Help on HP 82901 drive
ard at p850ug1.demon.co.uk
Sat Jun 16 17:26:00 CDT 2007
> >I think I'd first check I was getting a master clock on pin 24 (I think
> >this should be 1MHz), and then look at pin 26 (read clock) when the unit
> >was trying to read a floppy.
> On pin 26 there is absolutely nothing: it's always low.
> I traced it back, and found that there is nothing because pin 11 of LM361
> (output 1) is always low.
> I didn't go back it yet.
I've just checked my unit for you.
I get 1MHz on pin 24 of the FDC chip (as expected) and about 250kHz on
pin 26 (read clock). There is twice this frequency (500kHz) on pin 11 of
More interestingly I've checked the supplies to the LM361. I get +12V on
pin 1 (+ve supply), +5V on pin 14 (output stage +ve supply) _and_ -9.5V
on pin 6.
So there is a -ve supply (I'd half-suspected there was, knowing that
other HP drive units which use a load of transsitors + an LM361 as the
VCO/PLL also have a -ve supply). I've not yet discovered where it comes
from, it must be generated on the controller board (my guess is that
those 2 metal-can transsitors at the front left corner have somethign to
do with it).
Can you check the supplies on your unit. If the -ve supply is missing it
could explain a lot.
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