The Throwaround HP-12C

This is the reveal of the improvements to the sad HP-12C - I am quite pleased with how it turned out, I still need a battery door, but I have used some black insulation tape for now, which is fine.

There was some sanding done on the bezel to remove the gold finish and most of the scratches, I used automotive polish on the screen (very gently as it is a liquid crystal display) and some cleaning of the case of course and a little paint touch up and cleaning on the keyboard area. It was about an hour or so work to bring a little life back into the calculator.

TLC on an HP-12C

I want my belongings to be pristine, and I keep them that way, and that's the way I am.

However, this calculator caught my eye on eBay and I decided it was an unwanted orphan that needed some TLC, it was scratched up, the keyboard was filthy and scratched and it had no battery door.

I know.....

It did work though and I discussed with the Canadian seller what would work for both of us, because he needed to pay shipping, pay eBay and Paypal fees, and make a little cash. We worked out in Canada he could send via lettermail (you can send anything as long as it is light enough and fits through a 2cm slot) and an extra buck for fees and five dollars for him, so it worked out at ten bucks.

It has arrived and I have done work on it, my next post will be the resulting "throwabout" calculator for on my desk so that my pristine models can sit around and just collect dust.

Days between Dates : HP-12C

 How to calculate the days between two dates on the HP-12C calculator.

This is quite straightforward, and the default method of entering a date is mm.ddyyyy so the 19th September 2007 would be 9.192007 and todays date would be 9.202020 so step one would be to key in the first date and press enter to add it to the stack. 

Then key in the second date, press the blue "g" function key and then the EEX key which gives the answer, in this example, 4750 days.

Age of Excellence

I'll once again rewind back to 1985 when I was working at McDonnell Douglas in Long Beach, California and explain myself a little as to why I am having quite an obsession at the moment with Hewlett Packard.

I bought this off a Canadian seller a week or so ago, and it arrived yesterday and made me happy, as you can see, it is not for the HP-12C it is in fact for the HP-11C Scientific calculator. Including shipping this little gem cost me C$27 and it is worth every penny in my opinion. 

Spiral bound, 260 pages, printed in the USA.

It has the feel of a piece of history and it transports me back in time to the moment I did not buy one.

If you use algebraic calculators using infix notation, schemes such as AESH, AESP or the combination system of AOS then more than likely you will, at some point in your career, be shown how inefficient it can be when compared with postfix notation, specifically RPN or reverse Polish notation.

In the engineering building at McDonnell Douglas there seemed to be a fair few engineers with an HP calculator and the "new kids on the block" at that time were the HP-11C and the higher end HP-15C both of which were Voyager revision RPN calculators. 

In 1985 the HP-11C was US$75 and the HP-15C was US$120 which was a lot of money and even though most of my peers in the office were running RPN calculators, I decided to stick with infix and ended up buying a TI-66 AESP calculator which had a lot more program steps that the HP-11C and was a little cheaper.

In 1986, my car was stolen in the UK, and that was the last I saw of the TI-66

I have ordered an HP-11C from eBay, it should arrive in a few weeks, we will review that when it arrives, in the mean time I will hold my spiral bound manual and take a peek to see what I will be attempting when it arrives.

The Brazilian versus the Singaporian

 Another HP-12C arrived today, again a Voyager model and this time the serial number is 2943B84989 which means that it was built in (1960)+29 = 1989 and perhaps week 43 in the Brazilian facility. The 84989 is the unit number and I wanted to identify differences between this one and the newer Singapore unit I received a few weeks ago.

First off, the slip case has slight differences, but that might be expected with items made in different countries, material is perhaps a slight shade of brown darker, edge pressing is deeper. If I had not taken a photo of the Singapore unit I would probably get them mixed up, except for that edge pressing which you can see on the following photo.

Top is Singapore, bottom is Brazilian.

They are both showing the result of the self test, which is switch off, hold down the on button, press the X button, release the on button, release the X button for this particular model.

The calculators look identical, however, there are subtle differences. On the 1994 Singapore unit the HP12C logo is mirror finish, the 1989 Brazilian has a brushed texture, gold plastic woodgrain almost which I think is more stylish.

The keys on both units are of the double shot molded type, that means they are durable and will last a long time. 

The flipsides....

The Singapore unit has the CE91 logo and the earlier Brazilian does not, but it does have the 871B battery type printed. The earlier unit also has thicker feet and weighs 114 grams (133 with case) versus 112 (130) for the Singapore calculator. 

So, comparison wise very similar, quality appears the same and just a few slight differences which is certainly not true for the (downhill) progression of the HP-12C to the present day incarnations.

