You mean something like a first-order RC (or LC) low-pass filters in the speaker systems?
You can play with calculators and try it out: http://www.claredot.net/en/sec-Sound/low-pass … s-over-12dB.php

Is there a way to visualize what kind of frequency is being passed into the speaker? I can only hear it at the moment, so I don't know what is happening exactly on the wire.

The problem occurs between POST and a short time after the system boots. Eventually it stops altogether on its own, but is consistent between reboots.

Kahenraz wrote on 2022-11-12, 19:02:

Is there a way to visualize what kind of frequency is being passed into the speaker? I can only hear it at the moment, so I don't know what is happening exactly on the wire.

Do you have an oscilloscope? That would probably be the most straight forward way.

Another option would be to record the sound and use a frequency spectrum analyzer. Though depending on the frequency and relative volume, you'll need a mic capable of accurately capturing the sound.

I imagine what you describe as very high freq is only few kHz at most, so technically just a medium one. If they are only narrow peaks there is a possibility of doing a notch filter to make them much attenuated, but they'll work just as effectively for things you do want to hear so in the end you will hurt the sound anyway. You can try to do a small recording and then spectral analysis will tell what needs to be dealth with...

I guess it's supply ripple caused by periodic current spikes. This is likely to be a specific frequency, so a noch filter is a gold idea. Ideally, the noch filter would be bodged into the audio circuit before the speaker amp. I'm thinking about a series resonant LC combination at the speaker amp input pin to ground.

If Kahenraz has an Android phone, I suggest getting the Spectroid app for real-time audio spectroscopy.

This laptop has died, so the problem is on hold. I had it set on a table in front of me, idling in DOS, and the laptop powered off suddenly. It isn't responding to the power button, despite there being voltage on the power cable.

I've already invested time cleaning it up, buying a power brick, a new keyboard, and CMOS battery. This is very disappointing, and unexpected.

Sorry to hear.. 😔 Perhaps some mosfet/regulator failed. 🤷‍♂️
Later laptops have their own PSU, essentially, which is fed with a powerbrick that provides 15 to 18v or so ?

mkarcher wrote on 2022-11-12, 20:04:

If Kahenraz has an Android phone, I suggest getting the Spectroid app for real-time audio spectroscopy.

The Tricorder works, too. At least for a quick overview.
I occasionally use it because it simultaneously shows spectrogram and waveform.

mkarcher wrote on 2022-11-12, 20:04:

I guess it's supply ripple caused by periodic current spikes. This is likely to be a specific frequency,

Then the fact that switching frequency has dropped into audible range, way below normal operating conditions, could indicate that the voltage regulator was already on its way to Valhalla.

vstrakh wrote on 2022-11-13, 09:27:

[..] could indicate that the voltage regulator was already on its way to Valhalla.

Or Sto'Vo'Kor, if it was fighting a hopeless battle just before..

vstrakh wrote on 2022-11-13, 09:27:

mkarcher wrote on 2022-11-12, 20:04:

I guess it's supply ripple caused by periodic current spikes. This is likely to be a specific frequency,

Then the fact that switching frequency has dropped into audible range, way below normal operating conditions, could indicate that the voltage regulator was already on its way to Valhalla.

If the primary switching frequency goes down into the audible range, the regulator already is toast. I suspect that the audible frequency is either an interrupt rate waking the CPU from low-power idling at a regular interval, or a subharmonic of the primary switching fequency, possibly generated by the regulator starting to oscillate due to high output ESR. During POST and OS boot, low-power idle modes of the CPU are unlikely, so instable regulation caused by (nearly) dead capacitors around the core voltage regulator are a likely fault reason. I didn't suggest that in my first post, because while worn capacitors get better when they get warm during operation (this would explain why the noise disappears "by itself" after some time), they don't get bad by just rebooting the machine, but only when they cool down. So if the noise re-appears on a reboot when the machine is still at "operating temperature", this would be a point agains the capacitors as fault reason. Yet, if someone with an ESR meter is near, I suggest to have the filter caps of the core voltage regulators checked and if the ESR is high (above 1 Ohm), get them replaced.

There are only three electrolyte or polymer capacitors on the entire board; the rest are all either tantalum or ceramic. Nothing appears out of place visually, and I couldn't detect any shorts with some casual probing.

I removed the board from the laptop frame to inspect both sides and haven't been able to locate the porblem.

Kahenraz wrote on 2022-11-14, 02:39:

There are only three electrolyte or polymer capacitors on the entire board; the rest are all either tantalum or ceramic. Nothing appears out of place visually, and I couldn't detect any shorts with some casual probing.

I removed the board from the laptop frame to inspect both sides and haven't been able to locate the porblem.

The issue with compact laptop electrolyte or polymer caps is that you often get no sign that they are worn. Sadly, a standard multimeter isn't able to test for the "equivalent series resistance" of these capacitors, you need a specific device to test for it. Dedicated ESR testers usually work fine even for capacitors in-circuit. The generic $10 "universal component tester" you can get on ebay (common model number "LCR-TC1") will display the ESR of capacitors, too, but you usually need to desolder capacitors and test them out-of-circuit if you want to use a tester like that. Dried up electrolytic caps do not appear short, and might even measure nearly fine in a pure capacitance measurement.

I ordered a doner laptop of a similar model with a damaged screen. unfortunately, deposited appearing cosmetically identical, the two screens were incompatible.

I then started testing various aspects of the new machine and found that it wouldn't power on without the presence of a smaller secondary board at the bottom, which connects to the main battery. This gave me an idea. I swapped just this smaller board into the old machine and it worked! So it wasn't the entire motherboard that blew, but rather this smaller board that contains some kind of additional power circuitry.

I inspected the old power board one under my microscope but couldn't find any visible faults or shorts with my multimeter. I also removed and tested the capacitors, which were fine. So it's probably one of the more complex power IC chips. Which one it is, I have no way to tell. It's otherwise I going cosmetic condition.

I also found the source of the high frequency noise. It's not the speakers but the LCD inverter. Confirmed this by unplugging the speakers and could still hear the noise. I kept searching for it with my ear until I found that it was the inverter. This vintage of laptop would mean that it's a CFL inverter. I'm not sure what to do about this noise.

It probably makes the noise because of the CFL tube aging, the older they get the harder they seem to be needing to be driven and it starts straining the inverter. If there's any protection, the inverter gets shut off eventually, if not, there's blue smoke potential 🤣

Out of curiosity, I compared the two inverter boards between the two LCD panels I had, even though the panels themselves were incompatible. And surprisingly, the inverters had the same model number and appeared to be exactly the same. I swapped just the inverter over and the noise it gone!

It was definitely some fault of the inverter. Mystery solved.

That is good, I hope this system will last ~

I only have the one aftermarket power brick. There is a very real possibility that it's faulty and is the cause of the original failure. At any moment it might send a surge of power into this repaired unit and destroy it as well.

Hopefully not...

Some of those may be quite bad indeed, I have seen a few myself. Those usually tend to be really lightweight compared to any other proper laptop brick, you can almost feel that they're hollow inside.