HP-97 Decimal Point Display


Just restored an HP-97- and can't figger out for the life of me how to change the default 2 digits after the decimal point into more! The website sez the 67 and 97 have the same functions- but the 67 has a "DSP" key that allows you to set the number of decimals displayed. HELP!!!


The HP-97 on my desk has a key labelled "DSP" between the "." and the "+" on the lower right of the keyboard. It works to change the decimal point display to the number you desire.

I think that's the one you're looking for.



Oh, man. How did I miss THAT? I looked for over an hour!!!


The power switch is in the upper left of the keyboard:-)

Couldn't resist. Been there, done that.



The HP97 should have a DSP key as well. It's on the right hand keyboard, on the bottom row (IIRC it's just to the right of the decimal point key).
Press that, then a digit for the number of digits/decimals you want. You've also got the FIX/SCI/ENG keys on the top of the right hand keypad.
IMHO that DSP key is wrongly placed. Or rather R/S, in the bottom left corner of the left hand keypad is wrongly placed and should probably have replaced DSP on the right hand keypad. R/S is used a lot more than DSP after all.


I prefer to be anonymous but what means :


Signed : a poor french who don't understand all subtilities of threads posted here !!!


I think I know what these are...

IIRC: "If I Recall Correctly"

IMHO: "In My Honest(Humble?) Opinion"

And there's probably a lot more, don't be afraid to ask.


No reason to feel shame... you can speak two languages!

IIRC - "If I Remember Correctly"

IMHO - "In My Humble Opinion"

I have a friend from Germany that once saw a sign on a car that said "4 sale" (instead of "for sale"). He said our english words are already so short (compared to German words), and couldn't understand why we americans have this need to (improperly) shorten words further; he came to the conclusion that it must be simple laziness. What could I say, he was right. Same story with these abreviations.

Here is a link that lists a few more abreviations:



Of couse now that we have mobile phones with us Text messaging needs abbreviations to make the message compact to save money and time.

I personally hope that these abbreviations do not get into our dictionaries, but I have my doubts.


I just put an HP-97 back together....
everything works except for the card reader. Replaced the rubber wheel, cleaned everything.... Mechanically, it's fine. I adjusted the switches and checked 'em out- all OK. And it draws the cards smoothly through and then just reports 'error'. Cards work fine in my HP-67. No scratches on the head, used the head cleaner card AND tape head cleaner on a Q-tip-
Any ideas about further troubleshooting?


Does it fail for reading only (i.e. it won't read a card written on the 67), writing only (it'll not write a card that the 67 can read) or both?
A common problem with the 97 card reader is bad connections on that tapewire ribbon cable between the card reader PCB and the logic PCB. I've replaced the
cable in my 97 using 26 way IDC ribbon cable and connectors. A 13 pin SIL rightangle header soldered to the logic PCB (using the row of holes in the 26 way IDC socket nearest the top of the machine) and a 26 way header soldered to the card reader PCB (with the row of pins nearest the top of the machine soldered to the PCB pads and the other row of pins trimmed back).
Works fine, even if it's not 'original'.
Ohter than that,s check the cacpacitors on the card reader PCB and the supply decoupling electrolytics on the logic PCB.


If you can write a card and not read it the problem is almost always the small tantalum electrolytic filter capacitor (usually blue) on the reader board.

If you cant read and write then the problem is almost always the HEAD switch in the reader. I have noticed that they often appear to work when a card is inserted manually but not when being pulled through bu the motor. More adjusting to the reader switches may be on order.

The other common failure mode is the ribbon cable connections.


Thanks, David and Tony.
Still no joy.
Can't either read or write. No error when I try to write, but the HP-67 won't read what I wrote.
Caps check good in-circuit (Fluke 87), and the H switch goes from 6.3v with no card to 0v while the card goes through.
I do get a signal on both heads when I write. Can't detect anything when I read. (DVM on the head terminals) but that's probably in the microvolt arena.
Why do the two heads have different DC resistances, by the way? 47 ohms and 1.2Kohms...
Looking at the business end of the head, it does look worn- with a stereo microscope, I can see the gap and it looks clean, but the head appears not to be straight- it seems to be polished or ground convex in the vertical dimension. Not what I'd expect.

Can anyone offer more advice?


You have a bad head. Both windings should both measure the same. Also, the black wire should is the ground for the head casing.

Check the head wires. They are VERY fragile and tend to break where they connect to the circuit board. Also when working on a 67/97 always record the postions of the wires before doing anything. They vary between machines.

The filter tantalum cap often measures OK, but they have a problem with bad ESR values that are very difficult to measure. On the HP67 machines with the motor speed adjustment pot in particular you should always replace the cap.


You're rignt! Just confirmed it on my HP-67-

So- Does anyone know where I might find a good head and carrier assy? Or a "parts" reader with a good head?

Waiting with bated breath.....


I believe you can use an HP67 reader head assembly, and maybe even an HP65 one in the HP97. Which may help you find a replacement.


Firstly a general point. The problem with old electrolytic capacitors is high ESR (Effective/Equivalent Series Resistance). This is not something that's tested by the capacitance range on a multimeter (not even a Fluke). You really need to desolder the caps and test them on an ESR meter.
However, I don't think that's your problem -- I think the problem is the head. There should be 2 identical windings. Normally red/yellow is one winding, blue/orange is the other. And black is the screen (shield) ground.
The 2 windings should measnure the same resistance. I think one of yours is open and you're just measuring the 'resistance' of the other components on the board (mostly the 1826-0322 chip).
Do check the connections (and you might be able to repair a broken wire at the very back of the head -- there's nothing to lose by trying. Otherwise you need a new head/support plate assemhly, I think.
Incidentally, if one winding is open then the reader won't work at all. The encoding scheme used on these calculators is to record a '1' by a transition on one track and a '0' by a transition on the other track. This is a simple self-clocking encoding, but if one track fails for any reason (such as an O/C head) then all you get is '0's (or '1's), so it can't possibly work.


hi tony-
pretty clear that the head's bad. 1.2K right at the entrance to the head, thru the insulation on the wire. Toast. Lookin' for another head assy!
Thanks for your help- you and Dave guided me to the diagnosis.

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