303 clone and Q27 burn out (remedy?)

[Martin Czech]

> > Could some 303 owner check this ?
> 
> Did you want a 303 owner to check your whole description of the SCR
> function, or just that the flat portion of the saw is independent of freq. ?
> ;-)

The question is, how long is the flat time? Does it vary
with frequency?

> > The vca seems to be an ota, replacing it with an 3080 should 
> > make no difference,
> > if the resitor network gives the same amount of distortion. 
> 
> Maybe this is true, but I think it will be difficult to get the right
> distortion. I haven't tried the 3080 myself as I had a BA662, but I have
> heard from someone who did try the 3080 that it didn't sound right.
> The best sounding commercial clone (IMO) is the TBS303, and it uses the real
> 662.

If they are available, no question.

> > So there is only the filter left...  A funny circuit. A dual pot for
> > more output at resonance...  And the foot of the current source Q11
> > has no direct path to ground, as I would have expected.  It 
> > seems to be
> > fed by the current source (and AD output) Q39 via VR5.  This is really
> > strange... also VR3 seems to interact there...
> > 
> > Do you know the reason for this ?
> 
> The reason for this is that the envelope modulates the filter cutoff
> *around* the freq. it is set to.
> There is some description of this in the service manual where it explains
> why this is done - apparently the designer didn't like the way increased
> envelope mod on most synths increased the overall level of cutoff.
> This is another important characteristic of the 303. Of course there are
> other interactions in the filter depending on the accent setting.
> 

Yeah, feeding the filter with current from the bottom means multiplication...

There is such much interaction between the pots and the envelopes, it
seems to be impossible to cut this little silver box into 3 different
modules as i though first. To get the real sound the whole analog section
must be copied as is, I fear...

What about Q27 burn out ? Any improvements ?
(I don't mean the number of meltet plastic pnp's ;->)

m.c.