Any standard failure modes in old TI calculators?

How are the batteries? On some of those old calculators they worked to filter the power supply. Have you tried powering it with a lab supply? That one has 3 NiCd cells, right?

I've still got an SR-52 (the one with the magnetic card reader) around somewhere. But I never liked TI's keyboards.. felt cheap and were never 100% sure one and only one keystroke got through. Went HP and never looked back.

Best regards, Spehro Pefhany

Yup, three cells and they had leaked so they are on the way to E-waste. Luckily these leak into the bottom shell, not the electronics. Right now it's on a bench supply and a 100uF cap where the battery was. If I get it going again I'll give it three new NiMH cells and put them on a C/10 or a little less trickle. I know that's not too healthy for them but they seem to last a long time in cordless phones that way.

Same here, the HP-11C is my office workhorse. But I keep the SR-50 at the lab. Mainly because my parents gave it to me in the mid 70's when it was unbelievably expensive. I am forever thankful to them for that because it opened the door to many intricated projects. And because of the rather good quality LED it is much easier to read in bright light. No RPN though which throws me a curve every time.

Right now it's totally on the fritz again, totally erratic :-(

Note that there are about 6 mutations of the SR-50.

My guess(tm) would be that you have some manner of liquid or corrosion under some of the dome switch keys. The changes in key behavior are caused by electrolytic action. Disassembling the keyboard dome switches is a mess. However it's about the only way you're going to clean out the accumulated corrosion under the dome.

Nope. I can be done. Don't expect to re-use the clear plastic cover. I just used clear packing tape. I made a 1:1 Xerox copy of the switch contact board. I placed the packing tape backwards (sticky side up) over the Xerox copy, and placed the domes in their proper location. I then carefully stuck the tape onto the contact board. The first time I did this, it took about 5 tries to get it right.

Not that I know of. I've seen keyboard problems, dead leaky batteries, and flimsy plugs on the chargers, on the older TI calculators. However, I've worked on many more HP calculators than TI.

Good luck.

I've got the original, way on the left.

Possible. What puzzles me is that the thing worked fine 15 minutes ago and now is all dead again on the keys.

Yikes, that looks like a bear of a job.

The charger plug was the first thing that went but that was easily fixed.

Thanks. Not looking forward to this one though.

If one of the keys is 'stuck' on due to (say) spilt wine or perhaps a fine single-malt scotch (not that I'd have any direct experience with that sort of disaster), then the scanning stops and the calculator may appear dead, or may do something else (such as count up) if it triggers auto-repeat.

If something like that happened, you might be able clean it out or not. Once it 'tracks' the conductivity seems to stay. If you can get it open without wrecking it, it should be possible to clean. IIRC it's metal clicky domes in a polyester overlay on the TI calc's but it's been a LONG time, my memory may have faded.

I wonder if putting the keyboard through the dishwasher and then subjecting it to a vacuum to dry it inside would be less risky than trying to open it up.

I've also seen the keyboard PCB crack on remotes, but that's when they use the crappiest paper-based phenolic you can buy. Your TI probably has nice FR4.

Some remote controls just seem to spontaneously fail:

Best regards, Spehro Pefhany

Take another calculator and hold down one key, any key. Now, try punching the other keys and see what happens. On all 3 calculators in my palatial office, they all refused to respond.

It's not too horrible. However, there is an alternate procedure that might have some merit. Find a syringe with a rather fine needle. The type used for insulin injections is about right. Clean out the contents and fill it with alcohol or other evaporative solvent. Punch the needle through the plastic over and UNDER the dome switch. Inject the solvent, shake well, do your best to drain, and with luck, it should work. You can dry drawing out the alcohol with the syringe, but that will contaminate the contacts, requiring a purge between every cleaning. My guess(tm) is that there's only one key that's actually full of conductive corrosion.

