HP 41 high-pitch noise


Dear all,

I wonder whether some of you have made the same experience with an HP41CX: every time it runs a program or a function is executed I hear a noise of high frequency. This noise has its maximum at the back panel where the alphanumeric code is printed. Interestingly it is only present while the HP41CX is operating - while idle no sound. In contrast, my HP41CV does not show this.
Has anyone an idea where this noise does come from? Or even better - has anyone an idea how to eliminate this?

Thanks in advance


It is definitely comming from the Piezo transducer. The transducer is fixed to the back of the halfnuts and to the logic PCA of the fullnuts.

As to why it is buzzing during execution I would not know. However, try a master reset (ON + CLX) and then reload the programs. This has cleared the problem on a halfnut that 'buzzed' as soon as I reassembled it during restoration.

Hope it helps, Geoff

p.s. if the 'master clear' doesn'e work, remove the batteries and short the first and fourth battery contact. Then replace and try the 'master clear' and then reload the programs.


I used to make my HP-41 freak out back in the early 80s with various manipulations. A high-pitch noise was one of the results.

A simple master-clear should make it right. If not, I can't help any further...

The manipulation I'm talking about is to make the all-clear (clx + on) while the HP-41 is beeping, while the calculator is on. If you're lucky, the HP-41 will then squeal each time you press a key. You have to have the right timing to make it work though...


Just perform a BEEP or TONE (0..9)

This will reset the suitable CPU reg.




Always start with the simplest, least invasive fix. In this case, as Raymond states, reset the buzzer register with an overiding new beep command.

I find that the more involved one gets the longer the fix will take when in most cases the KISS principle is the best.

Cheers, Geoff

Edited: 18 Aug 2009, 1:05 p.m.


+1. Many of the 100+ synthetic TONEs leave the beep line high when they're done, so the power supply noise gets to the beeper. When the calc is idle, there's less power supply noise. Just BEEP or do a TONE(0...9) so the beep output will finish at ground instead of Vcc.


You've rediscovered "buzz mode", which is present on the 41CX, and on the final ROM revision (HFF) for the 41C and 41CV.


Thank you for your numerous contributions!

However, I turned the HP-41CX on to perform the recommended master clear and was surprised not to hear the noise anymore.
Apparently, a rest of two days was sufficient to resolve this problem spontaneously.
I was trying to follow-up the cause for that noise and came to the conclusion that an overnight run of the calculator was possibly responsible for the problem. It was attached in an HP-IL loop to run as a data logger. Perhaps a charge which might have developed in the circuits was the origin of the high-pitch noise.

Nevertheless, I will have an eye on this, and the next time I will try to resolve this by the generation of a TONE or by a master reset, and keep you informed.

Regards, Frido

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