Re: HP-71B with a dead piezo speaker - repairable? Message #18 Posted by Philippe Lasnier on 7 Mar 2009, 3:53 p.m., in response to message #17 by Philippe Lasnier
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";-)?
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