Board repair for subwoofer

Disconnect that board and throw it in the trash.

Call it a passive sub and run it with an external amp such as this one from fosi audio, might be good enough.

https://www.suncoastaudiodirect.com/products/fosi-audio-tp-02-tda7498e-subwoofer-amplifier-receiver-mini-hi-fi-class-d-professional-integrated-amp-for-sub-bass-shakers-220w?_pos=1&_psq=Fosi+tp&_ss=e&_v=1.0

@deep_333 

A problem is that there are two drivers, each with an 850 class D amplifier, in one sealed cabinet.  I would need to get two 850 W amplifiers on the same signal from the pre-amplifier, or some satisfactory combination that could drive both woofers from one RCA line.

You can find plate amplifiers from Dayton at Parts Express or fancier Hypex models at Madisound and miniDSP.  besides physical fit and power I encourage you to get something with good DSP capabilities so you can properly tune the sub.  If you are inexperienced, stay away from Hypex as their software is a PITA and stick to Dyaton or miniDSP. 

Im sure you mever use the 800W.  A Hypex 502 would give you 2x 500w if each is 4 ohms.  Plenty.

@drbond Were I in your situation, I would abandon the original plate amp in place, redirect the speaker leads from the old amp to new binding posts or Speakon connectors - essentially turning your sub into a passive sub - and drive it with an external pro amp, something in the 500wpc range and configured in parallel mode. Parallel mode takes a single input signal and feeds it to both channels of the stereo power amp, acting as an internal "Y-cable" if you will, thus allowing one mono source to drive both speaker loads independently.

Crown makes several models that would be perfect for this application. Their vintage Macrotech series amps are legendary Class AB monsters with a damping factor of 1,000, and their modern Class D amps have tons of punch and excel on bass duty. If you do decide to go that route, I think you’ll find bass tightness and slam have pleasantly improved. Good luck!