HP-35 Power Supply Waveforms

[cruff]

With the talk about the HP-35 being powered from a LiPo battery, I got interested in how the power supply actually worked. So I constructed a similar version from parts so I could probe it with the oscilloscope and understand how it works. I used the schematic from the HHC 2016 HP calculator power supplies presentation as a guide.

I managed to find a suitable toroid that came close to the inductance values of the schematic along with 2N3904 for the NPN and an old RS-2021 for the PNP transistors. Interestingly, I didn't get the circuit to behave and approach the design voltage outputs until I replaced C2 with a 47 nF capacitor in place of the specified 470 pF.

The diagram shows the waveforms captured at (yellow) Q1 base, (light blue) Q2 base, (purple) the "top" tap of L1, (dark blue) the L2/L3 tap. Specifically, I was interested in what drove the oscillation (a blocked oscillator). This is done by the saturation of the magnetic field which causes the voltage at the top tap of L2 to collapse, driving a negative pulse through C2 to the base of Q2, forcing it hard off (diode D1 acts as a reverse voltage clamp to protect Q2 from a large reverse base-emitter voltage). Q1 acts as the bootstrap for the base of Q2, starting the first cycle, as well as tuning the frequency to keep the Vcc voltage (+6) at the desired value.

When my test circuit was driven with 3.6V at 100 mA, the frequency was about 67 kHz, the extremum values of the waveforms were:
Q1 base: 2.457V, 3.250V
Q2 base: -1.187V, 1.25V
L1: 0.156V, 8.125V
L2/L3: 0.156V, 6.25V

And the resulting voltages were: -9.22V, 5.22V, 7.03V.