Amort strange behavior on HP 12C

[rarylson]

Hi.

I have a HP 12C Gold, and I'm getting a strange behavior when using the amort key.

For example, if I set payment mode to end (g END) and calculate a uniform serie compound with n=10 (10 n), i=10 (10 i), PV=10000 (10000 PV) and ask for PMT (PMT), I got about -1627.45 for each payment.

So, if I amort one period (1 f AMORT) to create a price amort table, the registers in the screen are X=-1000.00 (amortized value) and Y=-627.45 (interest payment). And if I ask the calculator the remaining PV (RCL PV), I got 9372.54. Evething seems ok.

And now I ask the calculator the financial registers, i=10 (RCL i), PMT=-1627.45 (RCL PMT) and FV=0 (RCL FV). However, when I ask n, I got n=11 (RCL n), but I expected to get n=9 because now there are nine periods to pay the remaining amout of PV=9372.54.

I think n=9 should be the expected value in the n register, because the new situation is equivalent a new uniform serie compound with PV=9372.54, i=10, FV=0 and n=9 (that would return the same PMT=-1627.45). Setting n=11 seems like a bug or an inconsistent value.

So, why n=11 (incremented by one) instead of n=9 (decremented by one and consistend with the remaining debt)?

[BartdB]

~~~ deleted~~~~ (see my updated reply below)

[Maké]

Hi!, @rarylson:

See ...

https://www.youtube.com/watch?v=T_Yqqwj_1JY

https://www.youtube.com/watch?v=efxpIbDGNlc

https://support.hp.com/rs-en/document/c01855972

http://homepages.wmich.edu/~balik/calculator/hp12cloanamort.htm

HP 12C steps to calculate Annual Mortgage Constant f - iSites

hp 12c financial calculator - Calculatrices-hp

[BartdB · Accepted solution]

Hi,

On the HP-12C you should not run the TVM before doing AMORT.  See p54 of the PDF manual:

http://h10032.www1.hp.com/ctg/Manual/c00363319.pdf

If you do run the TVM (e.g. to calculate PMT), then n must be set to 0  before doing AMORT

See p56 of the PDF manual:

If you want to generate an amortization schedule but do not already know the monthly payment:
1. Calculate PMT as described on page 46.
2. Press 0 n to reset n to zero.
3. Proceed with the amortization procedure listed on page 54 beginning with step 6.

It seems the 12C adds to n when the AMORT is used. e.g. if you don't reset n and look at the 10th period (10 f AMORT) then RCL n it will be 20.

So whenever you have calculated any TVM, first set n to 0 before doing AMORT.

Regards.

[rarylson]

Hi @BartdB,

Thank you for the response!!! It was a very good and clear explanation!

Adding more info... From the p54 of the PDF manual:

>To display the total number of payments amortized, press (RCL n).

So, when doing amort, n is the total of amortized periods, and NOT the number of remaining periods to amortize.

Again, thanks for the explanation!

[rarylson]

Thanks for the response. The link https://support.hp.com/rs-en/document/c01855972, for example, was very good and clear!

From the link:

> [...] every time [f] [AMORT]  is pressed, the following registers have their contents updated:

>

> [n] Contents updated to current number of payments amortized.