750C HDD doesn't spin up

[MooNWalker]

Subj. Pulling the HDD in the caddy as a whole single module from 750C and plugging it into 755C it spins up fine, but plugging it back into 750C results in a half-lit HDD indicator and no HDD sound of any kind. I suspect that there is simply no power to HDD connector on the motherboard, but I can't seem to find a pinout for it, so I'm a bit lost as to how to proceed with further diagnostic and repair. Any suggestions?

[Bondi]

First thing to do is to clean the contacts.
Did you try to plug into 750c any other HDD assembly?

[MooNWalker]

First thing to do is to clean the contacts.

The contacts look clean upon visual inspection, but I'll clean them anyway, with Deoxit.

Did you try to plug into 750c any other HDD asembly?

Yes. Same result - works in another machine, doesn't work in this one.

I have another 750C motherboard, but that one puts no power to display, even with the confirmed good DC/DC and interposer cards. I've tried comparing the two boards, but they must be of different revisions, because despite having the same FRU labels the circuitry layout is very different, especially around HDD connector.

[Bondi]

Did you also try to boot from a diskette? is the drive detected by bios?
AFAIR it's possible to test HDD and Mobo in BIOS.
From what you tell, I assume it's indeed something wrong with the motherboard.

[MooNWalker]

Did you also try to boot from a diskette?

Yup, works fine.

is the drive detected by bios?

Nope, the only sign of something being connected there is the half-lit HDD indicator LED.

AFAIR it's possible to test HDD and Mobo in BIOS.
From what you tell, I assume it's indeed something wrong with the motherboard.

All the tests in BIOS pass, no HDD test is available - the icon is simply greyed out.

I suspect the problem is only with the power circuit, +5V DC to logic board. The reason I think that is I found the HDD module that was in there originally when I bought the machine, tried connecting it, and unlike the HDD I tried earlier the HDD indicator doesn't light up at all (with previous tests it lights up, but only half way, and stays half-lit all the time). When I opened up the module I noticed that the cable is connected to the drive shifted by one pin, not covering one of the jumper pins, that is pins A and B are not covered, and there are unused sockets in the cable past pins 43 and 44 (see image below for reference). Based on the image, this configuration is likely shorting out pin 41 (+5V to logic board) with ground (pin 43 and lots of other pins), and that is the most likely cause of the fried power circuit there. If the power is shared between logic and motor that would explain why the drive doesn't spin up at all (though that could also be because drive needs both motor and logic board powered before motor will start spinning), and lack of power to the drive explains the half-lit HDD indicator with failure to detect the drive - a behavior I've seen with desktops as well if one connects the IDE cable but doesn't connect the power.

Laptop 44-pin IDE connector pinout:


I suppose I could try tracing the connections in the cable to figure out the location of the pins on the laptop motherboard itself, that should at the very least let me confirm whether there is power on the mobo connector... I'll just have to do that very carefully, to avoid shorting out anything else. And then afterwards I'd need to figure out how to find the fried components in need of replacement...

[goldeneagle]

Based on what's going on, I'd say the system board is bad. It sucks, since your other board doesn't put out video.

I know it might not be an option, but there is a PCMCIA boot, where you can use a compact flash card as your boot up device. I don't remember if this is limited to 512MB or not, since you could theoretically stick a 6GB IDE drive on that machine and get it to fire up (over 8GB is a no go).

On the plus side, taking out the system board is fairly easy, but not that fun if you can't find a suitable/fully functional replacement.

[MooNWalker]

Based on what's going on, I'd say the system board is bad. It sucks, since your other board doesn't put out video.

I know it might not be an option, but there is a PCMCIA boot, where you can use a compact flash card as your boot up device. I don't remember if this is limited to 512MB or not, since you could theoretically stick a 6GB IDE drive on that machine and get it to fire up (over 8GB is a no go).

On the plus side, taking out the system board is fairly easy, but not that fun if you can't find a suitable/fully functional replacement.

Yeah, I was unable to find the fried component on the board, but I was able to find a fully functional replacement. Unfortunately, it came with 1.03 BIOS, so it has this weird issue where any drive above 504MiB wraps around, so for 540MB drive (about 514.9MiB) the BIOS can see only ~10.9 MiB, meaning your boot partition is limited to that size. I need to finish rebuilding the battery pack before I can flash 1.43 BIOS. Fun stuff.

[goldeneagle]

I hope you're using a spot welder and rebuilding the battery pack exactly how it was before. Once you get that working, you'll have no problems updating the BIOS. Just make sure you fully charge and discharge the battery once or twice before you really start using it!