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HP 15C: Difference between revisions
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== Repair & Modification == | == Repair & Modification == | ||
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. For machines made in the transition period with provisions for both SMD and throughole construction, it is possible to cut the traces leading to the original capacitor or inductor, and solder new ones in place on the SMD pads. This mod is also applicable to the earlier machines as success has been reported on 1LF5 machines, which the screen and ICs are mounted on a separate PCB which is connected to the keyboard through a flexible link, although the exact procedure is not know. | 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. For machines made in the transition period with provisions for both SMD and throughole construction, it is possible to cut the traces leading to the original capacitor or inductor, and solder new ones in place on the SMD pads. This mod is also applicable to the earlier machines as success has been reported on 1LF5 machines, which the screen and ICs are mounted on a separate PCB which is connected to the keyboard through a flexible link, although the exact procedure is not know. | ||
In addition, for older HP-15C with throughole components, the bypassing capacitor that filters the powerline, and provide standby power for retaining RAM content while changing battery, is a 22uF electrolytic capacitor mounted inside the cutout under the battery compartment. For SMD type, this is replaced with a surface mounted 22uF mounted under the X-key position, far from the LC. This should ideally be replaced with a smaller capacitor near the LC and a larger one on the other pad, for both better filtering and longer standby time with increased standby current draw at higher clock rate. | |||
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, especially for small capacitor and inductors sourced through internet, should both be factored into when considering such a modification. | 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, especially for small capacitor and inductors sourced through internet, should both be factored into when considering such a modification. | ||
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| rowspan="2" |1LM2 | | rowspan="2" |1LM2 | ||
(855kHz) | (855kHz) | ||
SMD | SMD | ||
|Replacement of both | |Replacement of both | ||
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| rowspan="2" |1LM2 | | rowspan="2" |1LM2 | ||
(831kHz) | (831kHz) | ||
thruhole | |||
| rowspan="2" |Addition of inductor | | rowspan="2" |Addition of inductor | ||
|27uH | |27uH | ||