Vintage Synth Repair

Thanks! 

Great explanation. In the meantime I managed to find a suitable pot. 
I was a little bit hesitating because I always thought, that there is 
a special "Audio" type with better characteristics. But now I learned 
that the revlog one is this sort of Audio type...  I will try to 
evaluate this with the high quality one I found.

The pot has to be used not with an Minimoog (which I own also) but on 
an old russian synth, an Altair 231 (which is an almost perfect 
Minimoog clone).
The problem in the filter Emphasis (Resonance) control is that the 
Russians had no Reverse Logarithmic pots. So they used standard ones.
Due to that, there is happening almost nothing from 8h to 15h, and 
then, if you turn the pot for 1 mm more, it goes directly into self-
oscillation. As you described it.... 

So I will change the pot with the same value as on the Mini. This 
should solve the problem.
We will see.... (Should be interesting to hear how it sounds 
afterwards). 

Ambrosia





--- In vintagesynthrepair@yahoogroups.com, "duncan" <ferrograph@...> 
wrote:
>
> generally speaking, "audio" in the context of a variable resistor
> means "logarithmic taper". "rev" is an additional indication that 
the
> taper should be reversed.
> there's no harm in substituting a standard log taper pot or a linear
> pot of the same value in such circuits, but the behaviour of the pot
> once one starts to use it will be incorrect.
> 
> if you attach a resistance meter to a normal "audio taper" pot & 
watch
> the value as you move the pot, you'll see that all the action seems 
to
> be piled up at one end of the travel of the pot. 
> 
> now, in the context of adjusting the feedback of a filter circuit, 
you
> can see why you'd want the low-end of the pot's travel (working from
> fully ccw, or the bottom of a slider's travel) to make a lot of
> difference, & for the control to be of a higher resolution towards 
the
> critical part where the circuit begins to self-oscillate.
> 
> with a linear control, the fine-control as the filter goes into
> oscillation isn't so good. with a regular log pot, the control is
> almost unusable.
> 
> hth-
> duncan.
>