I have been trying to program an equation for discounted payback period into my 17BII (serial 3211S) and my 17BII+ (silver). In the forums, I have found Peter's excellent discounted payback solution in the following post:

http://www.hpmuseum.org/cgi-sys/cgiwrap/hpmuseum/archv019.cgi?read=153562

Curiously, this equation works reasonably well on the 17BII+ (reasonably because the answers are slightly off, but very close) but the equation fails as invalid after pressing CALC on the 17BII. I have triple-checked my typing, so it is not an entry error.

Any insight about this behaviour? Is it perhaps a memory limitation that allows equations of only a certain length in the original 17BII? (It shouldn't be, though, because the trig functions are much longer...)

Is this an unusual example the otherwise dodgy "plus" solver outperforming the original?

Is there an edit to Peter's tremendously useful formula that would make it work on the 17BII?

Many thanks in advance!