Minimoog keyboard circuit and transistor biasing

G'day all.
Have been knocking up a minimoog clone the last few months, and 90% complete. Big learning curve as I'm relatively new to the diy game.
Anyhow, check this out
http://www.fantasyjackpalance.com/fjp/sound/synth/synthdata/16-minimoog/002/907-cont-gen-key-schem.gif

The final sticking point - everything else is working beautifully, but I'm on the third build of the keyboard circuit without success -
Q23 does it's thing, but the base of Q24 is stubbornly sticking at 9.3Volts - R58 substituted all the way round a decade box with no change-  which faithfully ends up stored in C6.  Anything seem suss on the schematic, or does Q14 need to be involved in a chiken and egg kind of thing?
btw, happy to share some artwork - the filter and contour generator sound especially gorgeous, as does the oscillator - if only I could trigger it properly.

Cheers!

Q23/Q14 form a diff amp, the base of Q14 needs to have a negative feedback connection from the output of the CV amp (sets the gain of the circuit and improves tracking performance of the CV amp).  If the negative feedback connection is broken I could imagine the collector of Q23 being swung all the way up to the +ve rail.  Check connections around Q13 and Q10.  Do you have the external glide pot/switch wired up too?  Short terminals 8 and 7 together (far right of schem) to make sure your external glide circuit is out of the equation.  Voltages at bases of Q23 and Q14 should be pretty close to identical when CV is input from the keyboard by virtue of the negative feedback connection.

The minimoog has a two bus keying system - are you also giving it the trigger signal as well?

Base of Q23 should sit at +10V with no CV input from keyboard, pulled up by R53.  Is C9 shorted?  Is R53 open circuit?  When the CV is applied can you see it rising and falling at the base of Q23 as you press keys?

Base of Q24 (collector of Q23) being stuck at (nearly) the positive supply suggests that the Q23 isn't doing its job, and is cut off, either by some problem at the input side of Q23, or the negative feedback from the glide circuit.

Check connections/values around R52, and emitters of Q23 and Q14 as well.

cheers - very helpful- think i am getting my head around it a bit better. Triple checked the circuit - everything hooked up correctly, and tolerances all to within 1%. and yes, am triggering.


From what I can work out - I have approx 9.3 volts on the gate of Q10, but on the source is around -8V.
so feeding that into base of q14 (q23 sees 0 - 3.5v etc) gives the differential amp the maximum difference, and forces the output way up to the 9.4volts. ???  Am i on the right track?

What would be the reason for what's coming out of Q10?
And what should I be seeing on the gate of Q13? Anode of CR5 is 9.9V or -0.1 depending on the triggering, but does not seem to make a difference on the gate. I'm guessing it's too saturated from the dif. amplifier.
Cheers, Jim

Q10 is just a source follower - what appears on the gate should appear on the source, give or take a volt or so.  If you're seeing -8V on the source with +9.3 on the gate I'd say either Q10 is munted, there's something funky going on with the connections around Q10, or R18 is shorted/wrong value.

Just did a quick sim.  If I substitute a 39 ohm resistor instead of 3.9K for R18 I can get pretty much exactly what you're seeing.