I have a JBL PB12 sub, no power - prev wrenching sound on startup

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.

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

My ClassD module shorted out (buzzing sound/blown fuse) and not availabled any more so this is what I did: http://s807.photobucket.com/albums/yy352/budm/JBL%20PB-12%20mod/
I would suggest to buy new one.

More likely to be the Class D power amp module, but you should look through the service manual to make sure all the parts are installed correctly. My PB10 AMP module went out and you cannot get one anymore.
http://manuals.harman.com/JBL/HOM/Service%20Manual/PB10%20sm.pdf

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

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

More likely to be bad amp module.

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

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.

These subs have had many problems, all associated with the amplifier section. PB 12's with an "M" as part of the ser. no. were assembled in Mexico and these are the ones with the amp problems. JBL fixed mine and sent it back no charge, BUT it cost me nearly $100 to send it to them in the first place. I have another one that has had no problems at all. Try sending it back for repair. At best you'll get a rebuilt unit back, at worst a substantial discount from the company for a new sub.

exactly......I bought a Klipsh