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When my HP-42S broke down in 1998, one of my colleagues (also my former grad supervisor) gave me his broken HP-41CX and HP-19C. I described how I repaired the 41CX in another article. Here we'll take a look at the 19C. The HP-19C was one of a matched pair of high-end programmables in HP's "20 Series" of calculators. The 19C was the printing version of the 29C, which in turn was an improvement over the similar looking 25C. With the exception of the printing functions, the 19C and 29C shared the same keys, although in a different arrangement. The programming capabilities of the 19C (and 29C) were impressive, with space for 98 fully merged keystrokes, and 30 memory registers. Although these calculators lacked a card reader for permanent storage, both had continueous memory that would retain the most recently entered program and 16 of the 30 registers even when the calculator was turned off (a feature taken for granted today).
DEL key.
At the time that this calculator was new, I was thirteen years old and had never heard of HP calculators. Instead, I had a Commodore PR100 with 72 unmerged program steps and 10 memories, which falls far short of what the 19C can do. Of course, the Commodore cost far less as well, and I spent hundreds of enjoyable hours writing programs for it.
RepairsAs I mentioned in the introduction, my HP-19C required some repairs when I first received it. It needed a new battery pack, the keyboard had become unplugged from the main CPU board, and the tabs on the battery cover were damaged (almost broken off). I fashioned a new battery pack out of 1100mAh Sanyo NiCd cells. These were state-of-the-art in 1998, and had far more capacity than the original HP battery pack (which I think was 250mAh). Rather than rely on pressure to maintain a connection with the battery contacts, I soldered the battery pack leads directly to the terminals in the calculator.
The battery compartment cover has small tabs that hold the cover in place. These were almost broken off by someone attempting to force the cover closed. I repaired these by carefully straightening them, and then running a hot soldering iron along the crease to soften the plastic. The tabs are not as strong as they once were, but they are fairly solid. A piece of foam rubber on the opposite end of the cover keeps it securely in place. What About the Printer?
The printer on my HP-19C sort of works, except for two problems: two of the
print element dots are cracked, and I can't get any paper for it anyway. I cut
some appropriately narrow strips of paper from a roll of thermal fax paper to
test the printer, and noticed two rows of dots missing. Upon disassembling the
printer, hoping to find a loose connection, I discovered cracks in two of the
print head's heating elements. Testing with an Ohm-meter confirmed that these
were open-circuit. But even without the printer, this makes a nice desktop
scientific calculator.
One feature of the HP-42S that I missed was the equation solver, so I set out to write a simple one for the 19C. I submitted it to the Museum of HP Calculators, and it now appears in their software library. I've also reproduced it here. The HP-42S has a solver where you can provide a program for an equation of n variables, fix any n-1 of these variables, and solve for the remaining one. The solver uses the secant method, in which the two most recent guesses are used to define a line. The point where the line intercepts the x-axis is used as the next guess. When two consecutive guesses are the same, the solution has been found. I'm sure this solver is not as good as the one in the HP-42S, but it works sufficiently well for my purposes. It can get into an infinite loop on periodic functions, like sin(x). UsageUsing the solver is simple. First, rearrange your equation so all the terms are on one side. In other words, rewrite it in the form f(a1,...,an) = 0. Next, enter the equation as a subroutine with label 9. The parameters are represented by the like-numbered registers (i.e. a1 is in register 1, and so on). To solve for any one parameter, store values for all the other parameters in the appropriate registers, enter two initial guesses for the parameter you wish to solve for, enter the parameter number, and press GSB 0. See the sample problem for more details. Program ListingLINE KEYSTROKES COMMENTS 01 g LBL 0 -main entry point 02 STO 0 -store index of variable to solve for 03 R↓ 04 STO .2 -store second guess 05 R↓ 06 STO .1 -store first guess 07 STO i -compute f1 = f(R1,..,Ri1,..,Rn) 08 GSB 9 09 STO .0 10 RCL .2 -compute f2 = f(R1,..,Ri2,..,Rn) 11 STO i 12 g LBL 1 13 GSB 9 14 STO .2 15 RCL .1 -compute Ri2 <- (Ri1 f2 - Ri2 f1) / (f2 - f1) 16 × 17 RCL i 18 STO .1 -move old Ri2 to Ri1 while we're here 19 RCL .0 20 × 21 - 22 RCL .0 23 RCL .2 24 STO .0 -move old f2 to f1 while we're here 25 x↔y 26 - 27 ÷ 28 STO i -save new value for Ri2 29 RCL .1 -compare to previous guess 30 f x≠y -keep going until they're the same 31 GTO 1 32 g RTN Registers
Sample ProblemThe net resistance, R3, of two parallel resistors of resistance R1 and R2 is given by:
This can be rewritten in the form f(R1,R2,R3) = 0 as follows:
The following subroutine implements this equation: LINE KEYSTROKES COMMENTS 33 g LBL 9 -solver uses subroutine with label 9 34 RCL 1 35 RCL 2 36 × 37 RCL 1 38 RCL 2 39 + 40 ÷ 41 RCL 3 42 - 43 g RTN What is the resistance of a 5kΩ and 10kΩ resistor in parallel?
The answer is 3.3333kΩ. What resistance is needed in parallel with a 10kΩ resistor to give a 2kΩ parallel resistance?
The answer is 2.5kΩ. If the program cannot find a solution, it will eventually end up dividing by zero, which will display Error. For example, what resistance is needed in parallel with a 10kΩ resistor to give a 12kΩ parallel resistance?
It's not possible to put something in parallel with a 10kΩ resistor and
end up with a higher resistance.
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