[sdiy] Methods for Analogue Synth Mods

[Plutonique9]

>I was wondering how you go about finding mods in your synths - what
>type of methodology you follow.

The way i started out learning and understanding about modifying 
synths/drum machines is by reading and studying modifications already 
performed by others.

For example, when modifying my Tr-808, there was very little 
information written on the web pertaining to this, there were two or 
three on music machines, but they really didn't go that deep.

How i learnt to mod the 808 was by reading and studying all the 
modifications for the tr-606, comparing schematics of the 606 and 
808, a seeing how i could apply the mods for the 808. At first i 
really didn't understand schematics at all as well, but the more you 
start look at them the more you start to understand what is going on, 
even if you don't know 100 percent what is happening electronically, 
believe me in that its better to have them printed out and at your 
side when performing mods, than not having them at all. Apart from 
simple modifications that are logically straight forward, i make sure 
that i always have the schematics before i start modifying any of my 
synths/drum machines. Consider them to be like a street directory, 
and your machine is the city, trying to get to so n so st. without 
knowing where your going could end up leaving you lost, pist off and 
confused.

> 
>I'm aware things like resistance substitution boxes exist and I'd
>imagine that'd go a long way to determine the proper resistor or pot
>to drop in to change the sound. 

If you start with a larger pot than you you know you will need, it is 
always easy to scale down the pot to the right value after, that is 
by using a nother resistor in parallel with the pot. For example, if 
you have a 100k pot, and you need a 50k pot, by putting a 100k 
resistor in parallel with the 100k pot, you will end up with a 50k 
one, you know?

> 
>One question is how do you use something like a res. sub. box w/o
>pulling the existing resistor - or do you pull each resistor you're
>trying out? 

I always remove the resistors first, then experiment with different 
pot values. If you don't already know, solder braid (solder wick) is 
essential for removing electronic parts off of a PCB cleanly, without 
damaging the PCB.
 
>How can you be sure changing something like a resistor won't result
>in longer term damage - like changing a resistor and something down
>the line overheats after being on for a couple hours? 

No sound modifications i have performed has ever resulted in this. 
You should be alright if you stay away from messing with resistors 
and such that deal with powering the circuits....ie. positive and 
negative power supplies and such. Normally resistors overheating and 
burning up is caused by the +/- power supplies going to ground 
through short circuits and mistakes. You'll no when this happens, 
things will start smelling funning, the resistor will turn into a 
black crisp, as long as you catch it early though, it shouldn't do to 
much damage, then again what do i know.

> 
>For example, how do you know that shorting out a resistor wouldn't
>cause any damage?
> 
>How do you even know where to start? 

Read what others have done, read read read, experiment experiment 
experiment, your bound to make mistakes, but thats the price we pay 
for hacking our machines :)
> 
>Is there some kind of method you use to determine which resistors
>might affect the sound and how it might affect the sound or do you
>just kind of go around systematically left to right, top to bottom
>and try changing resistor values?

Now this is like circuit bending, dangerous for instruments and you 
operating over 9v, only cheap toys and casio's are forgiving using 
this methodology. You have to understand the circuit to realise what 
you need to change to alter different properties of the sound. For 
example, when modifying my X0X drum machines, i spent the good part 
of a night reading up on Twin-T filters (drum tones of the 
808/606/909), i started understanding how the circuits work, therefor 
i could apply them to my project.

> 
>I'm not talking about random circuit bending here - I'm talking
>about something like increasing the max LFO rate on an analog synth
>or changing the range of an OSC from an OSC Semitone pot. 
> 
>At this point, I can't read schematics very well but I'd guess this
>would help someone who could - but what about when schematics aren't
>available?

There are a lot of schematics for old synth on the net, seek and you 
will find, google is a good search engine for this. Post to this list 
and Analogue heaven, someone either knows where they are online, or 
has the ability to scan them themselves and send them on to you, 
there are many many kind and helpful people on both Synth DIY and AH.

> 
>Outside of simply changing resistance values you've got some mods
>that send the output of a resistor to ground - how would you figure
>something like that out?  The resistance substitution seems like it
>might be a fairly straightforward procedure but something like this
>would seem to require a pretty good knowledge of circuit design in
>general.

Yes sir e bob, electronics is no walk in the park, it can drive you 
nuts coming to grips why things work the way they do, but eventually 
it starts seeming a lot more clearer.
> 
>Hope this makes at least a little sense.
> 
>Thanks - John Potter
> 

Take it ez

Also note, that things like adding individual LFO outputs and 
envelope outputs and direct outputs for Oscillators ( making things 
more modular in structure) is a nice way to get into modifying 
synths. You just got to think about it logically.

For example, if you wanted an LFO direct output from one of your 
synths, the best place to look for the signal is right at the pots 
where you control, say, the pot that controls the depth of the LFO 
modulating the VCF. It seems logical that the LFO would directly go 
to this pot, where the pot itself would control the amplitude of the 
LFO through resistance, and then spit it out to the destination, in 
this case the output of the pot would be a control voltage input for 
the VCF. So Voila, from one pot you could make 2 mods, an LFO output 
and a VCF CV input, this is just a little imaginary example, but it 
normally holds true for most old analogue synthesizers, you know?

-- Plutonique9, Plutonique9 at symbiocom.com

http://www.mp3.com/Plutonique9