303 clone and Q27 burn out (remedy?)

[Fraser, Colin J]

> -----Original Message-----
> From: Martin Czech [mailto:martin.czech at intermetall.de]
> Sent: 06 November 1998 18:00
> To: synth-diy at mailhost.bpa.nl; Colin.Fraser at scottishpower.plc.uk
> Subject: RE: 303 clone and Q27 burn out (remedy?)
> 

<interesting research into 303 function omitted>
 
> I think the whole arrangement does only reach to the 12V supply because
> of the shut off latency of Q25 and Q27, these transistors are heavily
> saturated, it takes a lot of time to clear the base zone from all
> carriers. This also causes the flat portion of the saw wave and the
> time of this flatness should be independend of osc. frequency.
> 
> 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. ?
;-)
I didn't understand the SCR function too well myself - I just copied it. I
will look into it the next time I have the lid off.

> 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.
Changing just a 741 op-amp can make a noticable change to the sound of a
synth, so I think an OTA can too. 
 
> 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.

Colin f