Reversing the electrolytic cap in location C29 is sometimes causing a difference in sound because the cap is polarized, and it doesn't like being used backwards - thus using a non-polarized type like a polyester or non-polarized electrolytic should not show any difference, as electrically it looks the same either way it is inserted.textile wrote:How would a bi-polar EL or a polyester type go?
Can you do a simulation of these types?
Generally capacitors used in simulations are ideal, i.e. they consist purely of capacitance, whereas in reality a cap will have an effective resistance both in series and in parallel to it, and if the circuit is likely to be sensitive to such things, then you must explicitly include them in any simulation (and sometimes you need to go to the extent of including lead inductance etc. as well). There are many 'standard' models for caps that include such things, and when we start talking about modelling a reversed electrolytic, they become more complicated. On seeing guest's data, I simply decided that a resistor in parallel would suffice to show the kind of effect we were looking for - i.e. it would 'leak' lots of DC current and hence impact on the biasing. Incidentally, this means that probably the easiest way to 'replicate' this kind of effect in a x0xb0x will be to simply do as I did in the simulation, i.e. rather than reversing the cap physically, simply solder a 30kohm (or so) resistor across C29! (But my suspicions are that many owner's who might want to do this might not see that as mirroring the real thing in a TB-303, and will want to reverse the cap - the resistor method does of course have the advantage that you might not have to wait for several years - or however long it takes, I don't know! - for the capacitor to start becoming as leaky as bcbox's examples!)
Probably a mistake in laying out the PCB, pure and simple, and it has never been detected. If hundreds of units had started showing up with that cap blown, then I dare say they would have spotted the error, but since it seems the units do still function with C29 reversed, no one would have realised that anything was wrong.The question is of course is , why did roland do it.
That will come down to personal taste, but so far we have little idea (well I have little idea) what it takes to get the cap into a state whereby you could say the difference in reversing it is noticeable (see above)!Also is it recommended to reverse the cap. If the TB had it .. should the x0x too? It seems logical for a true clone.