Vintage Synth Repair

Hi, there! 

Can anybody tell me of which type is the Resonance Pot on a Minimoog, 
low serial number?

Should be a reverse taper pot, but how many KOhms, and what else  
characteristics?

Best regards, 

Ambrosia800

Hello Ambrosia800,

the service manual of the minimoog just says, that it is a rotary 
pot, rev audio, 50k Ohm. I don`t know, what rev means, but audio is 
right, because the filter-audio-output returns to the filter-input, 
so you need a very-good-quality pot. I don`t know more, because my 
mini hasn`t made any problems with the filter-section. But if I have 
some problems with one of my moogies (you can have a look/ear at my 
selfmodified prodigy at www.youtube.com/watch?v=OFFoLRvVPOk ), I 
would contact Rudi Linhard from www.lintronics.de . He is a great 
guy with a very-good knowledge about moog-stuff, and he is also from 
germany like me, which makes it much easier for me to communicate. :-
)

Best regards
Heiko



--- In vintagesynthrepair@yahoogroups.com, "ambrosia800" 
<s_carter@...> wrote:
>
> Hi, there! 
> 
> Can anybody tell me of which type is the Resonance Pot on a 
Minimoog, 

> low serial number?
> 
> Should be a reverse taper pot, but how many KOhms, and what else  
> characteristics?
> 
> Best regards, 
> 
> Ambrosia800
>

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.

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.
>

Update:

changed the Pot! Works fine! Unit now behaves like it should, with 
the Reverse characteristics...

Sound is good, only some tuning to do... this will finish the job.
Thanks! 


--- In vintagesynthrepair@yahoogroups.com, "ambrosia800" 
<s_carter@...> wrote:
>
> 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.
> >
>