HP 38C calculator disassembly?

Does the calculator have rubber feet on the back? There are often screws under the feet.

I had one of the original HP-35s (1972). I'd never heard of RPN, and thought "What's going on here?" After a few minutes using the calculator, I was permanently converted. Why would anyone use algebraic entry?

don't see anything specific re: 38c at the following site but it might give you an idea on how to proceed:

HP's are not easy to disassemble. Even with well written directions, you're still likely to damage or destroy your calculator. I'd suggest using some contact cleaner to get the crud out. If the membrane switch is worn out then you're only option is to replace the keyboard and replacing the entire calculator with a used one is likely to be cheaper.

HP still sells calculators with RPN. These sometimes include algebraic notation to simplify the entry of formulas when programming.

It should be noted that almost all "algebraic" calculators are hybrid, mixing RPN with algebraic entry.

A fair number of current HP calculator are bi- or tri-lingual - they support RPN as well as algebraic. The HP12C is RPN-only.

Yes.

You're going to have a problem getting the keyboard section apart. The case comes off easily. Two screws in the battery compartment and it breaks open. Watch out for the loose IC's. Take photos if you can't remember the chip directions.

The keyboard is held together by melted plastic blobs. Break them off and the keyboard section will come apart, but good luck getting it back together. You're better off immersing the entire assembled keyboard in some alcohol, and hope that the crud will wash away. Eventually, the alcohol (and water) will evaporate.

My batting average is about 50%. I've worked over about 4 of the series (HP31,32,33,37,38...). I fixed keyboards on two but also destroyed two others. The ones I destroyed were partially working when I started, and had more dead keys when I was done.

Yep. I have a small collection and still use them on a daily basis.

The HP35s has both Enter and "=" keys (although the "=" key is somewhat hidden).

| FWIW, I still think these HPs are wonderful. RPN forever! We don't | need no stinkin "=" key.

Ah yes, RPN vs. algebraic. Kinda the Mac vs PC wars of the 1970's. Or the Betamax vs. VHS wars of the 1980's.

Yep.

RPN is better, but there are many more algebraic calculators. Mac is generally better, but PC's are more common and cheaper. Beta has better image quality, but VHS didn't have a royalty attached and is therefore more popular.

Success through mediocrity.

Jeff Liebermann schrieb:

Hi Jeff

Where? I bought my HP35 in 1974 and haven't found this key till now!

Very interested

Reinhard

He should have said HP-35S. See above.

HP35s != HP35

AZ Nomad schrieb:

Many thanks to you and William! I did misinterpret the s as a plural s. Now I know that there is a HP 35 revival calculator ;-) ...

Best regards

Reinhard

The HP35 does NOT have an "=" key. The new and allegedly improved HP35s does. See:

It's above the +/- key, to the right of the Enter key.

The HP35s was suppose to be an update of the classic HP35 calculator. Unfortunately, just about everything is different. It's also full of bugs and annoyances:

I use mine effectively, but I'm still disappointed (and disgusted).

I think you are describing the early version of the 38C, and perhaps other Spice series calculators. At some point HP switched over to a new design which had all the chips soldered in the usual way. And that's what I have.

I didn't find the case to come apart easily at all. I ended up using the dental floss method of getting it apart. That's from the hpmuseum.org forum. The keyboard on mine has a metal dome for each key, and all the domes are held in the right place by a sheet of plastic tape. So I cut through the tape around the misfiring dome and removed it, cleaned it, and put it back. It seems to work fine now.

I took pictures of the keyboard and the bottom latch:

Sigh. Time to tear one apart. I couldn't find an HP38c in my pile, but did find three HP38e calculators. I tore one apart and tooks some photos:

Note that the chips are NOT soldered.

Good idea. However, I've gotten fairly good with the brute force method. I just pull the two plastic case parts apart until the locking hinge abomination snaps open. I've never cracked one. However, it does take some brute force and nerves of steel.

There's also no plastic tape domes over the keys. Instead, it's flexible PCB with domes molded into the plastic, with traces running all over the plastic. There's no sticky tape. The plastic is part of the sandwitch held in place by the edge clips. Cutting the plastic in the manner of your example would end up cutting a trace. Fortunately, removing the plastic dome sheet on my HP38e exposes the key contacts, which can then be cleaned.

Your HP38c and my HP38e are quite different.

Nice. Thanks.

Now to get it back together...

Yes, a completely different design. The early 38Cs were like yours too.

:-)

Well, it may be 25 years old, but I still love this calculator. And I didn't see any on Ebay. That's a bad sign I think.

I've been collecting and repairing HP calcs on the side for about 30 years and I've never seen one built like the one in your photos. Perhaps I'm leading a charmed life or haven't seen enough calculators. (Most common repair is rebuilding battery packs and cleaning out the damage caused by leaking cells).

It's back to gether and working with no broken plastic. Never mind that I initially installed the LED assembly backwards, missed on the slide switches, and had a difficult time with the battery board. It's been a while since I've worked on these, and you're right, the design sucks.

More like 30 years. Search again for hp35c and hp35e. There's one of each for sale.

Good guess. I just finished rebuilding my septic tank. Not fun and very expensive.

I moved the photos to:

Sorry(tm) about the grainy photos. I was in a rush and didn't bother getting better lighting or using the flash.

No soldering or sockets. Even the LED array is solderless. It's really a minimalist design.

The main PCB is actually a flexible circuit board, which relys on pressure from the clips along the edge, cellular foam backing, and stiffness in the plastic frame, to maintain electrical contact. It's generally a workable system, but requires considerable manufacturing precision. I've seen intermittents, but only when I do a sloppy job of re-assembling the sandwidth.

One nice feature is that it makes replacing active components quite easy. Unfortunately, tearing the HP-3x series cases apart is neither easy or fun. I don't think it was designed to be easily repaired.

There are also some nasty symptoms caused by this method of contruction. Here's a long thread on missing digits and segments on the display. Besides bad connections, I've found it can be caused by blown chips, probably by someone inserting them backwards (easy to do) or in the wrong position (also easy to do) and applying power:

Holy shit! Until I looked at your photo, I thought that you meant that the legs had corroded, or that the chips were socketed, or something like that. I'm amazed that they were that cheap when they were manufacturing them, or that they've lasted so long!