Minimoog Vcf

[uzs159 at ibm.rhrz.uni-bonn.de]

At 10:00 24.08.98 +0200, you wrote:
>
>> Well,I found that R20 resistor from Q6 emitter to -10 is 100k not 220k
>> And +10V should go not to col. of Q6 but to other side
>> of R6 47ohm resistor and + node of C9 100uF ?
>> Not shure about that last but if you look at moogmodular vcf, it
>> looks like that.
>>
>This makes sense. It is a kind of emitter follower (darlington).
>In this case the emitter resistors should be equal.
>And it is a differential to single ended converter. Again symmetry
>is required.
>
>Can any of the moog godnesses confirm ?
>
>m.c.
>

I'm by no means a moog godness, (merely a seeker of wisdom :)
it makes sense to me, but then if you look at the schem 
the R24 at Q7 at the opposite leg of the diffamp is 220k 
and not 100k. 
Wouldn't it make sense to make both legs totally symmetrical,
that would be R20=R24 and R3+R6=R7?
In my incarnation of the mini filter the problem seems to be 
that Q7 and Q5 have different temperatures due to the different 
currents. I have to either cool Q5 or heat Q7. 
When I power the unit up, it oscillates at maximum Q setting, 
but as soon as the components raise their temperatures the 
oscillation dies.
Connecting +10V to the other side of R6 didnt't help.

I changed the R20 from 220k to 100k and now the unit oscillates.
(Well only above a certain frequency but thats maybe due to my 
different cap values)

To me it looks like follows, one should thermocouple Q5 and Q7
(and maybe match them, the schem doesn't say anything here)
Or maybe I'll even use a trimmer for offset correction in the diffamp.

BTW does anyone know what value the CV summing resistors have ?

Bye
 -Rene