[sdiy] Methods for Analogue Synth Mods

[JP]

I've been into synths, drum machines, etc. for the past couple years so I know what I'm doing in that regard - the actual synthesis part - I've just started picking up a couple analogs (PAIA Fatman, FR-77) to complement the VA I've had for a couple years (Virus KB).  I'm getting more and more interested in the electronics side of things.  I've been reading some basic books on electronics, playing around a little bit w/ circuit bending a Casio SK-1, etc.  Started the PAIA a long time ago but burnt the circuit board and just haven't gotten around to ordering a new PCB.
 
I was wondering how you go about finding mods in your synths - what type of methodology you follow.
 
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.  
 
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?  
 
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?  

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

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?

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?
 
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.

Hope this makes at least a little sense.

Thanks - John Potter

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