 

Ebay arrival

This is another arrival this month, plus there are more on the way for my musings.

Another Hewlett Packard from the history pile, this one a faster business calculator that can be set as RPN or Algebraic entry. The HP-17B II which was introduced at the start of 1988, coincidently on the day we emigrated to Canada.

It is a Pioneer model, introduced in 1988 and the serial number is 3544S09832 which means that it was built in (1960)+35 = 1995 and in week 44 in the Singapore facility. The 09832 is the unit number which is a little spooky as the unit number from the HP-12C was 03298

The age difference between the HP-12C and this unit is approximately one year and nineteen weeks, and they were made in the same facility. The technology however is six years advanced and it shows when running the simple TVM calculation we have been discussing. The calculation is almost instant on the HP-19B II, perhaps half a second, whereas the older HP-12C takes about three times as long.

It was progress, and there were a great many improvements in the HP-17B II including many more functions, a built in clock and a powerful algebraic solver.

However, by the time this was released the real estate industry had adopted the HP-12C as the trendy desktop accessory, and no future business calculator would break its hold. Later variants of the HP-12C would be made with faster processors and it is still sold, albeit "Made in China" to this day.

HP-12C and the Canadian mortgage factor

Assuming all financial registers are cleared, according to the manual, the keystrokes to calculate the Canadian mortgage factor on the HP-12C are:

 

Key in 6 and press [n ].

Key in 200 and press ENTER , then PV .

Key in the annual interest rate as a percentage and press [+ ], CHS , then FV .

Press [i ].

Clear as mud, so what does that actually do?

 It is using the TVM formula to create the modified mortgage factor for a six month compound regime, prior to running the payment calculation.

 The first instruction places the number of periods, 6 months in this case, into the [n]   register and then 200 is keyed in and enter is pressed and then stored as the present value [PV]. The interest rate is entered, added to the 200 that is on the stack, the sign is changed and then it is stored as the future value [FV]. When [i] is pressed, the HP-12C runs the TVM routine with payment [PMT] as zero and calculates the Canadian mortgage factor.

 Simplified, for our previous interest rate of 6%, it runs the standard TVM calculation with n=6, PV=200, FV=-206, PMT=0 and calculates the modified interest rate to make that so. If the interest rate was 9%, n=6, PV=200, FV=-209, PMT=0 etc

 In the previous example, with the annual interest at 6% the above routine calculates [i] as 0.49386220 for use in the second TVM calculation, so key in number of periods (300) and present value (100,000) and future value (0) and press [PMT] which results in the correct Canadian mortgage payment of -639.81 (note the sign of the payment and present value are always opposite)

The only thing to be wary of is that there is a retained future value of -206 in the register from the manual key sequence and interest rate of 6%, so that has to be zero for the final Canadian payment calculation to be correct.

Code Monkey

 This might be an odd set of characters, but it will mean something to someone.

<< 200 / 1 + 6 INV ^ 1 -100 * >>

The << and >> indicate the start and end of a program for the HP48G calculator and the mode of calculation is reverse polish notation, or RPN, so you may ask what little secret does that piece of programming titled CINT unlock?

I've been playing about on the HP-12C calculator and the TVM functions calculate mortgage payments based on the USA, and perhaps the rest of the worlds, rules.

Loan Amount $100,000 with interest rate 6% per Annum, 25 year mortgage.

The HP-12C calculates that over the 25 years, the monthly mortgage payment would be $644.30

I asked my East Coast buddy, Graeme Wright, to send me his spreadsheet that we would use long ago when we had mortgages, the spreadsheet was always correct to the exact penny when compared to printouts from the banks.

It said the monthly payment would be $639.81

The question was, why the difference?

Canadian Mortgage payments are required by law to be compounded twice yearly, so for a standard TVM calculation to work, the equivalent monthly interest rate needs to be calculated. In the above example, for the USA formula, the monthly interest rate is 6/12 = 0.500 and if I return to my little piece of program code at the top, the CINT program code will do the following if 6 is the number on the stack (X) when it is started, remember, this is in RPN so it acts a little differently than algebraic logic and does not require brackets, so I'll recreate the algebraic equivalent and put them back in.

(((X/200 + 1)^(1/6))-1)*100 = 0.49386

I may have put too many brackets in there, but no harm in that....

This adjusted monthly interest rate, when plugged into the intrinsic TVM solver of the HP-12C produces the monthly payment of $639.81 which is all fine and dandy, but not so sadly here in my advancing years there is little use for this type of financial arithmetic besides the occasional daily amusement.

Just call me Chuckles.