HP-71B with a dead piezo speaker - repairable?



#2

Hi All,

I've got an HP-71B on which unfortunately the piezoelectric speaker doesn't work anymore. The best it does is emit a very faint "squeek" on a beep or tone command (and won't do it again until you wait for a few minutes).

I don't know if it is the speaker itself which doesn't work, or if it is the driving electronics.

Does anybody know something about this?

Also, where could I find some instructions on how to disassemble the HP-71B, or at least some pictures of the inside, as I'd rather know what to expect before I starting taking all the screws off :-)

Regards,

Philippe


#3

I recommend buying a T6-size Torx screwdriver for your project. If you have the metal-backed 71b with the two very small brass screws on the back- use caution- they are small enough to obey Hisenberg's Uncertainty Principle if removed. - My $0.02 stimulus package


#4

Ah wait <rumble><rumble>...damn! The smallest I've got is T9. I guess this can wait until I get the proper size. And yes, it's a 1984 model with the metal back, so I know I've got to be careful with these small screws.

Say, I wonder, assuming it's the piezo itself which is dead, if one from a dead HP-41 would do? (I've got one lying around) I guess I'll find out when I've disassembled the 71B.

Philippe


#5

Piezo elements are very common and inexpensive. If you don't have a greeting card with one that you want to use, buy one or even a hundred from someplace like Mouser, from the lower part of the page http://www.mouser.com/catalog/637/1814.pdf , figure "S". One of about 20mm diameter should be fine. I've used this kind of thing for many projects, even cutting them down with scissors to fit narrower spots. They never fail by themselves, but you do have to be careful not to damage the plating on the center part when you solder to it.

Edited: 22 Feb 2009, 9:29 p.m.


#6

Quote:
Piezo elements are very common and inexpensive. If you don't have a greeting card with one that you want to use,


I like the greeting card idea...


Piezo speakers are such a pain to solder. I suggest keeping the
leads attached to the donor speaker/squeaker and either connect
to the old leads or directly to the circuit board.

Ren

dona nobis pacem

#7

I'd first try to solder the wires to the beeper, on both the pcb and the beeper itself. Maybe you only got cold solder joints. Since the beeper vibrates when making tones, this may be stress to the solder joints. I found this problem on several 71 and 41 calcs, the symptoms were similar to yours.

The following link is pointing to the 71B service manual, it gives detailed disassembly instructions and shows the beeper's driving circuit: 71B Service Manual

In case you lost the two tiny brass flat head screws, these are size 00-90 x 1/8". You might find them in a model railroad shop.

Good luck!


#8

Thanks Rainer,

Ah! I knew I had seen this manual before. It's embarrassing because I remember now that it was at the exact place you pointed me to.

Thanks for the info. As indicated in Gareth's comments (thanks Gareth), and implied in yours, it's unlikely the piezo itself would have failed.

I'll check this out and let you know how I got on (I need to get the right screwdriver first though).

Philippe

#9

Rainer, Thank you for the link to the HP-71B service manual, I had never seen the manual. Now I have a lot to read. Thanks!!

#10

Hi Philippe,

I second all the advice you have received from our fellow forum members.

As you requested a picture of the innards, you'll find one below, showing the speaker location.

Pretty easy to access and solder after opening the Titan.

And no, you won't have all those red wires all around. I've wired them to replace the zebra connector which also frequently fails in the 71B as it does in the 41C.

Best

Etienne




Edited: 23 Feb 2009, 4:22 p.m. after one or more responses were posted


#11

Looks almost like mine :-) Would be interesting to know, how many 71Bs are still in the original state using the zebra connector. I got both of mine defective, and both perfectly work after I replaced the zebra connector with wires.

@Philippe: Hope your little gem is soon back to healthy condition.

@John: the site I linked to above has a lot of more service manuals and other docs, it's a great complement to the museum dvd of this site although it's not solely dedicated to HP pocket calculators.


#12

Hallo Rainer,

Over the seven 71B, I have repaired, 3 had a severely damaged zebra strip, such as this one:

On the 41 fullnuts, also prone to this kind of failure, the zebra strip can be replaced from a parts machine but one day, will always end up like this:

These were the times when Hp quit the connectors they used on valuable former models such as the beloved, unmatchable, elegant Hp-80:

Or maybe was this earlier with the Spice series...

Anyway, I only recall seing these connectors in Classic & Woodstock series but I stand corrected if somebody has a better info.

Best

Etienne


#13

Hi Etienne,

Thanks for the picture of the HP-71 innards - and now I'm going to be even more careful opening it up in case I damage this zebra connector...

I should know very quickly what's wrong with the piezo speaker.

Merci et bonne soirée ;-)

Philippe

#14

Well, maybe I should get my 41CV out of its box and repair it! I think the only problem I have is some corrosion to the zebra...maybe that is worse? Ultimately you have to wire it up? And how to prevent corrosion continuing...and what sort of soldering equipment is best?

#15

A zebra strip is an elastomeric compression-type connector with alternating conductive and non-conductive layers that go perpendicular to the PC board. It is not a flex-circuit-type ribbon. http://www.fujipoly.com/design/genDesignGuidelineArticle.asp?ContentItemID=dga_1010 shows the scheme.

#16

Progress report: zilch.

I did try, but all the five TORX screws have basically seized up. By soaking with WD-40 I managed to free up one of them. The other four will have to wait because I've eroded all the teeth on the screwdriver (cheap).


#17

Cheap tools aren't an option.

