HP-45 calculator


I came across this forum as I was searching for info on a HP-45 calculator. Actually my dad has one since about 30 years and is still using it. Unfortunately, and much to his frustration, he had a little “accident” with it. The calculator was hooked up to the charger (batteries were weak, but are till OK) and the unit was powered on. He was wearing some leather shoes at the time, walked over the carpet and when he touched the calculator he discharged himself!
The unit flickered, he turned it off & on again, it powers up but not all functions work properly. The bottom row seems all messed up – i.e. the decimal point does not function, divisions don’t work, the number Pi is gone and I think it does a few other weird things.
Having been in the electronic service business for a number of years (TV & VCR repairs), he would like to repair the unit himself. Does anyone have a suggestion of what could be done to fix the problem?

Thank you very much in advance!



I can't help directly with the troubleshooting, but last I checked 45's were fairly easy to find on ebay. He could pick one up as a replacement, or a source of parts if he decides to have some fun and troubleshoot/repair it himself.


It sounds like you killed one of the HP custom chips, most likely a ROM, but maybe the C&T (Control and Timing) chip. You have to get parts from another old HP calculator -- I think you can get the C&T from a 35, 45, 55, 70 or 80, but the ROM has to come from a 45.

I am pretty sure HPCC has a reverse-engineered schematic of the 45, so if you need help in identifying signals, etc just e-mail me.


It does sound like that one of the signals that scans the keyboard was damaged.

There is a chance that just opening the machine and cleaning all the internal contacts and particularly the CPU and keyboard with 91%+ isopropyl alcohol may clear up the problem. A lot of these machines had hand soldered joints and HP did not clean up the flux. Over time it causes problems with the high impedance signals within the machine. A little mechanical or electrical jolt can send it over the edge.

There are two screws hidden under the upper corners of the back label and two more hidden under the bottom feet (the feet are just flaps that can be lifted from one side).


Thanks for all of your suggestions!

Tried cleaning the contacts as you said, but unfortunately this did not solve the problem. Anyway, will keep looking for another HP45 either in good working order or one that can be used for parts.
What is the part, identification or location number of the chip(s) that most likely would have to be replaced?


Since all the chips are HP custom chips, the only source is another machine... which if you have then you would probably not be fixing the one that you do have. Most likely chips would be on the CPU card and not on the keyboard. Isolating it down to a particular one is probably not possible without just swapping chips... something that is likely to cause even more problems. I suggest that you try and find a working machine in poorer cosmetic shape than yours and swap the CPU board.


The advantage of doing component level repair is that you can often make one good machine out of 2 broken ones. In this case, it's likely that the A&R and RAM chips are good, so if you could find a machine with a fault in that area, you could use the parts from it to fix this one.

It is very unlikely the chips on the keyboard PCB have failed -- they're the display drivers (OK, the clock generator is in the anode driver, but since the machine does something, the clock is running). The fault is almost certainly on the logic PCB.

Since the machine powers up and does something it's unlikely the test points will be much help (the waveforms on them will look about right). So what I'd do if it was my machine is start by replacing the ROMs (starting with the one with the higher number, since I guess that appears later in the memory map, and it's likely the initialisation code, in the 'lower' ROM is intact). If that didn't cure it, I'd try the C&T chip.

I've desoldered and resoldered countless chips in HP handhelds without problems. It doesn't worry me at all (even on machines a lot rarer than a 45...)


Hello, just out of curiosity, what does 'a lot rarer' mean for a person like you ?

A prototype HP23 ? A Yellownut 41 ? A dual-led display HP65 ?

Puzzled !



I have this calculator that is working perfectly until a recent die-out. After being repaired (cleaning the corrosive circuits as I was told), one of the LED display (second from the right) died. Can this be repaired too? Thanks in advance for any tips.


In a 34C or other SPICE series machine with the pressure contacts almost all display problems are caused by a bad connection.

The other problem that can cause a single bad digit is either a bad LED module or a bad cathode driver (in the big 40 pin chip in a Spice machine). Only source is another machine.

Spice series LED modules are not very reliable. They tend to get bad connections due to corrosion working its way under the display lens. They cannot be fixed.


Well, I've stripped down a 95C 'for fun' (i.e. not because I needed to) -- and before you all jump on me, I'll add that it did all go back together afterwards. That's probably the rarest _HP calculator_ I've worked on (and it wasn't mine). When it comes to minicomputers, peripherals, and workstations, I've worked on machines where there are only a couple of known examples around...

Possibly Related Threads…
Thread Author Replies Views Last Post
  Repair HP 45 Nanoman 3 1,376 04-12-2013, 05:40 PM
Last Post: BobVA
  Recharging original batteries in HP 45, 65 and 25 Michae Altmann 1 1,251 03-05-2013, 12:21 AM
Last Post: DavidShenk
  HP 45 versions james summers 18 4,847 04-14-2012, 02:36 AM
Last Post: Eric Smith
  The 45 with a 15C twist Matt Agajanian 5 1,784 04-05-2012, 08:43 AM
Last Post: Jeff O.
  My new kid has arrived! It's a 45! Matt Agajanian 5 1,843 04-05-2012, 05:46 AM
Last Post: Harald
  21, 45, 32E--which should be next? Matt Agajanian 14 3,700 03-25-2012, 02:48 AM
Last Post: Thomas Radtke
  If the 21 came after the 45...? Matt Agajanian 5 1,900 03-24-2012, 05:45 PM
Last Post: Matt Agajanian
  Strange HP-45 Michel Beaulieu 4 1,631 03-01-2012, 04:34 AM
Last Post: McAllan
  Custom HP 45 Jim Gossett 4 1,560 05-06-2011, 08:16 AM
Last Post: Mike Morrow
  New HP-45 Emulator for iPad Namir 4 1,506 03-27-2011, 11:13 PM
Last Post: sjthomas

Forum Jump: