HPZ840 Fan noise

[SDH]

https://youtu.be/nGhSa29oNVo

Check out this YouTube Z840 video from HP. It is a bit over an hour long but has a table of contents so you only need to see what you want. I did not know this existed... handy, and it shows what the different cables look like.

[mhx4]

The fan noise problem on my HP Z840 is solved.

The suggestion by Dgroves that it was caused by a temperature sensor hidden in the speaker cable was correct.

Some notes:

1. On my HP Z840 the on-off switch is extremely complicated to access. I therefore decided not to order the new HP cable advised in this thread and directly replace the sensor transistor, which is near the speaker and much easier to manipulate.
2. The right side panel of the Z840 must be removed. How to do that can be found by following the links in this thread. You will need a rubber hammer and a piece of wood, because it doesn't come off easy. Putting it back is fiddly but once you've done it there is nothing interesting to tell.
3. I did not believe the transistor could be broken (there is no stress on it), so I experimented a bit with it first. Putting a 5nF capacitor in parallel immediately made the fan run down to an acceptable level. I assumed there was quite a lot of noise being picked up and assumed the cable shield had gone bad or something fell off the (refurbished) motherboard?
4. When I prepared to solder in the 5 nF (a week later) it didn't work: the fans howled full-blast again. However, at bootup I had seen messages about fan failures, maybe because I was careless when removing and putting back the power supply and fan assembly. I made the decision to replace the transistor (which proved to be a 2n3904) with a new 2n2222, and *also* soldered the 5nF in parallel. After carefully re-seating the power supply and CPU fans the system booted up silently and stayed that way... Unfortunately, this means I am not certain if the 2n3904 really had failed, or that the 5nF did the trick. I suggest to experiment with a small capacitor if a replacement cable does not work.