Never!


#18

I agree, but I had just borrowed a set from the office which turned out to be a cheap one :-) I'll go and buy myself a proper T6 screwdriver...


#19

I'd like to let you know how I got on. I've got partial success...read on...

We had left it with screws stuck, and no screwdrivers...

After a few days I managed to get some proper T6 screwdrivers, and the WD40 having done its magic in the meantime, I was able without effort to remove the remaining screws.

After opening up, it was easy to locate the beeper. By the way, my HP-71B being a 1BBBB version from 1984, with the metal back, it didn't quite look like the picture that Etienne posted on 23/Feb. For a start, there wasn't this zebra connector prone to failures - I had a flex cable joining the PCBs on short sides of the two halves of the shell. (I'd happily post a picture, but the only way I've read to do that is to ftp the file to the site's incoming directory, and wait for David to move the file manually. Isn't there a more automated way?)

The wires from the beeper to the PCB looked ok. Nevertheless I re-did the soldering on the PCB side, then tried. It worked, of sort; faint beeps. Then it stopped working. I jiggled the wires, resoldered on the beeper itself, tapped and prodded. Eventually it worked again, and I became suspicious that the beeper itself was simply "glued-up" from not being used in 10 (or 20) years, so I wrote a short program to exercise various frequencies and volumes, and let it run for 2 hours.

So far, so good. The next morning, it still worked loud and clear. The next day, it still worked loud and clear. I didn't check for about a week, and tonight it wasn't working again, just like when all this started. Just a faint 'pic' sound. A few BEEP commands later and the sound is quickly better and clearer. Over the last few minutes, it's been exactly like that: it works eventually, then I leave it for a few minutes and it's back to square one again.

I don't know if this is an electronic problem. From the symptoms, and especially from the fact that it initially worked for hours, and was ok the next two days, it feels like it's the beeper which is past its best-by-date (and I haven't got an oscilloscope to check the signals).

I'm reluctant to remove it until I've got a replacement, but what's its technical data (apart from "coming from a greeting card";-)?


#20

Quote:
I'm reluctant to remove it until I've got a replacement, but what's its technical data (apart from "coming from a greeting card";-)?
Different ones will have different resonant frequencies, but those will be changed by the surface it's taped onto or the enclosure you put it in anyway. They also have different capacitances, but it won't matter at all for this application. You could just get one that's about the same size and it will be ok.

But although one could get damaged in an accident, they do not go bad simply with time. You seem to have a different problem.


#21

Quote:
You could just get one that's about the same size and it will be ok.

Pardon my ignorance, but besides frequency and capacitance, these things also have rated voltage, e.g. 12V, 15V, 30V. Does that really not matter?

Quote:
But although one could get damaged in an accident, they do not go bad simply with time. You seem to have a different problem.

Ok, but any idea what that could be?

Thanks for the help.


#22

Quote:
Pardon my ignorance, but besides frequency and capacitance, these things also have rated voltage, e.g. 12V, 15V, 30V. Does that really not matter?
They're kind of like a speaker. More signal voltage makes them put out more volume. The 71 won't put out enough signal voltage to damage any of them though. If exact sensitivity or issues of sound quality (low distortion, good frequency response, etc.) were a consideration then yes, you'd be looking for details; but that's not the case. I've built countless workbench prototypes of our products over the last 20+ years using these piezoelectric elements so the beeps could be heard without always having to use headphones which will be used in the final product in the cockpit, and never seen a bad one, never seen one go bad, etc.. I just drive them with an AC signal coming from 5V CMOS logic. These prototypes are usually to prove the software before the product is built, so all I need from the piezo element is the annunciator beeps, much like the 71 produces. I know I could take out any of the old breadboards and blow the dust off them and fire them up and the beepers would still be the same as I left them many years ago. In fact, sometimes I have re-used them from old things I didn't need anymore.
Quote:
Quote:
But although one could get damaged in an accident, they do not go bad simply with time. You seem to have a different problem.
Ok, but any idea what that could be?
If it's the piezo element at all, it had to have been bad right out of the factory (which I doubt); but if that were the case, I think you'd see a crack, delamination, or something like that. I think you have a problem on the circuit board, but I don't have any info on what's in there. Has the unit experienced some sort of accidental abuse? I would suspect something like a cracked surface-mount solder connection (although those are more common in the newer products made with solder with no lead in it), or a cracked chip capacitor (which is something you don't need in series with a piezo element anyway). You might need to resort to fixthatcalc.com since you don't seem to have any test equipment.

Edited: 8 Mar 2009, 9:44 a.m.


#23

Ok, thanks Garth.

There was nothing I could see in terms of cracked solder connection or anything else like damaged capacitor or the piezo itself, but of course, besides the obvious, I may not be looking in the right place.

Effectively, the conclusion is that without testing equipment, a full diagnosis is beyond me.

I've thought of fixthatcalc, although I think that they are still not accepting new orders. Because I'm in the UK though, this makes fixing this rather uneconomical.

The service manual gives the full instructions for diagnosing speaker problems, but I need an oscilloscope. Of course, I could get one, but unless I'm going to use it for other repairs, this is an expensive proposal. I'd also need a proper workbench, etc. (Sigh)

You asked about accidental abuse: I wouldn't say that there was any. There is an indentation on the metal bezel above the display obviously from a knock, but it's minor; that's about it.

Oh well. I may carry on poking at it from time to time, but I think I will have to accept the problem.

Thanks for all the help.


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