RUBBER KEYPAD under SPICE calculators

Hi guys,

there is supposed to be a rubber pad between the keys, and the dome switches, in a spice calculator.

But I learned something, its not that many units have got it. Its definitely better (quieter, less rattling, probably dome switches will last longer). I guess they thought of it only near the end of production?

Somebody said U can make your own out of old bicycle tubes.

What if a person wanted a more "correct" and consistent material than that. Did anybody find it?

Occasionally I buy from "Boker's" and they stamp large quantities of washers, in any and all materials. And I am before too long going to order some flat-stamped round rubber washers from them.

I could find out a suitable material, if anybody's interested. ONE MORE TIME..... if anybody could supply me with a rubber pad I would pay $$ for it. I want one sitting in front of me while I try to line up replacement material. The only rubber pad material I own is INSIDE OF my only working 34C, and NO I am not taking it apart just to look at it again. Hope to hear from somebody, and beyond that, I am going to find a good source of the material by the square foot.


Can't help with the original, but here's a source for just about anything by the foot:

Go to Raw Materials then select rubber. Their thickness ranges are a bit limited, but you can usually find something that will work. This is where I found 41 and Voyager feet material.


Get thee to a Home Depot garden center. They sell lining material for garden ponds. First type is 1/16" think EPDM roofing membrane. Makes nice rubber footsies for your your machines. The other is a thinner polyethylene or vinyl material. Looks like the right thickness for keyboard membranes. Only think is the material is sold by the liner foot (it is about 12 feet wide).


So a minimum purchase at H-D would make 8,228 HP41 feet?

Even at a 50% yield, (I can't cut straight) I'll still have enough to do a 1,000 complete 41 footies transplants. Yeah, that 'ought to hold me for a while.

Dang, I never have learned how to think in production quantities, no wonder I don't have any money left at the end of the week.


Hello David, Randy, Luiz

if one of you has already bought the pond liner,
and been cutting it to pieces, I got a good idea.

Send me 1 square foot of the material. You can
even cut it smaller so it fits into an envelope.

I will pay my way. Let me know. But then I dont
have to buy so much I can see it from outer space, and I dont even know if I like the material yet.

I STILL need an authentic pad, so I have something to compare to.

HEY LUIZ I know U got one of the genuine rubber pad pieces, can U send me a real one? I'll pay.

- Norm


Hey, Norm;

in the name of the big friendship and cooperation spirit around here, you got it. I'll send a keyboard rubber pad as a gift. You deserve it for your devotion and need to keep the original parts in your HP34C, no pun intended :)

Send me your actual address and I'll mail it to you ASAP.

I've been doing all I can do for as many MoHPC contributors as possible, and I've been receiving a lot in turn as well. Doc Mike Meyer knwos what I mean. And I am sure this means a lot to you.

I have been reading your posts in search for for the keypad, now you mentioned my name and asked for help. You got it.

I'm waiting for your e-mail with your actual address.

Best regards, my friend.

Luiz C. Vieira - Brazil

(I know I should write you in private, but I'm sure people will be happy for you as well)


Hi Luiz, thank you very much for this idea. I am grateful.

Try e-mailing me direct.

My e-mail is

When I tried contacting you the e-mail didn't work.

So if you send me something I will reply..


Hey, Norm; you're welcome.

The e-mail address was wrong: I typed "con" instead of "com". The e-mail here works fine, sorry.

Please, would you try again? Be my guest!

Luiz C. vieira - Brazil


I opened up my 34C's last weekend to lubricate the switches and generally clean up the one that I never opened before. I used the dental floss for the first time and it works great! I used a double loop (a single loop broke the first time I tried it) and tied the loop to a screwdriver handle with a triple half-hitch (I remembered a "dumb criminal" story about a couple of convicts who made a rope out of dental floss and cut their hands up escaping from an upper-story prison window.)

My original unit is the soldered type. I had opened it before with a modified crack-of-doom and twisting operation which resulted in breaking the narrow lip along the front edge, which I had glued back as well as I could. The dental floss re-cracked the end of my glue joint but otherwise didn't affect the front edge. I hadn't remembered some details, such as that the main assembly is a regular fiberglass PCB (its S/N is 2051S)- with so much discussion about the flexible PCB in the solderless design, I though the only difference was some solder! Then I opened the other unit and saw the solderless design up close for the first time (its S/N is 1934A).

My newer, soldered unit has metal disks under clear sticky tape for the keyboard click, like the Voyagers. It also has the thick rubber sheet between the keys and the tape over the disks. The older, solderless unit has a sheet of plastic over the keyboard with printed traces and the clicky domes molded into it - actually it looks vacuum formed. There is a third layer of plastic between the sheet with domes and the main flexible PCB, with holes under the domes where the column lines make contact with the row lines. There are also five holes where the column lines on the outer layer are "spot soldered" to the main flexible PCB. Now I see how much greater is the travel of the older keyboard. When switching back and forth between them, the the older, long travel keyboard seems tiring, but after using just it for a while, it seems normal.

The sheet of rubber in the newer, soldered unit isn't just flat on my unit. It has disc-shaped regions molded in over each keyboard disk.

My older unit had corrosion on the power switch but was otherwise pretty clean. I removed the corrosion and cleaned the rest of the flexible PCB pads - and the IC and display and P.S. assemply pins - with a "Pink Pearl" pencil eraser.
It worked when I lubed the switches and put it back together except some segments of the LED are open. I found that my multimeter diode test range is good for testing the LED display - it puts out just enough current to light up the segments, and the voltage readout should show up segments that are marginal - in fact I thought the minus sign segment was somehow different because it registered a higher voltage when I first lit it up (all other segments had registered about 1.5V), then I found the other terminal that connects directly to the minus sign cathode and determined that the display assembly has a regular diode connected between the two outer terminals - what's that for? It's between the power supply positive (after the power switch) and negative inputs - like a shunt reverse polarity protection diode, except without a fuse - but why is it part of the display?

I traced out a schematic of the solderless unit including the power supply and compared the two units. I noticed two differences in the power supplies: The newer unit has a 10K resistor instead of a diode to provide the standby power from the battery - I imagine that keeps the memory alive with a lower battery voltage - also, the capacitor that holds some charge for the standby power when the battery is out is 22 mf in the older unit and 33 mf in the newer one.

My newer, soldered unit doesn't "flash" (at least not as much) and the older one does, but I don't believe it has anything to do with deterioration of the IC's - they have different IC's. The units have two IC's in common - the two 8 pin packages that are not connected to standby power: 1MA1-14 and 1MA1-15 (-0015 on the older unit but I imagine it is the same as -15). On the older, flashing unit the other 8 pin is 1LB5-002; on the newer unit it is 1LB9-02. The 40 pin IC on the older unit is 1820-2162-A, on the newer unit is is 1820-2162 (no A). The 40 pin IC's are made by AMI and the datecodes are 7906 and 8047 respectively. Who knows if the A means they are different?

Regarding flashing, the difference between my units is this: when executing a function like Sine, the older unit flashes random commas and the minus sign; the newer unit is completely dark until the result is displayed. Both units flash random numbers, commas, periods and the minus sign when a user program is running.

BTW, both units return the same number for the Calculator Forensics test as the 41CX and the 15C - does this mean that the Spice models contain some kind of Nut CPU and firmware?

I also made some current measurements. The battery charging current is 150 mA. This would be the .3C "quick charge" 5 hour rate. With the battery almost discharged (just before the low battery indication starts) the unit draws 150 mA with the display set to FIX 9 just after power on (all zeros) and 190 mA after STO ENTER (all segments lit). With a freshly charged battery, the unit draws 125 mA with all zeros and 160 mA after STO ENTER. Also with a freshly charged battery, it draws 65 mA displaying "1." and from 60 to 75 mA executing the Integration function - the higher current occurs during evaluation of the function in the user program, when random numbers are displayed.

One final thing about my older, solderless unit: it has a mistake in the printing on the keyboard bezel - above the "A" key, both "FIX" and "DEG" are printed in gold! Also, the blue arrows in "->D->R" and "->H.MS->H" have a bit of gold at the very end of the tails.


Hi, Ellis;

I was wondering about to find some time to track Spice's power supply to draw their schematics... If I promse to do it with the E-series (non-continuous memory) and post it here, would you share what you have? Ahn?

Would you be as kind as for sending me the schematics? Please?

Thanks alot!

Luiz C. vieira - Brazil

Edited: 18 June 2003, 8:33 p.m.


Sure, I'll email it to you after I clean it up.


thats a lot of interesting detail. U say that your solderless one flashes, and the soldered one doesn't.

