Correction 1: Testing more closely, it does not appear that DIR is being held high all of the time. What's happening is actually that the BIOS is doing a recal operation during POST test, which is supposed to seek outward until it hits track 0, but since it actually seeks the other way it never hits track 0 and never reverses direction (DIR low). If I step up or down in Image Disk, I can see that the DIR is going high and low accordingly, but the drive is only seeking inward.

Correction 2: I incorrectly said above that it only steps "outward" rather it only steps "inward" - towards the middle, towards track 79.

If the schematic is correct then this drive has a separate amp/driver chip for the stepping motor: HA13421AMP (IC103). I assume you've checked that the DIR signal does indeed reach the main logic control chip. From there it's 3 signals to the amp, two are for stepping (STM1, STM2) and one is for current reduction when stepping is done (STPY). Since you have a scope you should make sure these signals are sent properly and reach the amp chip. If so the amp itself might be busted, wouldn't be the first time Sony coil drivers failed due to being worked too hard (looking at you, various PS2 laser coils chips).

Other then that you should make sure the coils in the stepper show similar DC resistance and there aren't any problems with connectors or soldering between the motor and the amp chip. Stepper motor can do weird things if the coils are not energized properly. Also check power delivery to the amp chip, the resistor network there is only to provide the lower current when stopped and should not be much of an issue, the main power comes through that jumper SL105 which you should investigate. I would assume it's soldered in the correct position with a piece of wire but do check anyway.

Testing these chips in-circuit is not really possible, so if the control signals look OKish then I would assume the amp is damaged and must be replaced. As I've said Sony does have a history of their coil driver designs crapping out after some time.

Deunan wrote on 2024-06-27, 08:53:

If the schematic is correct then this drive has a separate amp/driver chip for the stepping motor: HA13421AMP (IC103). I assume you've checked that the DIR signal does indeed reach the main logic control chip. From there it's 3 signals to the amp, two are for stepping (STM1, STM2) and one is for current reduction when stepping is done (STPY). Since you have a scope you should make sure these signals are sent properly and reach the amp chip. If so the amp itself might be busted, wouldn't be the first time Sony coil drivers failed due to being worked too hard (looking at you, various PS2 laser coils chips).

As you mention below, testing in-circuit on this board is a bit tricky since those two chips are located on the top side of the board which is inaccessible when the board is installed. Would have to figure out a way to rig it up so that the motor control ribbon cable and the stepper motor cables can both be connected while those pins are free to probe. I haven't seen the top side of the board yet so not sure if it might be possible to tack solder test leads or not.

Deunan wrote on 2024-06-27, 08:53:

Other then that you should make sure the coils in the stepper show similar DC resistance and there aren't any problems with connectors or soldering between the motor and the amp chip.

Yeah, tested the coils and both windings are good (I believe it was about 5 ohm DC on both).

Deunan wrote on 2024-06-27, 08:53:

Also check power delivery to the amp chip, the resistor network there is only to provide the lower current when stopped and should not be much of an issue, the main power comes through that jumper SL105 which you should investigate. I would assume it's soldered in the correct position with a piece of wire but do check anyway.

That had occurred to me too and I double checked that all of those A/B pads matched the photo provided by the OP of that earlier post, and they do. Unfortunately, the locator numbers on that schematic don't match to the ones on the pcb, so I'm not 100% sure which one that is... at least until I pull the board.

Deunan wrote on 2024-06-27, 08:53:

Testing these chips in-circuit is not really possible, so if the control signals look OKish then I would assume the amp is damaged and must be replaced. As I've said Sony does have a history of their coil driver designs crapping out after some time.

I suppose the question would be if that's an available part to source. This drive was listed as "NOS" and came in a sealed plastic bag, so I'd have to assume that to be true and never been use... doesn't show any signs of like discoloration or dirt. So if it's dead, it died in storage or blew at the first power up. Interestingly I swear the first time I powered it up it seeked correctly but since it took a few cycles to realize what was going on, I wouldn't stake a bet on it. I was hoping for a bad connection or bridged solder (it had some dodgy looking hand work on it, from the factory?) but haven't found any which means it would have to be on the top side, which feels less likely.

640KB wrote on 2024-06-27, 14:45:

As you mention below, testing in-circuit on this board is a bit tricky since those two chips are located on the top side of the board which is inaccessible when the board is installed.

It's PITA because of all the work required but if you can't access the measure point otherwise consider soldering some short wires and putting the PCB back in it's place. Just be careful not to short the loose leads to anything.