I'm asking if the pad which I have labelled "negative preamp supply?" connects to Vp1 or Vp2. That square multi-function IC is the spindle motor controller and VCM controller. It is also a PMIC which generates the onboard supplies.

If Vp1 doesn't appear briefly after power-up, then I would suspect a failure in the MCU or motor controller. That's because the MCU commands the motor controller to switch this supply on and off.

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A backup a day keeps DR away.

Yes, it connects to vp1.
On the updated photo I've marked vp1 and vp2 with distinct colours (orange-ish and green-ish) and used the same colours near the motor controller.

I don't see any voltage on vp1 after powering up.
damn, I've hoped the big ICs didn't take the hit :/

The motor controller is powered from both 5V and 12V. It generates the lower voltages required by the MCU.

The MCU appears to be connected to SATA power pins P1, P2, P3. P3 is the Power Disable pin, but I don't know what the others do, if anything (I thought they weren't connected?). Normally these pins receive +3.3V in older systems that predate the redefinition of P3, so perhaps your PSU misadventure impressed the wrong voltage on this end of the connector. It might be worth measuring the resistance between ground and each of P1-3, and the resistance between these pins and Vio (1.8V).

Here is the datasheet for one of the early SMOOTH motor controllers (L7250):

https://pdf.dzsc.com/L72/L7250.pdf

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A backup a day keeps DR away.

I understand you mean the orange pins from attached image?
I'm 99.9% positive those pins were never powered. I used a 4 wire sata power connector (YBRB) and pins 1-3 are not wired in my connector (also I can't put the plug wrong because of the "L" shaped connector.

I've measured resistance beetween GND and each P1,P2,P3 individually -- all max out.

One thing that made me wonder when taking more measurements is that when checking resistance between the test point at 0V (attached below) and GND, it shows a short. Is that expected?
Normally when testing points around inductors to GND i see some resistance (low, but still 2-6Ohm, but the one at 0V is <1Ohm. FWIW the 1.5V test points (also around inductor) show OL to GND.

That short circuit is expected. That inductor is part of an inverting boost converter (negative switchmode regulator). The diode is inside the IC. I wasn't sure of this until your measurements confirmed my suspicions. Thanks.

See https://www.hddoracle.com/viewtopic.php?p=596#p596

I think most of the inductors on your board would have resistances much less than 1 ohm.

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A backup a day keeps DR away.

Thanks for the more detailed explanation on these voltage regulators.

so I understand that we're missing -5V on the Vp1. If IC was working properly, should I see constant ~-5V on Vp1 even when no drive is attached?

btw what about the spindle motor power delivery? shouldn't at least that part "work" (ie. platters rotating even if actuator arm won't move)? I'm guessing the MCUs are "smart" and they wont spin up the disks if preamp isn't communicating in the first place?

I'll try to get my hands on some cheap, faulty (with damaged platters) drive that has the same motor&board controllers and replace them.

The -5V supply is used by the preamp IC on the headstack. If the MCU doesn't detect the preamp, which is the case when the PCB is disconnected from the HDA, then it switches off this supply. That is, the MCU switches on this supply, looks for a preamp, doesn't find it, and then switches off the supply. In fact, in some current models this voltage is -3V.

https://hddscan.com/doc/HDD_from_inside.html

https://hddscan.com/doc/images/hdd_main_parts/preamp.jpg

I should say that I don't know for certain that your drive behaves in the way that I expect, but that's how some earlier models behave. There are also other designs where the negative supply is always switched on, regardless of the circumstances.

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A backup a day keeps DR away.

I've attached the HDA, powered it up and measured voltage on vp1 (i've scraped myself a test point on the other side of the pcb) but still 0V.

I'll post an update once I get my hands on a compatible pcb (ie. board with same main controller and motor controller), take some measurements there (without HDA) and see if my bga skills are any good

I suggest on some of these slightly more recent logic boards that a more modern digital multimeter can be helpful. I have a cheap but recent multimeter and it is handy.

Checking for shorts is another aspect of finding faults. My cheap multimeter black can be connected to ground (screw hole) and then the red lead can poke around to see if any shorted out parts are present.

SATA disks are slightly different from the older EIDE. I have a USB/SATA cable in addition to enclosures. The cable allows for easier probing of a given disk.

Often overlooked is checking the power to the drive motor. If it is too low the disk will not spin. So check the path to the SATA connector.

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Orico DS500C3-US-BK 5-disk 3.5" SATA, iTGZ 4x M.2 2280 Thunderbolt enclosure NVMe/SATA, etc. etc.

update:
I've found a donor board from another (faulty, though with damaged plates) drive (different HDD model, but same board and thus ICs). I've tested the voltage on the Vp1 without HDA attached and there's constant ~-3V there.

I've started replacing the motor IC when I realised the pads on this IC have some weird offsets... good luck to me reballing that using an universal stencil *sigh*. looks like i'll have 88 balls to put manually on the chip.

Reballing is a tricky at best. Stencils are available for a wide range of BGA configuration.

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Orico DS500C3-US-BK 5-disk 3.5" SATA, iTGZ 4x M.2 2280 Thunderbolt enclosure NVMe/SATA, etc. etc.