First check the Power supply and adjust +15 and -15 rails using R25/24
Watch when de-soldering components that you don't lift the PCB tracks. They are pretty fragile on this synth.
Get yourself the schematics. They are in Russian but will at least throw some light on the internals
Of course this synth had a few problems with it when I got it.
One of the upper 'B' notes was stuck on. On upper voice registers 2 and 4.
This turned out to be a bad diode KP521. I used a DIY audio probe to trace this down to voice area Z11 on the PCB.
This diode was replaced with a 1N4148 and the upper 4' voice issue was fixed.
The bender range was way off, so adjusted with these two trimmers:
Although my synth was ok, many EM-25's have keys that do not release. This can be down to a bad capacitor which sits under the corresponding key.
Another issue was some bad solder dry joints. A dry resistor solder joint was responsible for range upper 2' not working.
The EM-25 has a 'slide' control with interval. On my synth this doesn't work. I assume it's the same as the 'glide' found
on western synths. I have the transposer board out and I think it's a faulty logic IC.
K561TM2 IC = 4013A Dual Flip Flop
or one of the other 3 IC's in the area.
K554SA3 = LM311 DIP14
K553UD2 = LM201 DIP14
K561LA7 = 4011A
Update - I actually found out that these controls only affect pedal operation and were not faulty after all. DOH! It was fun to decode the IC's though. Thanks go to Karolis for confirming this.