Vintage Synth Repair

I've got KORG PE1000 and could repair some problems like not working  
tones and other usual stuff on the instrument which was not used for  
long time.


Now everything works except one vibrato oscillator - LFO.


There are three LFO's, first one controls first three tones on the  
keyboard, second one the next three notes, third one the next three  
notes and this pattern is repeated throughout the keyboard. So I have  
now vibrato on keys 4 to 9, then 13 to 18, etc. Coupler notes one  
octave higher follow the same pattern but shifted, because note one  
octave up use different LFO. Very clever idea, which helps to get more  
animated sound - all LFO's have the same speed and depth (controlled  
by pots) but different phase.


So I suppose one LFO doesn't work. By measuring voltages at LFO  
outputs (points 50, 53 and 54 in schematics on the page 7) it was  
confirmed - while LFO 2 and 3 have changing triangle wave on the  
outputs and voltage changes from approx. 6.5 to 7.5 Volts, LFO 1  
output gives steady 7.11 Volts.


I have attached Service Manual and I'd like to ask more experienced  
electronic gurus here:


- Which component in LFO circuit would you considered faulty with the  
highest probability? I don't want to change all of them... Could be  
only one half of LM3900 working and the other faulty? Or is it  
transistor?


- How I can find which component is faulty? I have digital voltmeter  
and oscilloscope....  Can I find wrong component by measuring and  
comparing voltages in different locations of the LFO 2 or 3?


Thanks for any hint...


Daniel Forro


P.S.: If attachments are not allowed here, I will send Service manual  
by PM, or it can be found here for download:


http://elektrotanya.com/?q=showresult&what=korg%20pe-1000&kategoria=&kat2=schematics