Sorry for OT but I'm sure somebody of clever members here could help me.
I've just got Korg PE1000 from a friend and could repair some problems like not working tones and other usual stuff on the instrument which was not used for long time. As you probably know it's vintage analog polyphonic ensemble from 1976 (full polyphony, each key has its own oscillator), kind of electronic piano or string machine. Very nice.
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 by three notes, 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 directly 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 between approx. 6.5 and 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? Some resistor or capacitor?
- 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