HP33C - press #7 but get #4


I recently spilled some coffee on my calculator. I disassembled and cleaned it with some alcohol.

When I ...
press #7 I get a #4
press #8 I get a #5
press #9 I get a #6

I was thinking that it might still be wet under the keypads on the circuit board. Would baking the circuit board at a low temp dry it out if that is the problem?



do these keys also generate double touch?

I want to dare another question, but I am almost sure you're gonna blame for that: did you disassembled the keyboard so you could clean the calcualtor? If so, (God help me!) are there any chances you build... the keys... in... say... dif... ferent... positions?

If not, please, forgive the questions. It's so obvious that they are in the same column and neighborg lines that I could not help myself asking you about it. The numeric keyboard should look like this:

[7]  [8]  [9]
[4] [5] [6]
[1] [2] [3]
[0] [.] [R/S]

As you supposedly rebuilt the keys facing the back of the front half ot the calcualtor, MAYBE you reversed the order (I once did it, but I saw what I'd done before closing the calculator).

If everything is fine, I cannot understand the error, unless you have a short-circuit in the keyboard lines (but I cannot figure out a way it would reverse both lines of the keyboard).

Best regards.


Hi, Jeffrey;

I assumed the keys have reversed their functions, but I read your post again and you mention a one-way changing, say, you get 4 when pressing [7], BUT you probably get 7 when you press [7], right?

If so, by some reason, there is a short-circuit in two lines of the keyboard: one for the [+] [4] [5] [6] row and the other for the [-] [7] [8] [9] row. In time: please, check for [-]: do you get addition when you try to perform subtraction, too?

Sorry for my previous post; I was about to delete it, but I'll keep it to remind myself I also write silliness, too. Well, at least not everytime.



This is a very common problem in spice and woodstock machines that have some kind of contamination (often invisible) on the keyboard. If you have removed all the gunk, it should go away once it drys out. A low oven (say 120-140F) overnight is a good start for drying out.

On woodstock machines I have seen this when battery spewage gets into the keyboard holes. Often they keys have a dull crunchy feeling when you press them. Try soaking the keyboard in vinegar for a few minutes until any bubbling stops, rinse VERY well, shake it very vigorously, blow with canned air, wipe the keyboard dry, repeat the shake, blow, dry cycle until no more water appears, dry in a low oven. Presto... nice, clean, snappy keys. If any keys have bad contacts clean them through the holes in the keyboard with a tiny wire brush made of three wire brush bristles twisted together and trimmed square.


I took everything apart and cleaned it thoroughly. I dried it in an oven overnight. However, I still have the same problem. Could I have shorted out something in the chip when I turned the calculator on when it was wet? If so, can I buy a replacement chip for my HP-33C?


I am still pretty sure that the problem is a contaminated keyboard. I have seen the exact problem many times and this was always the cause. Sometimes the mystery gunk gets under a key dome and shorts out the etches there where you cannot see them. I use 91% isopropyl alcohol to clean my internal circuitry with with. Keep it off the case and display lens.

The only source of chips is a scavenged machine.



there are two basic types of Spices, as for the inner "guts" (building "blocks", if I may). One type, the one wherew IC's and LED assy is just "sanwiched" without solder, is a lot easier to clean than the other, with soldered IC's. In fact, I have cleand some spices of the "sandwich" type and I have an HP31E that is not this type (I did not disassembled its keyboard "yet").

David, do you have any experience on disassembled the keyboard of soldered-type spices? Are they easy to deal with or not?

Thank you.


The soldered keyboards have the metal disks that are held down by a big piece of adhesive tape (like the HP41). I don't think there is an easy way to fix them. They do have the little holes under each key where you can do the wire brush trick. Unfortunately, the 4 and 7 holes are under the 40 pin chip. As a last resort effort you could try ultrasonic cleaning followed by a long, low temperature oven dry. Hopefully the adhesive tape will not be damaged by the bath.



as you mentioned, MAYBE there is still some gunk right below these keys, and as we have two cooper trails shared with [4], [5], [6] and [7], [8], [9] rows, chances are the problem is under the main IC.



I have not followed the traces, but yes it could be under the chip. It is interesting that in the woodstock machines that show this error that the 4/7 combo is the most common followed by the 6/9.


It's fixed!

I disassembled the calculator [again] today. I peeled the clear adhesive tape with the with the metal spring dots off of the circuit board. I used an acid brush with a water & dish soap solution to gently scrub both sides of the board. I rinsed the board with IPA. I carefully inspected the keypad areas on the board to ensure that they were not covered with any of the residual sticky adhesive. I dried the board for 20 minutes in a low temperature oven [50C]. I also carefully cleaned the metal dots on the clear adhesive tape with IPA and a cotton swab. I didn't put the adhesive tape in the oven for I feared that it might shrink. I reassembled the calculator and everything worked just fine. Thank you Vieira & David for the suggestions.

I do have another question - I have been using 2 AA alkaline batteries connected in series to power the calculator. The supply voltage of the 2 AA batteries is 3.0 volts. Everything seems to work OK. I know that the original battery pack specification was 2.5 volts. Is there a risk of damaging anything with the 3.0 volts supplied by the 2 AA batteries?


Oooh, a lot of risky work there to get it running properly. I'm glad the adhesive remained useable and did not tear, etc.

A lot of people have used alkaline cells in the machines without problem. Just remember to never plug it into the charger.

Possibly Related Threads…
Thread Author Replies Views Last Post
  HP33C help with display madjack 3 2,080 12-05-2013, 05:58 PM
Last Post: madjack
  This is not the yellow press ... Walter B 101 17,318 12-17-2012, 10:19 AM
Last Post: Maximilian Hohmann
  HP news in the press Randy 1 1,049 05-08-2012, 09:18 AM
Last Post: Walter B
  Updated: Some press articles about the new 30th anniversary 12c Gene Wright 13 3,811 09-01-2011, 08:57 PM
Last Post: gene wright
  HP Press Release about the 30th Anniversary 12c and 15c LE Gene Wright 27 6,576 09-01-2011, 07:18 PM
Last Post: Glenn Dowdy
  TI 17bii solver: after you press the CALC key Don Shepherd 1 861 11-10-2007, 01:41 PM
Last Post: Bill (Smithville, NJ)
  HP35s in the Press (Spectrum Magazine Oct 2007) Chris McCormack 5 1,770 10-13-2007, 01:54 AM
Last Post: brian healy
  HP 35s Press release Howard Owen 0 721 07-13-2007, 06:29 AM
Last Post: Howard Owen
  33S key press problem kc 10 2,523 02-05-2007, 12:42 AM
Last Post: Forrest Switzer
  HP33C - Dot/Comma link Mike T. 3 1,537 12-29-2006, 07:36 AM
Last Post: Mike T.

Forum Jump: