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ST251-1 loading PS down

U

uriahsky

Member
Joined
Jan 23, 2012
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37
I have a pcb tester built around a 286 with a ST251-1 in it. The ST wont spin up in the computer but it did spin up with another PS that i have. It might be the PS but when i measured the resistance between the 12V and gnd i only got about 30 ohms which seems kind of low. Does any know, or can anyone check what the resistance is between those two pins? If that is low is there a known fix for this problem?
Thank you
Russ
 
It's pretty much the rule that hard drives use more current while spinning up. According to the technical specs, the ST251 requires between 2 and 2.5A on +12 when spinning up and only 0.5A when running.

SInce it's not a purely resistive load, an ohmmeter won't be a lot of help in determining consumption.
 
Chuck is right, but since I just happened to have an ST251-1 beside me I measured the resistance and yours does seem anomalous; I get around 4000 Ohms both ways on the 12V supply pin.

The 5V side does read around 50 Ohms; are you sure you're on the right pin?

There was a post in another thread recently about replacing some caps on the power connector; you might try that.
 
Ah yes electronics worst enemy chemical reactions, some caps need replacement just about every 9 years or so because the chemical process in the dielectric.
 
I would like to say thank you for this thread, I just read that and remembered a tandy hardcard with a tm362 drive on it that would keep the machine from powering up.
I replaced the 2 caps closet to the power input with tantalums and it powers on just fine now. I'm taking it home to try on my 1000sx.
It's been sitting on my bench for months, and this thread allowed it to live again.


Later,
dabone
 
Yes, I made a mistake it was on the 5v line that I got the low resistance. I did the cap replacement and it worked and the drive now spins up, but I guess there were other problems and I think there was also a short on the mother board and a cap popped on it. I did find another motherboard so I will give that a try and hope that the controller isn't damaged.
Thanks
Russ
 
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