I've managed to find the main fault with my TDS6604 - one of the trigger chips was fried, the infamous Tek # 156-8278-00
I've also managed to locate a couple of Acq. boards for TDS6604, but I'm not sure what parts, if any are common with TDS6804B.
Unfortunately around about the time that the TDS6804B was released, Tektronix changed their policy and Acq. boards were no longer sold as spares.
Even worse the part number is not even listed in the 6804B service manual.