HP 15C: Difference between revisions

Later models of HP-15C, powered by a 1LM2-0001 processor derives its clock signal from a parallel LC oscillator. Roughly around 1986 this circuit changed from throughole components mounted on the screen side of the PCB to using SMD components on the backside of the PCB. This allowed easy modification as only four rubber pads and four screws are required for access to it. As delivered, this circuit is composed of a 180uH inductor and 180pF capacitor. Since the oscillating frequency of a LC circuit is determined by f = 1/[2 x pi x sqrt( L x C )], swapping in a smaller capacitor or inductor will expectantly increase the clock frequency of the HP-15C, achieving an overlock. This procedure is also applicable to similarly constructed Voyagers. A caveat, however, is a 12-20pF (value of 15pF has also been reported) parasitic capacitance has been reported as a result of the circuit design, with the larger value likely a result of manual soldering of SMD components. This, as well as component variability, should both be factored into when considering such a modification.

The quality factor of a series RLC circuit is Q = 1 / R x sqrt( L / C ), and ideally, care should be taken to maintain a similar (if not larger) L:C ratio for modifications. A high quality factor indicates a less damped oscillator that dissipates less energy per cycle and oscillates more consistently around its central frequency.

[https://www.hpmuseum.org/cgi-sys/cgiwrap/hpmuseum/archv016.cgi?read=99306 Gerson W. Barbosa] reported, in Septermber of 2006, of:

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