Hi,
I'm trying to get this circuit working again - see PDF for schematic. It was working in the past.
It's a midi -> control voltage circuit, for making music. So midi pulses coming in at J8 and J7 are converted to voltages in the range 0-10V.
There are two parts to the circuit, one for notes, and another for controlling other elements, like filter cutoff. The outputs are at J1 and J2.
The problem is that the top part of the circuit for notes is not working. I get constant 10V at J1.
Everything is fine at the other controller - J2 varies from 0 - 10V according to input.
The RFB in the DAC U3, at pin 9, shows a constant -9V. That seems to be wrong and I don't know where that's coming from.
The components seem to be OK, and I tried swapping the ones from working part of the circuit with the non-working part. Same results. I'm presuming that the PIC IC and the crystal are good because of the working half of the circuit? I programmed a new PIC and put it in, thinking that was the issue, but of course it is possible that I have damaged it in my explorations.
VCC, V+ and VREF are good in U3. 5V, 12V and 10V respectively.
Any ideas?