Bought this about 3 months ago - as usual I paid too much for it. The 650 is notorious for its scarcity and they seldom show up in any condition, this one being pretty much trashed. It was advertised as not working, no other details given. But it's a 650 and I saw potential. The left channel was blown badly. I have a feeling this thing sat in this state in a very dirty place for many years. I knew I was in for a big job when I removed the bottom cover and chunks of emitter resistors fell out, saw several parallel resistor repairs, 10,000uf filter caps in place of the original 18,000 uf-ers, a 33k ohm resistor where a 100 ohm should be and so forth. Here's the condensed version of the process - repaired the left channel, blew up the left channel again, repaired it again and made some upgrades, made exact same changes to right channel, then commenced with a complete overhaul and makeover. Pictures This is how it looked when I got it - notice the splotchy pink/purple fading to the "bronze" faceplate. Yuck. More on that later. Left channel damage - 3 of the 4 Sanken outputs blown, 3 of the 4 emitter resistors, plus board components. All 8 outputs were replaced with OnSemi MJ21193G/94G. Repaired the damage, replaced all 4 emitter resistors and got the channel working. Ran it on the bench for a few days, then got the bright idea to try an ebay 2sb536 for one of the drivers that had blown originally. That lasted for about 20 minutes, then this happened. (lesson learned, no lecture necessary!) This took out 3 of the 4 OnSemis, a couple transistors on the board, and of course the ebay special. This is an early unit - serial # 330xxx. Later versions have this board with significant changes. Re 17,31 &32 were upped from 1/4W to 2W,the big axial cap Ce9 was eliminated and Re30 was replaced with a jumper. I upgraded the 3 resistors (3W), but left Ce9 as I wasn't comfortable eliminating it. All other resistors were swapped out for 1/2W metal film. The right channel got the exact same treatment. Once both amp boards were working properly, I moved on to the rest of the overhaul. Power supply rebuilt. The nice thing about this board is that it's easily removed from the amp. There is a lot crammed on to this little bugger. The regulators run wicked hot, so care needs to be taken not to lift a solder pad or two. I had one slightly lift but it didn't tear and I was able to save it. Tone and pre-amp boards recapped - no other changes made. This is when to do a thorough pot and switch cleaning. The bass pot on this was horribly dirty but was able to successfully clean it. Note the white trace outlining - later boards were dark green. Also, this one has the X08-1470-10 pre-amp board. Later models used a different board. Contrary to popular belief, you don't have to unbolt and manipulate the transformers to replace the filter caps in these. This makes it very easy to get to the cap clamps and also easy to replace the relay and diode which are on the rectifier board. (clip the wires to the filter caps before getting the amp into this position - ahem) An overhead view of how grimy this thing was.