I have one solderless that flashes when you enter Sin(45) and another solderless that does NOT flash when you enter sin(45) . I absolutely do NOT care for the extra flashing and consider it a serious problem, although maybe it was just something from the first 3 months of production, I just can't stand it and start looking for the next unit rather than put up with all that ....

Luiz and I determined that the "flashy" ones will blink at each push of "2" if you enter 2.22222222
but not for any other digit.

Try the 2.222222 and let us know if it flashes ?

HEY LUIZ: Maybe the firmware is in one of the 3 little 8-pin chips. Maybe they really did make some firmware that blinked too much. They must've got rid of it fairly early on. What do you think, maybe the problem is from software that was stored as firmware in one of the 3 little chips ?


Hi Ellis,

BTW, both units [HP-34C's] return the same number for the Calculator Forensics test as the 41CX and the 15C - does this mean that the Spice models contain some kind of Nut CPU and firmware?

Not necessarily. This just means that the exact same algorithms were used; the underlying hardware can be different. The same Forensics result goes all the way back to the HP-27. So calculators from three different series -- Woodstock, Spice and Voyager, plus the 41 -- all give the same results. But this does not mean that the instruction sets are the same, although they are undoubtedly similar.

- Michael


Hey, Norm;

I think you have a point here. And the best way to determine what is going on for good is to find two Spices of the same type (solderless is better) provided that one of them blinks the "2" and the other doesn't. Then it's time to exchange components and check at which moment the blinking exchanges, too.

Because of a destiny contingency, it happens that I'm gonna be with some Spices of the same type this very weekend. Maybe we're all lucky enough they fit our needs and I can resume the experiment successfuly.

I'll let you know. Please, warn Captain Zener about any emergency call... I'll be scanning the power supply of the E-series spices (non-continuous memory) and I'll check for both: tantalum capacitors and comma/digit separator selection. Their power supply is different, as you reason about it.

Best regards Norm, folks. Thank you for your advice; I appreciate it.

Luiz c. Vieira - Brazil


Hey Luiz,
If you get 2 such units, and you swap their firmware chips,
I want to make a suggestion ....

The main litmus test for the 'blinking' problem is two things:

(a) the 2.22222222 test, everytime you type "2" it blinks, the other digits do not (like 1.11111 or 9.99999 is normal).

(b) Sine or Cosine will blink while its computing the answer.

YES, if you can clearly demonstrate its all in the firmware chip, that would be fascinating to me. BTW, I wonder which chip is the firmware chip anyway??? There are 3 little chips, and then there is the big chip.

The big chip must be a microprocessor with a special display driver.

The little chips must be memory chips, that hold the program, the register, the stack, and the firmware.

I bet you they used cheaper smaller IC's on the cheaper models. For example only, a 512 byte memory was cheaper than a 1024 byte memory. People played around with that a lot, back then. The 512 byte chip was 50 cents cheaper than the 1024 byte chip, so they'd get all excited and build 2 models (instead of use the 1024 byte chip in all models). Kinda dumb really, would be better just to make them all use the big chip for consistency.

SO ANYWAY Maybe all chips are just fine, its just that one of them has substandard firmware..... HMMMMMM ....... then maybe I could find a chip that has the good programming in it... not real likely.

I wonder how the heck they found 8-pin memory chips that would surely be downloading on a serial bit stream.

Obviously then they programmed the microprocessor to accept that sort of data.

Fascinating...... absolutely fascinating.
Beats the pants off some laptop or PDA with a 30 Gig hard-drive, I can get more done with the 34C.


Hey, Norm;

thank you for your enthusiastic e-mails; both for the HP34C and for the experiments. And, hey: there are people in here that put me in their pockets... I know, I learned a lot with them. I'm just too nosy and curious!

One thing to add: if you examine the PCB's layout, you'll see all three 8-pin DIP chips are in parallel EXCEPT for the topmost (the one closest to the LED display): its #8 pin is separately connected to the power supply. After posting some notes about it some months ago, I remember reading that this is the RAM place, where it is constantly fed (continuous memory). The other positions are ROM places, no order.

If you reason a bit about it, there is no specific place for the 8-pin DIP chips if you have an E-sereis Spice, and if you switch places in a C-type, you'll loose memory contents when you switch the calculator to OFF.

I think this should be also posted.

Best regards.

Luiz C. Vieira - Brazil

Edited: 20 June 2003, 10:42 a.m.

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