Vintage Synth Repair

Does somebody here any experience with the repair of Electone EL organ?


I've got EL500 (1998) with some problem (from a friend who wanted to  
throw it away) and would like to revive it. The problem is -  
instrument sounded about 15 minutes, then it got frozen, and the next  
attempts to get it work did only this:
- display backlight got on but display was blank
- muting relay on the amp board was activated after three seconds
- instrument didn't start, was locked before the boot


Hard reset nor an attempt to enter Test mode didn't work.


I couldn't find Service manual on the net, but supposing that the  
other EL are build in similar way - which was right - I could open the  
machine following the Service manual for EL90. In the hope that  
cleaning it inside and reseating the connectors and ICs in sockets  
would be enough...


So I have cleaned all from the dust, disconnected boards and put them  
out one by one, check carefully for bad caps, cold solders or other  
problems, cleaned all connectors, repaired slightly deformed membranes  
of tweeter speakers (checked if they are not shorted), cleaned FDD,  
reseated three Mask ROMs on the main board...


I didn't disassembled panel switches board, display, keyboards, small  
boards with connectors, knee lever switch, pedal keyboard, foot  
controller, board with fuses near the transformer...  All looks well,  
like new, even after 19 years.


Then I have assembled all, and after switching on the instrument  
sounded around 15 minutes, then again locked... Then I tried Test  
mode, this time it worked, but I could do only RAM and ROM test.  
During Rhythm Tone Check it was again locked... (but all rhythm sounds  
sounded well when I tried them before).


I tried to disconnect power cable to amp, but instrument didn't  
started at all, no display backlight - probably OS can recognize the  
missing amp and doesn't start.


Now I'm almost lost and don't know what can be wrong there. I suppose  
it is known problem, and the technician with the experience with  
digital Electone would know what it is and how to repair. Maybe  
something on amp board, or problem with the main board or OS Mask  
ROMs? But why... that friend is not aware of any reason why it should  
start such behavior suddenly, they got it in recycle shop, and could  
use it for few years without any problem. Inside everything looks  
pretty well, like new.


I couldn't find any info except somebody had no sound at all but  
instrument worked, and it was some resistor fuse between two pins of  
SANYO amp IC... I don't think it is my case because instrument behaved  
quite normally after switching on, and locked itself after 15 minutes...


I can disassemble and check all the other parts, or replace bigger  
electrolytic caps on amp board, and on another board near the power  
transformer and fuse board, which makes some voltage filtering  
probably... but it can be unnecessary and will not help...


I would be grateful for any hint or advice, or link... Of course  
Service manual copy would be great...


Thanks in advance.


Daniel Forro