minimoog model D serial 10175 below VCO2 dead

[dancelwerk]

Hi to all, i buyed a Minimoog broked recently, works all less VCO2, is dead, i am thinking on change all transistors and ics related with VCO2, but perhaps somebody can explain to me how to VCO2 generates the waveforms, in the minimoog manual service only explain the new boards, but i have the old version serial 70xx i can read it very well there is smashed in the serial number only 70 is visible.

I don't know where I have to start route the signal. In schematics for this model i read many number labeled "A" in one side of schematics and in the other side "B", i don't know wich are the inputs and outputs, i suppose a inputs and b outputs, i think the vco2 is dead in the beginning
of the generator

is the core of the vco2 in IC2, IC4 and IC 5????

Any ideas?.

Thanks in advance.

[thealien666]

Hello.

If I'm not mistaken, the control voltage part of OSC2 is effectively made up of IC2, IC4, and IC5. It receives the control voltage from different sources (keyboard, pitch bend wheel, tune potentiometer, etc...), thru summing resistors, and inverted by IC4. Then this is applied to an exponential current converter, made up of parts of IC2, and IC5 acts as a temperature compensation circuit to try and keep things stable via a Tempco resistor.

So if there are no sound, from all waveforms, coming out of OSC2 chances are that IC2 might be defective. But to be sure, better replace all 3 ICs.

The actual oscillator part is based on a precision sawtooth generator, from which other waveforms are derived.

Hope this helps.

[dancelwerk]

I will check this and will replace the ics related, I keep you informed, thank you very much.

Fernando.

[Kevin Lightner]

Just a quick note from experience: Suspect the ICs and components last and physical connections first.
That is, make sure the edge connectors are making good contact and there's no broken wires or cracked solder connections.
I would look very closely at the osc board and the soldering.
See if any components have been bent (shorting others) or loose (cracked soldering) before replacing parts.
Same for the octave and waveform switches: make sure they're not shorting against anything.

Parts DO go bad, but I've repaired more Minis from taking this approach than making the assumption components are the problem.
There are hundreds of connections in a Mini.
So perhaps wiggle things or flex the board a bit and see if it's not some intermittent physical connection first?

Past that, I'd suspect opamps, tranny arrays, the dual FET or main integrator cap (polystyrene and usually clear.)
A scope can also be helpful as OSC 2 might well be working, but could be oscillating beyond hearing range.
This is often from a bad opamp, but is easily checked with the right test gear.

Good luck!
K

[dancelwerk]

problem solved, Oscillator 2 is alive again, was the ic4, an lm741, changed and fixed.

Thanks to all for the comments.

Fernando.