Vintage Synth Repair

I'm working now on EL500 from 1998 which I've got from a friend who  
declared it as non working. After 10-15 minutes of work it stays  
frozen, locked, and following switchings on are unsuccessful - output  
muting relay clicks after 3 seconds, and instrument doesn't boot. Next  
day the same...


I could perform Reset, it didn't help. Test mode showed ROM and RAM  
OK, I couldn't try more tests.


I did cleaning inside, everything looks well, like new instrument.  
Pulled out amp board, main board with two sub boards connected (XG  
daughterboard, effect board) and some other small boards, checked  
caps, checked solders, resolder some, repaired little bit damaged  
membranes of tweeter speakers, reseated three big mask ROMs in  
sockets... no help.


I suspect usual problem with aged components - namely electrolytic  
caps... The problem can also be dependent on warming some components,  
maybe... I'm not sure. Somebody's advice was to replace Xtals...


So for start I'm thinking about the replacing all bigger electrolytic  
caps in PSU and other supply circuits (they are distributed on more  
boards). After purchasing schematics I could found pretty hidden small  
switching supply and pull it out. It looks well.


In this context my question is: can I switch on this PSU outside the  
instrument, that means without usual load, and measure output  
voltages? Is it safe? I don't want to damage it...
Output spec is:
+5 V/5 A
+12 V/0.3 A
-12 V/0.4 A


It's not Yamaha, but Sharp product. It's interesting to find whole  
block inside Yamaha product not made by Yamaha itself. Before I have  
found it only in Yamaha TX816/416 FM rack modules, and it was also PSU  
from different manufacturer.


Thanks for any advice or hint what I should check or replace.


Daniel Forro