HP34C Fault


At the start of the Christmas break I took the opportunity to swap cases/boards in my HP33C and HP34C. Fortified with the knowledge I'd gathered from this forum and heedful of the warnings I'd seen I approached this task very carefully and with some trepidation.

In the case of the HP33C I only had the case from my original unit which had died about 20 years ago, it had been of sandwich construction and had become so umrelaiable as to be totally unusable. I did try to solder the chips to the circuit board to rectify things but with no knowledge on how vunerable NMOS was to anti static damage it was almost enevitable that it never worked again. This time I made sure I used an anti-static mat, wrist strap etc!

I'd kept the case in the hope that one day I would be able to obtain another well worn HP33C second hand to be able to use to 'repair' mine. Thanks to the assistance of others on this forum I was able to do so and having swapped the cases I now have a very nice clean unit, which is back where it belongs - in my breifcase.

My HP34C was however rather well worn and had started to look a little sad, and though it still functioned perfectly well I obtained another better looking unit as a spare, just in case. I found that this unit though in very good cosmetic condition was also showing signs of its age and in particular the keyboard action was not quite as crisp or positive as my older unit.

So having successfully swapped the cases of the HP33C I decided to do the same with the HP34C. On opening up the units I discovered that someone had lovingly and very carefully carried out some quite extensive repair and restoration work on the spare unit I had aquired to restore it to working order. I expect this machine though well looked after had been quite heavily used which explains why the key action on some keys didn't feel as positive or as crisp as my other machine - though they are both of the sandwich type construction.

Again I successfully swapped over the cases and how have a very nice HP34C on my desk at home (it is just too complicated, and heavy to carry round with me!).

No problems thus far, however, in the last couple of days the donor machine seems to have developed a faulty contact somewhere that prevents the B, GTO, ENTER, -, +, X and / keys from working.

The affected keys are all obviously all in the the same collumn so I'm proabably looking for just one bad contact somewhere. I know it works as it still passes it's self test - but only if I push the processor chip against the circuit board quite hard.

One thing I did notice was that the donor machine had thin rubber(?) membrane over the key contacts while my original did not. I kept the membrane with that front panel when reassembling the machines and as as a result the donor system is now in my old case without a membrane covering the keyboard contacts. I suspect that this _might_ be the reason why it has stopped working properly.

I have two questions -

Does anyone know what the membrane is made of and how thick it is..?

Where should I look for the faulty contact?

Thank you

Mike T.


As you correctly guessed, those keys are all one column. The keyboard rows and columns go to pins on the 40 pin processor chip, so I would suspect a bad contact there first. If you're _very_ unlucky, it'll be a problem in the keyboard flexible PCB where it's wrapped round the metal plate, but I've never had this.

You might want to get the HPCC schematics CD-ROM (or ask Dave Colver nicely for the appropriate diagrams). This disk contains a reverse-engineered schematic for the 33C. All -C spices have essentially the same schematic (different chips, and different keyboard buttons...), so this will be useful to you in finding just which pin of the processor corresponds to that column.


The processor chip has been soldered on to the circuit board (obviously to overcome the intermittent contact problems that my original HP33C suffered from). Before I risk making things worse by prodding about to see if any if I can find any dry or cracked joints I'd like to know if the problem could be a connection lose between the two layers on the keyboard side...?

Does anyone know how to I get hold of HPCC schematics CD-ROM?


Mike T.


To get the above, send e-mail to secretary@hpcc.org, and he will tell you how much (and how) to pay, etc. I think it's mentioned somewhere on the HPCC web site too.


Thank you..

Possibly Related Threads...
Thread Author Replies Views Last Post
  HP 32sii Fault Carlos Estinos 9 2,286 09-26-2013, 01:06 PM
Last Post: Csaba Tizedes (Hungary)
  RPN hp20s and lcd hp34c proposals Jose Poyan 11 2,371 04-29-2011, 08:17 PM
Last Post: Scott Newell
  Hp 71b power fault Sean Connor 1 740 09-25-2009, 11:46 AM
Last Post: Bart (UK)
  HP34C Black Scholes Model Ed Austin 2 880 05-18-2009, 07:40 PM
Last Post: Ed Austin
  HP 41CV intermitent fault Nigel Bamber 3 1,108 04-20-2009, 01:12 PM
Last Post: Geoff Quickfall
  HP34C Manual - 30 years ago Ed Austin 12 3,401 11-20-2008, 05:19 PM
Last Post: Ed Look
  HP97 fault Michael Tortorella 12 2,341 09-13-2007, 04:46 PM
Last Post: Chan Tran
  HP34c decimal point/comma interchange for display JaSon 2 949 04-30-2007, 05:58 AM
Last Post: Mike T.
  HP34C Keyboard Problem Mike T. 3 989 08-26-2006, 01:16 PM
Last Post: Mike T.
  HP34C Pyerre 14 2,525 06-06-2006, 02:11 AM
Last Post: Eric Smith

Forum Jump: