HP75D Speaker replacement


I bought on eBay (yeah, I know, I know) an HP75D, installed in a "pod". I played around with it for a while like that, but never could hear any "beeps". So I took it out of the pod, and noticed a hole drilled in the calculator case for a set of red and black wires...

Well, the prior owner must have been playing the beeps through his stereo or something, because the HP75D had NO SPEAKER INSIDE.

I bought a little .1w speaker at Radio Shack, which Nearly fits, being about 1mm too big around to fit in where the old speaker was to fit. I have filed a bit on this speaker, and would be installing it, except I am fairly convinced this thing SHOULD be capable of more volume than it has.

In fact, a "beep" command puts out a sound at just a bit above the edge of my hearing. I think there is a possibility that the transistor in that area could require replacement, but I am guessing I need to know where to find HP transistors or their replacements.

Incidentally, while the audio section of the calculator appears to be toward the back of the unit, there is a lonely little potentiometer towards the front. I thought of tweaking it, but then thought, oh, maybe that's for the power supply, or the card reader, or the display-- if you don't know WHAT you're tweaking, Don't....

So, my question is: does anyone have a schematic or have any ideas about what's under the hood of this baby, so I can restore it to health and musical glory?


You should have checked the message archive before you started your project. The 75D did not contain a speaker. Only the 75C has a speaker. The wires just dead end in the 75D. I think the inclusion of a modem prohibited the use of the speaker, but I could be wrong. I almost went through the same exercise when I got one. Check the old message threads.


Sorry, Don, that was my first post here and I thought I had read quite a lot on the HP75D before I bid on it, BUT that was a little gotcha I missed (odd peripheral, that pod).

Now that I search back to March 17, yes, I see it, thank you. Guess that means I should not have expected a module cover nor a battery door, either. ;-) And I didn't get a barcode wand... would anyone happen to know the model number of Wand to look for?

Back to my original question: does anyone happen to have a schematic, transistor i.d. chart, or any idea what that one potentiometer adjusts?

The wires may have led to a jack on the pod--- that's fine. I still need info on repairing this to 75D NON-POD status.

If they put a jack on the pod, did they sell a HP Outboard Speaker Unit?

BTW, the modem is in the pod itself. There are no changes, as far as I can tell, to the HP75D other than emasculating the speaker. I was a little taken aback by this, it seemed rather kludgey compared to what I've seen of other HP preipherals (that hole drilled on its side, etc).

The speaker I chose from Radio Shack was a dynamic 8-ohm job, primarily because it looked a lot like the internal shot in the Museum's photo collection, but also because the manual for the unit mentions it can reproduce 100-1400 Hz well, but draws a lot from the battery below 100 Hz. I see someone mentions a piezo speaker in the thread where this was discussed earlier-- I'll give one a try just to see if the volume improves.

A piece of aluminum, cut to size, is operating admirably to hold the battery pack in tight. As you can tell, this is for use, not a glass case. The annunciator function is rather important to me.

Thanks for all.


I have a HP75D on a POD. But there aren't any changes compared to a normal HP75D. The modifications you mentioned weren't carried out by HP. I also have another HP75D which I took apart as some of the screw post were broken. I could send you the speaker from this machine. Unfortunately I don't have a spare battery door as my POD machine also came without it. The volume of the buzzer is rather low compared to other calculators (HP41, HP71). Hope this is of some help. Cheers Daniel :)


Doh!! As I re-read your response, you are telling me that HP never DID the output-jack thingy that had Don and I straining to hear angels.

Okay... then these are prolly HPs owned by enthusiasts with pods who figured out how to feed their cat via audio-relay and HP75's with modems. Fess up, inventors.

Two? Or the same machine? :) Oh heavens, could there be more out there like this? The inhumanity of it all!!!


I've purchased two "pod enslaved" HP-75D's from two different sources (one for $10 BTW). Both have the speaker removed and the little headphone jack in the pod.

A couple of software things to check:

1. Make sure you issue a BEEP ON command

2. BEEP takes two parameters, frequency and duration. BEEP by itself issues a very brief chirp. Try something like BEEP 100,10

Mark Hardman


I checked all the HP-75Ds I have, three have a working beeper, one has none. This one was installed in a POD with the earphone jack. One of the others came also in a POD, but without the jack, and has a working speaker. And I checked the Expansion POD reference manual, it does not mention any speaker jack, so I think these connectors are not installed by HP. And the HP-75 speaker is a piezo element, not a conventional speaker.


Thanks to all of you helping this thread.

Okay, I shall give a piezo speaker a try (I think I have one somewhere here).

I find it funny that so many pods were "modified"-- it must have been to some good purpose... I am wondering if it was just for those inputting barcode to be able to with an earphone, hearing the entry but not disturbing others with the beeps.

Were the 75Ds used a lot in retailing, perhaps?


I presently have to 75Ds, both in pods. They came from different sources (one of the pods, in fact, has a prototype id in place of the serial number), yet both pods have the audio jack, installed in an identical manner, and neither 75Ds have the internal speaker. Given that there are other differences (one of the pods has no modem, for instance) I think it's fair to say that there were several variants (i.e., option so-and-so) of the 75D+pod combo sold by HP, some with a speaker, some with the jack.



I also have a surplus 75D with pod and wand. This machine has no speaker and the afforementioned output jack. I am sure that these machines were once part of some companies point of sale or inventory system. When they scrapped the system, several hundred machines appeared on the surplus market with these modifications installed (not by HP)



The serial number of the barcode wand is HP 92267B. Cheers Daniel


Wow, Daniel. Very generous offer. Thank You. Would take you up on it, and may still if my "mod" doesn't work, but I think this little Radio Shack speaker is actually going to fit, and I don't want you parting out what I may want to buy from you later. ;-)

Model 273-092, 8-ohm mini-speaker, 1-1/8" diameter. It has a flange round it which a Dremel-wheel removes with gusto. It appears that, installed in the speaker grille-hole, the volume comes up (at least a bit).

I'm still pondering whether any damage might have occurred to the audio output transistor, which appears to be the direct-drive to this speaker. I have no way of knowing this, just puzzled by the faintness of sound.

If I put it near my ear, just turn the machine on and issue a BEEP command, I can hear a Somewhat small BEEP, followed by an EXTREMELY faint whisper of a "machine sound" afterward, sometimes the whisper dropping pitch like the sound of a cartoon bomb, finally ending as a low "growl". Hitting enter or another key halts the whisper dead-away. I'm pretty sure that if the output of the audio section were up to snuff, the gain difference between this electrical interference pattern and the real Beep would be quite large. Hah-- my theories don't sound very scientific, do they? :D

Haven't yet tried measuring the output, but not sure if that would give me insight into this anyway: proper impedance? How to determine if all is right, when the right specs are kind of an unknown?

Daniel, if sometime you happen to be around your pod's audio jack with an AC voltmeter... on say a tone at 500 Hz....

BTW: WAND INFO: Great!!! Now to find that wand, and then figure out how to print programs in barcode!!!! LOL (aren't old machines fun?)


I havn't read the other responses yet, but the intended element was probably a pizo element. A cheap replacement may be a CRYSTAL earpiece with the bit that goes in your ear screwed off.

The 75 was probably designed to drive a high impedance source, and the speaker is low impedance. With such a mismatch you'll get very low volume and there is the possibility of damaging the driver circuitry.


Regretably, I never saw an HP 75 C/D by myself. But I noticed that the Museum has an X-ray view of an HP 75, and also an interior view. Those may help you in identifying the pads where the speaker leads were soldered, and also the kind of speaker used. I first saw the X-ray view, and the outline suggested a piezo buzzer. But the interior photo, in full color, suggests a dynamic (conventional) speaker. This last one may be a medium impedance one (not 4 or 8 ohm, it may be about 100 ohm or so). It looks similar to those used on headphones. Please take this idea just as a suggestion, you should find more "solid" evidence...

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