Problem with PSpice Simulation

I'm having trouble with a PSpice simulation for a relaxation oscillator posted in a previous message on sci.electronics.basics: "PUT Oscillator?"

Terry Pinnell was able to model the oscillator succesfully, using the following netlist:

*Spice netlist for Circuit: > D:\Docs\Electronics\CIRCUITS\C­M\PUT-Diode.CKT > D1 V1_1 C1_1 D1N914 > V1 V1_1 0 DC 6V > Q1 Q2_2 Q1_2 V1_1 Q2N2907 > Q2 C1_1 Q2_2 0 Q2N2222 > C1 C1_1 C1_2 20nF > R2 0 Q1_2 2.2meg > R1 C1_2 Q1_2 22k > .SAVE C1_1 C1_2 V1_1 Q2_2 Q1_2 @d1[p] @d1[id] @v1[p] v1#branch @q1[p] > @q1[ic] > .SAVE @q1[ib] @q1[ie] @q2[p] @q2[ic] @q2[ib] @q2[ie] @c1[p] @c1[i] > @r2[p] @r2[i] > .SAVE @r1[p] @r1[i] > * Selected Circuit Analyses : > .OP > .TRAN 1u 50m 0 1u > * Models/Subcircuits Used: > *1N914 100V 80mA Si Switching Diode pkg:DIODE0.4 A,K > .MODEL D1N914 D(IS=7.075E-9 RS=0.78 N=1.95 TT=7.2E-9 CJO=4E-12 > VJ=0.657 > + M=0.4 BV=100 IBV=0.0001 ) > -- > Terry Pinnell > Hobbyist, West Sussex, UK

This simulation works correctly for CircuitMaker (using Spice3f5/XSpice). A picture of the results is at http://www.terrypin.dial.pipex­.com/Images/Relaxation1.gif

I tried the following netlist in PSpice Lite Version 9.2:

Oscillator >D1 V1_1 C1_1 D1N914 >V1 V1_1 0 DC 6V >Q1 Q2_2 Q1_2 V1_1 Q2N2907A >Q2 C1_1 Q2_2 0 Q2N2222 >C1 C1_1 C1_2 20nF >R2 0 Q1_2 2.2meg >R1 C1_2 Q1_2 22k >.OP >.TRAN 1u 50m 0 1u >.lib eval.lib >.probe >.end

This circuit does not oscillate. V(C1_1) is 4.9504V, V(C1_2) is 5.293V, VB(Q1) is 5.293V, VB(Q2) is 772.971mV. I(D1) = IC(Q2) = 89.884mA. I(C1) =0. I think that IC(Q2) should discharge C1 but all the current goes through D1 instead.

I was wondering how this circuit is supposed to work, and what possible changes I could make to the PSpice netlist to derive the correct results.

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Terry Pinnell
Hobbyist, West Sussex, UK
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Terry Pinnell

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