Proteus/Isis Simulation Problem

I am trying to simulate a comparator PWM oscillator in proteus, however it will not oscillate. Simulating the exact same circuit in Multisim oscillates with no problem. The reason I need to use Proteus is the various components to be used after the PWM e.g. IRF1405, DC motor etc are not available in my version of Multisim. I have attached a schematic.


Any ideas anybody?.

Thanks in advance.

Rick.

First... Change the cap to a modeling primative.... Then load simulator options for better convergance... It should work.

BUT!! It goes terribly slow.... For simulation change the 10nf for a 1uf and use a much slower PWM...

What simulation period did you use?
It simulates in LTspice, but needs ~137mS before oscillations start.

Thank you for your replies. I changed the Cap and the convergence as Ian suggested and the circuit does indeed simulate. As for "terribly slow", I think that was an understatement . Changing the Cap value does help and It does at least offer some proof of concept. Now on to actually building and testing a real circuit.
Thanks again.
Rick.