitors which loook swollen, they look like Blue cans, Faulty and / or leaky capcitors can cause the thump.

Bad Class D power amp module, no replacement for it, I mod my, http://s807.photobucket.com/albums/yy352/budm/JBL%20PB-12%20mod/

Service manuals:
http://manuals.harman.com/JBL/HOM/Service%20Manual/PB10%20sm.pdf
http://manuals.harman.com/JBL/HOM/Service%20Manual/PB12%20sm.pdf

More like to have bad Class D amp module, common problem with PB-10/12. I modified my, http://s807.photobucket.com/albums/yy352/budm/JBL%20PB-12%20mod/

Service manuals:
http://manuals.harman.com/JBL/HOM/Service%20Manual/PB10%20sm.pdf

http://manuals.harman.com/JBL/HOM/Service%20Manual/PB12%20sm.pdf

Service manuals: http://manuals.harman.com/JBL/HOM/Service%20Manual/PB10%20sm.pdf

http://manuals.harman.com/JBL/HOM/Service%20Manual/PB12%20sm.pdf

Take some voltage reading according to the manuals.

Hiya, could be the power supply or possibly the amp circuit that has gone. If there are electronic guys in your area, they should be able to sort it out relatively easily. Get a quote first, make sure it is in economical repair as well, but it can be repaired

Hello there..Thats normal only if your sub volume and the pre amp is turned up..Your amp is the cause of the speaker jump..Turn amp volume sub level down a few and check your wire from sub to amp..If all of that is good is a normal operation for pre amp warm up in sub if level is turn to max..Try unplugging amp freom sub then turn sub on stand alone(just sub outlet plugged in and no wires from base amp plugged in..If sub stops the its the amp making the problem..If it still is there then the pre amp is either dusty on inside or the outlet plug that it is plugged in doesnt have a good ground..Hope these help to solve your problem..Have a great day...

I’m happy to assist further over the phone at https://www.6ya.com/expert/joe_a3369546f8067589

I have the same subwoofer, my amp crapped out as well. I am going to take out the JBL amp and replace it with a dayton sa240. They are not close in measurement but I'm going to seal the old amp hole with mdf and cut another hole for the new amp.

if your serial number starts with ha then it's probably a bad capacitor C72 . 6.8uf 100v np electrolytic. replace it with a higher voltage capacitor of the same value.