If you don't care about looks and resale value, you can drill a small hole in the side (not the top) of the dome. It will take some care not to hit the switch contact on the contact board or go through the board. You can then use the syringe trick quite easily. Just wrap some electrical tape around the drill to keep the drill from going in too far. A Dremel tool should work.

I've had to mold my own rubber plugs:

It really doesn't take immersion or spills to fill a dome key with liquid. High humidity will do the job quite nicely. Just move the calculator from a cold vehicle, into a warm office a few times and condensation will do the job. Gold plated domes and tin contacts are also not a great galvanic combination.

One of my many side businesses is calculator repair. For example: Troubleshooting is easy. If it moves, it breaks.

If the keyboard domes were vented, that might work. However, without a way for the suds to get under the domes and back out for drying, I don't see it doing much good.

Yep, it's FR4. Later cost reduced models were phonolic.

What size nail were you hammering with the remotes?

Right, the polyester is glued right to the PCB, right? Should seal it against ingress of fluids, but also prevents cleaning.

Bitch chewed it. Female Golden Retriever, to be specific. I'm sure that there are people who would try to return something like that under warranty. After all, Panasonic didn't have to make it so tasty and bone-shaped.

Best regards, Spehro Pefhany

I have an HP35 that still works! And I still use it.

Joerg schrieb:

Hello,

if you are really used to RPN, you have problems using another calculator without RPN.

Bye

True, and I do. But it does train the brain so I like the challenge, just like driving a car with right-hand steering. I also am sometimes forced to use non-RPN because I often don't take my HP on the road. The usual, some innocent EMC job and then they need me to also fix internal noise and calculate a filter, on whatever calculators they have. Lately on guy handed me his smart phone turned calculator. No RPN either and then you have no time to load a new app.

No joke: This can help in at least pushing out diseaese like Alzheimer's.

Unfortunately that's the case. Jeff has described how to fix it but it seems to be a rather onerous procedure.

That looks like the "Golden Retriever effect" :-)

At least there's a pinout for the scan chip:

That would be another option. I don't think these SR-50 have much of a resale value and I am not concerned about that anyhow. I could just buy a new calculator for $30-$40 but it has some sentimental value to me. That sentimenatl value has its limits though :-)

It's polyester (acetate) film is glued to the PCB to hold the domes in place. After 40+ years, the glue tends to be rather dried out and brittle. It should just fall off after a little prying with a spatula. The film is rather thin. I would not expect it to survive. I've tried using household cellophane film as a substitute. It works, but gluing it in place was tedious and a bit difficult. If I did more of these, I would probably make a fixture for locating the domes. If I didn't care about repairability, I would probably use RTV to tack the edges of the domes at 2 or more points, and not bother with the acetate film.

That's what you get for training the dog to fetch the remote.

I'm in the repair biz and I see lots of human damaged equipment. Methinks it would be in the public interest to offer classes on how to properly handle and care for electronics, entertainment devices, computahs, etc. I've seen some amazingly abused and trashed examples of consumer misuse, some of which they attempted to return under warranty. Judging by what I've seen at the recyclers, using a TV remote as a hammer is actually quite common.

I couldn't find one that's available for parts, but here's a working SR-50 for $47.50:

In my TI LED calculator parts box, all I could find were two TI-55 calculators, which are quite different inside.

The old joke is something like "I replaced the handle, and my brother replaced the head, but it's still my fathers axe". I don't think he'll mind if you just replace the keyboard.

John Larkin schrieb:

Ciao

Reinhard

I'd expect some evaporated electrolyte condensed somewhere it doesn't belong. I expect you'll have to clean all the boards very thoroughly.

In the case of an HP35, that would be '1+', I suppose.

Jeroen Belleman

I carry my HP11C (emulator) everywhere I go. Well, I don't carry my cell phone around the house. ;-) Loading a new app on my cell phone takes all of 30 seconds.

I can't stand "algebraic" calculators. No two are the same. There are always minor differences in the way operations are chained.

Yeah, suicide is faster.