|HP41C power supply|
Message #21 Posted by Ellis Easley on 30 July 2003, 5:38 a.m.,
in response to message #19 by Harry (Germany)
I've been looking at the 41C service manual on the Museum CD5 (schematic on page 36 and power supply Theory of Operation on page 10) and comparing it to the re-drawn 41C schematic that has been available for a while and I think the re-drawn schematic introduces some errors. For one thing, the 14 pin bipolar chip labeled "LCD-DRIVER" on the re-drawn schematic is actually the power supply chip (U2). One of its functions is to provide three voltages (nominally 1.1, 2.2 and 3.3V on pins 4,3 and 2) to the actual LCD driver chip which is part of the Display Assembly. The power supply chip also temperature compensates the three voltages, which is required for consistent display contrast over temperature. Another function of the power supply chip is to monitor the input voltage (the power supply chip gets its operating power on pin 14 from C1 and through CR1 from the battery) and if it falls below 4.2V, it pulls its pin 5 low, which is connected to CPU pin 6 (LLD) - the text says this pin's function is low battery voltage detection.
The Theory of Operation section doesn't go into the details of how the power supply chip generates the 6V Vcc operating power supply but it describes how the CPU turns the power converter on by driving a current into pin 13 (Vci) (from CPU pin 8 through R1) and how once the converter output voltage reaches 6V, the power supply chip drives its pin 6 (Vco) low to signal the CPU (on pin 7) that power is good, with a capacitor (C5) storing the low level (between power converter switching cycles, I think).
My guess is that the resistor (R3) and capacitor (C6) on pin 7 control the time constant of the power supply chip's power converter and that pin 10 (IND) is the converter switching output, driven low to store energy in the inductor (T1), and then when the current is turned off, the counter-EMF of T1 is rectified by CR2 and filtered by C3 to generate Vcc, which is sensed by the power supply chip on pin 9 to control the timing of the switching output on pin 10 to maintain Vcc at 6V.
The service manual Theory of Operation doesn't say anything about the resistor (R2) connected between power supply chip pin 12 and ground (although there might be something about it in the troubleshooting section). The signal is labled "ISET" which would generally mean "current set". Some oscillator chips use a constant current source to charge a capacitor to establish a time constant, so this pin might control that aspect of the power converter. Or the current could control the internal voltage references that determine the output voltage and/or the low battery voltage and/or the three LCD voltages. Or, given that it is named like a current input and the pin next to it (Vci) is described as a current input, the current flowing from pin 12 to ground might establish the current level into pin 13 that represents a logical "1".
There are a lot of differences in the component reference designators on the service manual schematic and the re-drawn schematic - this might be because the two schematics represent different revisions of the logic board. There are also differences in the pin numbers on the board-to-board connectors on the two schematics. It might be that the person who re-drew the schematic assigned his own pin numbers, if they aren't marked on the PCB's (which could also explain the different reference designators). Also, the re-drawn schematic doesn't have the two transistor circuit that controls the DATA signal from the Phase 2 clock. Since both signals are connected to the CPU, maybe the transistors are eliminated by a revision of the CPU.