I would first ask how the seller(?) knows that the "rectifier" has failed. First of all, I'm sure that it wouldn't be considered totally impossible, but it is darn near a very rare improbability. Second, the Concert series uses three separate power supply rectifiers (see the schematic posted on this site). If any one has failed, it would probably be the one identified as CR1, as this is the rectifier for the power amp power supply.
A rectifier can fail in two ways... it either opens or it shorts. I wouldn't be too concerned about an open failure, but a shorted failure can send AC into the power transistors (assuming it is CR1) not to mention the power supply electrolytic which will go up in smoke rather quickly. If you can, open up the amp and check the electrolytic (the big can type thingie) for any bubbling, splatters or strange smells.
If it is the rectifier, it shouldn't be too difficult to replace. I can't remember exactly... but this amp uses a lot of spade lug type connections and that might be true in the case of the rectifier. Otherwise it might be soldered in. At any rate, it is bolted to the bottom of the chassis and should come out pretty easily.
Hope this helps.
