[sdiy] Problem with DCO

[Tom Wiltshire]

Hi All,

I've been experimenting with a DCO design based on the Roland JX8P  
circuit:

http://www.electricdruid.com/DCOTestDesign.gif

Broadly speaking, it works, in that I can get a ramp wave out of it  
if I adjust the 47K pot on the left. This pot and op-amp buffer will  
be replaced by a DAC when I get it working better. I've got reset  
pulses generated by a PIC being fed into the base of the transistor  
at 440Hz for the purposes of the test. That part is fine.

The problem is that the output goes from 0V down to -0.5V and then  
seems to saturate. What's going on? How can the op-amp output be  
saturated at only -0.5V when it's running on +/-15V rails? Everything  
I've read suggests that the output from an integrator ought to carry  
on as far as the op-amp output can take it, so I was expecting a ramp  
from 0V down to -15V or thereabouts. I'm sure there's something I'm  
missing here...

Thanks for any clues,
Tom