HP-41CX fullnut noise!!!


I still continue searching my main board problem. I cleaned every contact, i check every diodes, condensators and resistor the best i can. I reassembled the calculator and it works great for about 1 minutes : great improvement over the 1 second i was use too...

I realize another things : in the minute of good working, nothing anormal is heard but the calculator is slowly beginning to give a distortion noise comming from what i think is the main board. When the noise is steady, the calculator stard do freegames again (about 1 minutes).

I thought it was the piezospeaker giving that noise so i removed it. reassembled the calculator and the noise is still there!!! WHAT ELECTRONIC component can do noise in the mainboard of a HP-41CX mother board?!?

Thanks for help!

Edited: 21 Oct 2003, 5:27 p.m. after one or more responses were posted


The only other component I think that can make a noise is a coil. But I don't know it the halfnut have one, the fullnut have one in the switching power supply.

Good luck!



It was my immagination : when i remove the piezo, no noize can be hear... So the noise is from the piezo.

When i turn on the calc no noise (calculator 100% fonctionnal) and slowly a quiet creaking high frequency noise noise is heard mor and more continuously. a minute after, the noise get steady and calc get buggy LCD and key.

Edited: 21 Oct 2003, 5:57 p.m.


It must be the power supply regulator! There is a coil in the board that is switched in high frequencies but if you can hear it the regulator can be defective (lower frequency) and the voltage may be altered (erratic behaviour).

This can be caused by a discrete component (probably a capacitor) that is too old. Can you check the components or the voltage? It can start good and drop to a altered voltage (the point that you hear the noise).

Good luck



I have never had or even held a 41C of any kind, but some calculators' rechargeable batteries emit oftentimes a high frequency whine. But in my cases, I had to hold the calculator somewhat close to my head to hear it.


The battery or a battery pack? I have some TIs that uses a battery pack with two NiCD cells and a power converter with a coil. That coil can emit a high frequency noise, I think...




Nelson --

I hope that you're right about the symptoms Michel hears, being cause by a failed discrete component. My 1984 fullneut 41CX has nearly identical symptoms -- intermittently works properly, then a high-pitched ringing broken by clock pulses, key-presses causing shutoffs, and erratic display.

Perhaps www.fixthatcalc.com will be able to make this one functional for little more than the standard $60 fee. I don't mind paying them -- I'm not an electronics technician or a 41C* expert.


I have acquired a pretty rotten fullnut 41C some time ago that had the same problem, but only itermittent. I eventually found out that the contact between the battery/module flex connector and the keyboard PCB was the problem. Repeated thorough cleaning of this contact with contact spray has finally resolved the issue, the machine now works pefectly.

I found this out because a) the same problem occurred with this calc after I had exchanged the main PCB with a known good one, and b) because I could cause or prevent this behaviour by pressing the case halfes together at various spots near or between the ON/USER and PRGM/APLHA switches.

I guess the other contact (between keyboard PCB and main PCB) could cause similar problems, plus a few others. I'd try to get these as clean possible.

Hope this helps, and keep us posted!

Cheers, Victor

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